large surface biological/chemical decontamination …
TRANSCRIPT
LARGE SURFACE BIOLOGICAL/CHEMICAL
DECONTAMINATION
by
JONATHAN NELSON BLACKWELL, B.S.
A THESIS
IN
ELECTRICAL ENGINEERING
Submitted to the Graduate Faculty
of Texas Tech University in Partial Fulfillment of the Requirements for
the Degree of
MASTER OF SCIENCE
IN
ELECTRICAL ENGINEERING
Copyright 2002, Jonathan Nelson Blackwell
ACKNOWLEDGMENTS
The exploration into the large surface decontamination of biological and chemical
agents performed over the last 15 months could not have been completed without the help
of multiple people and departments at Texas Tech University (TTU). I offer special
thanks to Dr. O'Hair and Dr. Dickens, in the Electrical Engineering Department at TTU,
who gave me the opportunity to work on this project and have been constant sources of
advice, direction, and the main driving force throughout the project. I would also like to
recognize and thank Mike Hoffman for his significant contributions and work on the data
acquisition system. Mike was a great help when performing the tests and keeping me
company while we listened to the noise of 60,000 volts kill spores. Recognition to the
long standing efforts of Dr. Kristiansen and the rest of the faculty and staff working in the
Center for Pulsed Power and Power Electronics should also be made. (Dino Castro,
Daniel Garcia, and Lonnie Stephenson were especially helpful throughout the duration of
the project.) The facilities, equipment, and resources wdth which I was provided are a
result of countless years of work by these people.
Dr. Fralick and his lab assistant Kathryn Royce, who work in the Microbiology
Department at the Health Science Center at TTU, have been instrumental in providing the
biological agents, preparing the test specimens, transporting the specimens to the
Electrical Engineering lab, and evaluating the specimens after exposure. Dr. Dasgupta
and Dr. Karthikeyan Sathmgnan, a post-doctoral researcher, were responsible for
preparing, transporting, and evaluating the chemical agents used during the experiments.
Dr. Dasgupta and Dr. Sathmgnan work in the Chemistry department at TTU. The
judgment and professional experience of both Dr. Fralick and Dr. Dasgupta have been
invaluable resources throughout this project.
On a personal note, I would like to offer special thanks to my wife. Heather, who
has been a constant source of love and support. I would also like to thank my parents for
the love, support, and guidance they have offered throughout my life. They are
responsible, in a large part, for the person I have become.
11
TABLE OF CONTENTS
ACKNOWLEDGEMENTS u
ABSTRACT v
LIST OF TABLES vi
LIST OF FIGURES vii
LIST OF ABBREVIATIONS ix
CHAPTER
L INTRODUCTION 1
1.1 Background 1
1.2 Project Objective 3
IL EXPERIMENTAL SETUP 7
2.1 Generation of Killing Mechanisms 7
2.1.1 Pulsed Corona Discharge 7
2.1.2 Heat 17
2.1.3 Hydroxide (OH) Radicals 21
2.2 Data Acquisition 22
2.2.1 Oscilloscope Settings 23
2.2.2 Agilent Data Acquisition/Switch Unit. 26
2.2.3 Solutions to Electromagnetic Interference (EMI) 28
2.2.4 Power Supplies... 30
III. CHEMICAL AND BIOLOGICAL AGENTS 32
3.1 Biological Agents 32
3.1.1 Background 32
3.1.2 Preparation 32
3.1.3 Coupon Analysis 33
3.2 Chemical Agents _ 34
rV. CHEMICAL AND E. COLI TEST RESULTS 35
4.1 Method for Calculating Results 35
4.2 Chemical Experimental Results 37
111
4.2.1 Malathion Exposed to the Pulsed Corona Discharge... 37
4.2.2 Parathion Exposed to the Pulsed Corona Discharge.. 38
4.3 E. coli Experimental Results 41
4.3.1 E. coh Exposed to the Pulsed Corona Discharge 42
4.3.2 E. coh Exposed to Only Heat. 43
4.3.3 E. coU Exposed to Heat Then the PCD 45
4.3.4 E. coli Exposed to the PCD With Water Vapor 46
V. SPECIFIC APPLICATION TO THE ANTHRAX PROBLEM. 48
5.1 Motives and a Re-Evaluation 48
5.2 Bacillus Experimental Results 49
5.2.1 Bacillus Exposed to the Pulsed Corona Discharge 49
5.2.2 Bacillus Exposed to Only Heat 50
5.2.3 Bacillus Exposed to Heat Then the PCD 52
5.2.4 Bacillus Exposed to the PCD With Water Vapor. 52
5.2.5 Bacillus Exposed to the PCD With Water Vapor when Covered 54
VI. FUTURE RESEARCH 56
VIL CONCLUSIONS 58
REFERENCES 59
APPENDDC A. RESULT TABLES FOR ALL TESTS 60
B. SLEDE LAYOUTS FOR ALL TESTS 67
C. POWER CONDITIONING UNIT (PCU) 73
IV
ABSTRACT
Contamination by microorganisms such as E, coli and Anthrax has been a source
of concern in the agricultural and food processing industries for many years. However,
the increasing presence and availability of the more robust microorganisms, such as
Anthrax, present a significant threat to the safety and quality of life to which we are
currently accustomed. As the knowledge and technology to manufacture and weaponize
lethal sfrains and different suspensions of biological and chemical agents become more
available, the knowledge and technology to detect and desfroy these biological and
chemical agents must also be developed. Methods currently used to decontaminate
biological agents include elecfron beams, exposure to very high heats in excess of 130°C,
exposure to Ultraviolet (UV) light, and the use of liquid solvents such as bleach, just to
name a few. Based on previous research, another very effective decontamination
technique is to expose the agent to an arc jet plume, which exhibits multiple killing
mechanisms but is energy inefficient. It is of great interest to explore additional
decontamination techniques that decontaminate more rapidly, are more energy efficient,
do not harm the contaminated surface, and do not leave behind a residue which must later
be cleaned.
The pulsed corona discharge (PCD), which generates many of the same killing
mechanisms as the arc jet, is thought to be a viable solution to the decontamination of
biological and chemical agents; especially when combined with environmental enhancers
that increase the amount of water vapor in the air or that increase the temperature of the
specimen. A combination of experiments has been performed with the PCD utiUzing the
environmental enhancements previously mentioned on both chemical and biological
specimens with the main goal pointing towards the decontamination of Anthrax in
various environments. Additional experiments have been performed to determine if
common barriers, such as paper or plastic envelopes, would inhibit the killing efficiency
of the pulsed corona discharge. The most immediate benefits of the research performed
with the pulsed corona discharge could be applied toward the destmction of Anthrax in
contaminated mail.
LIST OF TABLES
A.l Malathion Exposed to the Pulsed Corona Discharge 61
A.2 Parathion Exposed to the Pulsed Corona Discharge 61
A.3 E. coh Exposed to the Pulsed Corona Discharge 62
A.4 E. coh Exposed to Only Heat 62
A.5 E. coh Exposed to Heat Then the Pulsed Corona Discharge 62
A.6 E. coli Exposed to the Pulsed Corona Discharge with Water Vapor 63
A.7 Bacillus Exposed to the Pulsed Corona Discharge 64
A.8 Bacillus Exposed to Heat Then the Pulsed Corona Discharge 64
A. 9 Bacillus Exposed to the Pulsed Corona Discharge with Water Vapor 64
A. 10 Bacillus Exposed to Heat Only 65
A.ll Bacillus Exposed to the PCD with Water Vapor when Covered 66
vi
LIST OF FIGURES
2.1 First Wire Array Stmcture 10
2.2 Second Wire Array Stmcture 11
2.3 Layout for Heat Exposure Tests 19
2.4 Temperature Variation of Hot Plate at 80°C 21
2.5 Schematic of the Data Acquisition System 24
2.6 Location of Current and Voltage Monitors in A2 Unit of PCU 25
2.7 PINetworic 29
2.8 Opto-Isolator Circuit 29
4.1 Destmction Process of Malathion 38
4.2 Malathion Survivability When Exposed to the PCD 39
4.3 Percentage of DDTP Present When Exposed to the PCD 39
4.4 Parathion Survivability When Exposed to the PCD... 40
4.5 Paraoxon Present During Exposure to the PCD .40
4.6 Parathion Chromatograms .41
4.7 E. coh Survivability When Exposed to the PCD .43
4.8 E. coh Survivability When Exposed to 50°C .44
4.9 E. coh Survivability When Exposed to 75°C .45
4.10 E. coli Survivability When Exposed to Heat Then the PCD. .46
5.1 Bacillus Survivability When Exposed to the PCD 50
5.2 Bacillus Survivability When Exposed to 100°C 51
5.3 Bacillus Survivability When Exposed to 125°C .51
5.4 Bacillus Survivability When Exposed to Heat Then the PCD 52
5.5 Bacillus Survivability When Exposed to the PCD with Water Vapor 53
5.6 Bacillus Survivability When Placed Inside the Cushion Mailer 55
B. 1 Layout for Bacillus K, L, N, O Tests 68
B.2 Layout for Bacillus X, Y, Z, AA, BB, CC, DD, FF, KK, LL, NN,
0 0 and Parathion M Tests 68
B.3 Layout for Bacillus J, M, P, S, and E. coli Q Tests .69
B.4 Layout for Bacillus V and W Tests 69
vii
LIST OF FIGURES
B.5 Layout for Bacillus T and U Tests 70
B.6 Layout for Bacillus EE and GG Tests 70
B.7 Layout for Bacillus HH Test 71
B.S Layout for Parathion K, Malathion H Tests 71
B.9 Layout for Malathion I Test 72
B. 10 Layout for Parathion H Test 72
C I Circuit Diagram of the Thyristor Charging Unit Al 77
C.2 Circuit Diagram of the High Voltage Nanosecond Pulse Shaper A2 .....77
C.3 Circuit Diagram of the Control Unit A3 78
VI11
LIST OF ABBREVIATIONS
A ampere: the SI unit for electiical current
AC Alternating Current
AWG American Wire Gauge
BNC Bayonet Neill Concelman [Electronics]: Connector used with coaxial cable, named after inventor
DC Direct Current
GSa/s Giga-Samples per second
Hz Hertz: the SI unit for frequency
IR Infrared (as in IR temperature probe)
J Joule: a unit for Energy
kJ kilo-Joule (1000 Joules)
kV kilo-Volt or 1000 volts
kVA kilo-Volt Amp (1000 Vohs Amps)
MHz mega-Hertz (1,000,000 Hertz)
mL milliliter (0.001 liters)
mm millimeter (0.001 meters)
ms milhsecond (0.001 seconds)
ns nanosecond (1 x 10' seconds)
nmole nanomoles (1 x 10' moles)
Q Ohm: the SI unit for electrical resistance
PCD Pulsed Corona Discharge
PCU Power Conditioning Unit
SOS Semiconductor Opening Swdtch
fxs microsecond (1x10"^ seconds)
UV Ultraviolet (as in UV light)
V volt: the SI unit for voltage
IX
CHAPTER I
BACKGROUND AND PROJECT OBJECTIVES
1.1 Background
On a daily basis, some portion of the nation's population comes in contact with
microorganisms and chemical compounds that are potentially lethal in the appropriate
concenfrations. Some of the more common hosts in industry to these agents include
feedlots, insecticides, and pesticides. Unfortunately, the probability of agents such as
these being used as weapons has also increased. As the presence of microorganisms such
as Anthrax and compoimds such as Parathion and Malathion become a more intmsive
presence and threat to the safety of the general public, technologies and decontamination
techniques must be developed which are increasingly energy efficient, decontaminate
more rapidly, do not harm the surface to be decontaminated, and leave no residue.
Throughout industry, many different decontaminafion systems are used which utilize
different sources of energy. Some of these destmction techniques include the use of
electron beams, killing with excessive amounts of heat, and liquid solvents such as
bleach. The efforts of this research group have been focused on determining the
capability of a pulsed corona discharge to desfroy biological and chemical agents. The
pulsed corona discharge is believed to generate killing mechanisms which will resuh in a
more energy efficient, rapid, non-destmctive solution to the problem of contamination by
biological and chemical agents.
The technology for this research was first investigated at Texas Tech University
(TTU) Center for Pulsed Power and Power Electronics in the late 1980s with NASA
sponsored research on arc jets, one of the satellite size electric space propulsion
technologies. The main parties involved with this research at TTU include Dr. E. O'Hair,
Dr. J. Dickens, and graduate students and lab technicians working imder their
supervision. The details concerning the background of the technology have been provided
by Dr. O'Hair. The primary focus of the research was directed toward reducing the
cathode erosion such that the operating time in the space environment could be extended
to at least 1000 hours. Both 10 and 30 kW size arc jets operating at 100 and 250 amps
1
DC were investigated. The gas propellants used were nitrogen, hydrogen, or a
combination of the two at a flow rate of a few tenths of a gram per second. At these
current levels and small flow rates the nozzle exit gas temperature was very high, on the
order of about 10,000 °C. The plasma that was produced consisted of elecfrons, ions,
atoms and molecules. In addition to these elements, an elecfric field. Infrared (IR)
radiation, and Ulfraviolet (UV) radiation were also produced.
In 1997, the US Air Force, in an STTR solicitation, requested proposals on the
destiiicfion of biological and chemical warfare agents. The research group at Texas Tech
University (TTU) fonned a team v^th MONTEC, a Montana small business. The team
was awarded a one year contract to investigate the decontamination of biological and
chemical agents on large surface areas using a high temperature plasma.
The biological agent used during the experiments with the arc jet was a strain of
Bacillus spore. Bacillus spores were employed as the test specimen because they are
very good simulants for Anthrax, a well-known biological warfare agent. The spores
were plated on fiberglass slides. The exhaust environment produced by an arc jet is very
destmctive of aerosol deposited biological agents. Results showed that these agents
could be desfroyed, using air as the propellant, at a rate of 3 mph without causing any
damage to the surface. The proposed operational concept was to develop a 5 ton tmck
size mobile unit, using a row of arc jets positioned a couple of inches above the surface.
The decontamination unit would move fast enough to "clean-up" a road or mnway in a
reasonable time but not so fast that the residue agents would become airborne and blown
away.
The technical decontamination objectives of this research were met. However, to
operate a 3 meter long row of 10 to 15 kW arc jets would require a tmck-mounted
electric generator that could produce over 200 kW of power. The supersonic high
temperature gas plasma of the arc jet contains multiple killing mechanisms which, when
used individually at sufficient power levels and duration, have desfroyed surface
deposited chemical and biological agents. While the arc jet exhaust is a very effective
method for decontamination, it is not a very energy efficient solution. The two main
drawbacks with using the arc jet large surface decontamination system are: that it will
require a large supporting electric generator and that it probably generates a more
desfructive environment than is required. These two drawbacks have lead to the first year
of research supported by SBCCOM and perfonned by the group at TTU.
A proposal was submitted to, and funded by US Army SBCCOM for the first year
of a three-year effort to research and develop the concept of a mobile field deployable
chemical and biological large surface decontamination system. In addition to effectively
decontaminating chemical and biological agents, the decontamination system should also
preserve the surface that is being decontaminated, leave no residue that requires clean-up,
decontaminate 15,000 square meters per hour, require no non-standard expendables, have
a long maintenance free "shelf-life," and have the ability to operate for longs hours with
minimum maintenance.
The technology exists in single sources such as: electron beams, UV lights, pulsed
corona discharges, etc., to reproduce portions of the arc jet environment. Electrically
driven sources like these listed will meet the general consfraints addressed in the previous
paragraph. From the published data there are no individual sources that meet the rapid
decontamination rate objective. So, the question that must be addressed is: What
combination of sources in series or concurrently can produce the desired result, i.e.,
"acceptable" surface decontamination with only 0.05 seconds of exposure, at a
"reasonable" ratio of square meter/kW-hr?
1.2 Project Objective
Past research has shown that electrical technologies can be very effective tools to
rapidly decontaminate large surface areas of biological and chemical agents. For the first
year, certain tasks have been addressed to provide the infrastmcture for future work on
electrically driven biological and chemical decontamination. The selection of energy
sources and setting up the experimental apparatus are the first goals that have been
reached. Following the development of a flexible experimental set-up, the group has
determined the chemical and biological simulants to be used, who will prepare them, and
who will evaluate them. After making the above decisions, the first year's experiments
have been performed with the intention of perfecting and evaluating sources, data
acquisition instrumentation, agent handling, and results evaluation. The final work has
been to address the problem of mail contaminated with anthrax as a specific need
application of the equipment and processes developed over the last year.
The group that is referred to throughout this paper consists of faculty, staff, and
student workers from three different departments at Texas Tech University. Dr. O'Hair
and Dr. Dickens are the faculty members in Electrical Engineering who have directed
their expertise towards supervising the development of the pulsed corona discharge,
adding the environmental enhancements, and all other equipment necessary to collect
data for energy calculation and system monitoring. They have also been responsible for
organizing the efforts of the Chemistry and Microbiology departments to provide and
analyze the test specimens. As an Electrical Engineering graduate student, I have
worked imder the supervision of Dr. O'Hair and Dr. Dickens. In the Microbiology
department in the Health Sciences Center (HSC) at Texas Tech University, Dr. Fralick
and his student assistant, Kathryn Royse, have been responsible for acquiring, preparing,
fransporting, and evaluating the E. coli and Bacillus specimens after exposure. Dr.
Dasgupta and a post-doctoral researcher. Dr. Sathmgnan, prepared and evaluated the
Malathion and Parathion test specimens for the project. Naturally, the faculty and staff in
the Chemistry and Microbiology departments used their backgrounds to help make
critical decisions concerning appropriate test conditions and test specimen sfrains, when
necessary. Collaboration among all three disciplines was necessary to properly interpret
results obtained from different exposure techniques.
The first objective of the project is to explore the effectiveness of the Pulsed
Corona Discharge (PCD) in decontaminating large surfaces. The PCD is created by
pulsing a high negative voltage of about 60,000 Volts through a wire array generating an
approximately uniform discharge between the wire array and a ground plate. The voltage
pulse has a pulse width of about 20 nanoseconds (ns) and the pulse repetition frequency
is 100 Hz. Killing mechanisms generated by the PCD include a very high electric field,
kinetic energy of charge particles (electrons and ions), IR radiation, UV radiation, and the
production of OH' radicals. The specimens of interest are exposed to the PCD by placing
them on the ground plate, below the wire array. An important condition to note is that all
experiments are run at atmospheric pressure in order to more accurately imitate the
environmental conditions in which the system is to be used.
In addition to investigating the effectiveness of the PCD, the group will also be
evaluating methods to enhance the capability of the PCD to desfroy biological and
chemical agents. The main goal is to explore the possibility that a combination of the
PCD and another killing mechanism, or some environmental enhancement, can resuh in a
more energy efficient method to decontaminate the agents. The combinations of killing
mechanisms that have been explored include the PCD combined wdth thermal energy and
the PCD combined with water vapor. Thermal energy and water vapor are also tested
individually in order to establish a point of reference that helps determine the
confributions of each killing mechanism when combined wdth the PCD. The biological
agents have been exposed to all combinations of exposure mentioned above. The
chemical agents have not been exposed to heat or humidity. The different agents are only
exposed to conditions that the researchers feel will lead to meaningful real world
applications.
The biological and chemical agents chosen as simulants are thought to represent
agents that are already known to be weaponized and tend to be robust in survivability.
The chemical simulants are Parathion and Malathion. These compounds are commonly
used in insecticides, but in increased concentrations can be very dangerous. These
chemical agents are felt to be good simulants for chemical agents that may be
encountered in the field. The biological agents that have been tested include Escherichia
coli (E. coli) strain 802 and Bacillus stearothermophilus. The E. coli has been chosen as
a preliminary test specimen to make sure that each stage of the testing will kill the E. coli
before using the more expensive and much more robust bacillus spores. The bacillus
spores that have been chosen simulate the well-known biological agent. Anthrax.
The experiments throughout the first year of research have been stmctured to
perfect and evaluate the electrically driven energy sources in addition to determining their
effectiveness as killing mechanisms. Multiple experimental apparatus have been
developed and problems concerning data acquisition have been addressed. One of the
biggest challenges conceming data acquisition for electrical performance has been to
minimize the amount of noise induced on the signal and power lines that is from
elecfromagnetic interference (EMI) generated by the pulsed corona discharge. Methods
and protocols for handling, transporting, storing, and evaluating the chemical and
biological agents have also been developed and fine-timed. Finally, a method for
calculating energy levels of exposure and comparing the killing capability of the different
"energy" sources based on this energy level and the survival of the simulants has been
established. Basing the survivability of a simulant on the amount of energy to which it is
exposed allows one to compare the killing capability of one energy source with the
killing capability of a different energy source.
In all tests, the main parameter that is used to determine electrical efficiency is the
total energy applied to the specimen. Because of equipment hmitations, the time duration
of exposure is on the order of minutes and a certain amount of "rest" time must be
allowed in between shorter exposure intervals in order to allow the equipment to cool.
For this reason, the results one will view will be in terms of total energy applied to the
specimen for a given duration. At this point, it is assumed that increasing the frequency
of pulses using the proper Power Conditioning Unit (PCU) and removing these "resf'
periods will resuh in a more realistic time frame for decontamination while maintaining
the effectiveness of the PCD technology. However, this is an assumption left for later
investigation.
A specific need has developed during the past year prompting the group to
research the effectiveness that the PCD technology has when applied toward the
decontamination of mail. It is of interest to explore the different types of materials that
one may encounter in the mail system and limits that the different materials may impose
on the effectiveness of the pulsed corona discharge. Experiments combining the PCD
with heat and with water vapor have also been performed during the investigation of mail
decontamination. Materials tested include 20 lb. printer paper in varying numbers of
sheets, cushion mailer envelopes with shredded newspaper as the cushioning, cardboard
mailer envelopes, and plastic mailer envelopes. The collection of data under these
various conditions is an important resource for further project development.
CHAPTER II
EXPERIMENTAL SETUP
2.1 Generation of Killing Mechanisms
2.1.1 Pulsed Corona Discharge
As mentioned in Chapter I, the investigation of individual killing mechanisms
such as the PCD, conceming their potential to decontaminate biological and chemical
agents, stems from the previous research performed using an arc jet. Like an arc jet, a
pulsed corona discharge generates multiple killing mechanisms. The main elements
thought to cause the destmction of biological and chemical agents include the presence of
an extremely high electric field, the kinetic energy of elecfrons and ions bombarding the
surface of the specimens, IR radiation, UV radiation, ozone, and hydroxide (OH)
radicals. The tips of filaments extending toward the ground plate due to the partial
breakdown of air that exists during a corona discharge are one source of this very high
elecfric field. The tips of the breakdown filaments have a great deal of field enhancement
and in turn expose the biological samples to a very high electric field. The field
enhancement of the wdres in the wire array is another source of the very high electric
field. For this reason, particular interest lies in the viability of biological and chemical
specimens when exposed to a pulsed corona discharge (PCD).
Since the 1960s there has been an interest in the effects of a pulsed electric field
on the viability of microorganisms, mainly bacteria, in liquid. Schoenbach explains that
"experimental results obtained over a large range of electrical and microbiological
parameters, point towards an irreversible formation of pores in the cell membrane as
mechanism for lysing" [1]. These studies show that both the magnitude of the applied
electric field and the duration of the pulse affect the viability of the microorganism.
There is some question about how large a role the electric field plays in the tests nm with
the pulsed corona discharge because the pulse duration is so short (nanoseconds)
compared wdth the pulses used by Schoenbach (microseconds range). Schoenbach, while
speaking of the theory of irreversible elecfroporation, states that "only when we approach
the range of very short pulses at high elecfric fields, must the conventional theory be
modified to include internal effects of electric fields" [1]. ft has been assumed throughout
the tests with the PCD that the effects of the applied electric field are cumulative. The
resuhs presented in later chapters will support this assumption. The repetition rate for
these tests is 100 Hz. ft has been necessary to expose the specimen to one minute of
PCD, and then let the power conditioning unit (PCU) rest for 5 minutes. Another one
minute of exposure is applied after the PCU has cooled down and this process is repeated
until the desired duration of exposure is accumulated.
Research conducted by Mounir Laroussi demonstrates the effectiveness of using a
uniform glow discharge at atinospheric pressure to sterilize biological media. "The
plasma generated by the atmospheric pressure glow discharge ... is a source of elecfrons,
ions, excited atoms and molecules, active free radicals, and radiation (from the infrared to
ultraviolet)" [2]. In a similar way, the PCD is also a source of electrons, ions, excited
atoms and molecules, active free radicals, and radiation. Tests by Laroussi give sfrong
support that the effects of the electrical field and the charged particles in an atmospheric
glow discharge play a significant role in the destmction of the specimens in the
atmospheric glow discharge. The tests performed at Texas Tech with the PCD have not
been compared to an equivalent level of UV radiation. Laroussi comments on the
contributions of UV radiation by saying "... that the UV radiation generated by the
plasma was not the only agent of sterilization [and that charged] particles and active free
radicals appeared to play a significant role" [2]. Based on the work by Laroussi, the
assimiption that the UV radiation is not the dominant killing mechanism in the PCD has
been made. The PCD has been pursued because it is thought to be an energy-efficient,
electrically driven energy source capable of decontaminating chemical and biological
agents and could be easily realized as a mobile unit for field use.
The main considerations in generating an appropriate pulsed corona discharge are
uniformity and reproducibility. The generation of a uniform pulsed corona discharge
enables one to assume that the multiple specimens being tested during each test nm are
exposed to the same intensity electric field. If the PCD is non-uniform, one must
consider the relative locations of the simulants underneath the wire array in addition to
the duration of exposure when comparing the viability of the specimens. Because the
tests have been mn over an 8 month period, one must be able to reproduce similar
conditions each time a test is mn or unwanted variables are introduced into the
experiment. Limiting the test environment to as few variables as possible is essential to
drawing accurate conclusions about the effectiveness of the PCD in killing the biological
specimens and breaking down the chemical specimens. Various wire spacings have been
used to address the issues of generating a uniform discharge and providing a high degree
of reproducibility.
Two different stmctures have been used for supporting the wire array. The first
stmcture suspends the wire array above the ground plane with no physical contact
between them. The wire spacing is varied by threading the wdre through a copper rod
with holes in it every 0.25 inches and back toward a sheet of plexi-glass with slits cut
every 0.25 inches into the bottom of it. Lifting the ground plane closer to the wire array
using a scissor jack varies the distance between the wire array and the grotmd plane. This
stmcture is limited in its ability to vary the wire spacing to distances less than 0.25 inches
and its ability to curve the edges of the wire array away from the ground plane. The
distance between the wire array and the ground plane is also very difficult to keep
uniform across the entfre exposure area because the scissor jack has a certain degree of
flex in it. This stmcture requires that one thread the wire array with one piece of wire,
which makes tightening the wdres of the array and replacing a broken wire cumbersome
and time consuming. The first stmcture was used for tests from June 15,2001 through
October 17, 2001. A drawing of the first wire array stmcture may be seen in Figure 2.1.
The second and present wire array stmcture addresses the lack of flexibility
experienced with the first stmcture when trying to vary height, wire spacing, wire size,
and curvature of the wdre array (Figure 2.2). The second stmcture employs the same
concept used when stringing guitars. Instead of a single, very long wire, multiple wdres
are used to create the wire array. The individual wires are thread through the center of a
bolt, which can then be used to tighten the wire by turning the bolt and securing it with a
pair of nuts. The wires are thread through a set of fiberglass templates secured at either
end of the array that have a minimum spacing of 0.125 inches, which is half the spacing
allowed by the first stmcture. The fiberglass templates used to space the wires are easily
and quickly machined enabling one to reduce the spacing further should one desire to do
so. The consistency of the spacing of the wire array is much better because the fiberglass
templates at either end are much more accurate than the spacing slits used with the first
stmcture. The stmcture is secured directly to the ground plane using nylon bolts at each
comer enabling one to adjust the distance between the wire array and ground plane wdth
greater accuracy and consistency. The curvature of the wire array can easily be varied by
inserting curved fiberglass forms underneath the wires at each end of the array and
tightening the wires to conform to the curves of the form.
Figure 2.1: First Wire Array Stmcture
The main weakness with the second stmcture is the tendency for breakdown to
occur at the edges of the stmcture where the edge of the plexi-glass stmcture is in
between the wire array and the ground plane. It is at this location that surface flashover
plays a role in premature breakdown along the edges of the stmcture that supports the
wire array. Even though the stmcture does not extend all the way to the groimd plain, it
does provide a smooth surface between the two conductors along which charged particles
can more easily accumulate, boimce across, and continue down to the groimd plane. This
creates a lower breakdown voltage along the edge of the stmcture than exists between the
wire array and the ground plane in the area intended for exposure. A patch of silicon is
10
placed on the ground plane beneath these trouble spots to minimize the occurrence of this
breakdown.
Figure 2.2: Second Wire Array Stmcture
An array of wires closely spaced will generate a discharge that is approxunately
uniform assuming that the wire array is sufficiently separated from the ground plane. A
wire array is favorable to a parallel plate geometry because the field enhancement
experienced at the edges of a parallel plate will discourage the generation of a uniform
discharge throughout the volume between the plates. When generating the PCD using the
array, four main variables have been taken into account. The distance between the wdre
array and the ground plane, the spacing between the vsdres of the wire array, the size of
the wire, and the curvature of the wdre array each affect the uniformity of the discharge.
While the wire array approximates a parallel plate geometry, it also takes
advantage of the field enhancement of the wires generating a corona discharge along the
length of each wire. As the array is moved away from the ground plane, the effects of
charge diffusion cause the individual discharges from the wires to overlap. The overlap
of each individual corona discharge causes the electric field experienced at a distance just
above the ground plane to be approximately uniform.
The level of field enhancement is determined by the geometry of the electrode
configuration. For the wire array, a cylinder-plane geometry is assumed. The formula
11
for the field enhancement factor is presented in Equation 2.1, where d is the distance
between the wire and the ground plane and r is the radius of the wire [3]. The field
enhancement factor is muhiplied by the mean field strength to get the maximum field
strength in a non-uniform field. The maximum field sfrength occurs at the wire. The
mean field strength is based on the voltage per distance for a given geometry. The
average pulse voltage used during the experiments is around -60 kV at a distance of about
1 inch (or 2.54 cm). These conditions yield a mean field strength of about -23.6 kV/cm.
Given that 22 AWG wire has a radius of 0.032 cm and is separated from the ground plane
by 2.54 cm, the field enhancement factor (f) is about 16.4. With a field enhancement
factor of 16.4 and a mean field sfrength of-23.6 kV/cm, the maxunum field sfrength is -
387 kV/cm at the wire. There is not an immediate arc to the ground plane under these
conditions because the maximum field sfrength dies away fairiy quickly in space and
because the voltage pulses are only about 20 ns in duration, leaving little time for a short
circuit path to develop to ground.
r-li
0.9d
r + d f = ~ 7 T - A (2.1)
r J
As the wire array is moved further from the ground plane, the wires in the array
tend to act less like a single wire above a plane and the wire array begins to exhibit more
of the properties consistent with a parallel plate geometry, namely field enhancement at
the edges of the wdre array. According to Equation 2.1, one can also see that as the
distance between the wires of the array and the ground plane increases, the field
enhancement factor increases, but the maximum field strength decreases. By multiplying
the field enhancement factor by the mean field strength, the only influence of the distance
(d) that is left is in the denominator of the equation. This causes the maximum field
strength to be inversely proportional to the distance between the electrodes. As the wire
array is moved closer to the ground plane, the field enhancement due to the individual
wdres of the array becomes more significant, there is less distance for charged particles to
diffuse, and the discharge becomes less uniform and more concentrated under each wire.
12
Both the first and second wire an-ay stmctiires suspend the array between 0.8
inches and 1 inch from the ground plane depending on the spacing between the wires of
the wire array. The grounded mounting ring at the face of the PCU is 1.55 inches from
the aluminum bar that is attached to the high voltage terminal. If the distance between
the wire array and the ground plane exceeds or even closely approaches the distance from
the grounded mounting ring to the aluminum cylinder, a portion of the energy used to
generate the PCD will be lost to corona discharge and arcing between the aluminum
cylinder and the mounting ring. Considering the factors explained above, it is estimated
that the maximum possible distance between the wire array and the ground plane is about
1.4 inches, using the second wire array stmcture.
The spacing between the wires of the wire array and the size of the vsdre must also
be considered when adjusting the wire array to generate a uniform PCD. If the vvdres are
more closely spaced, the charge distribution across the array becomes more uniform
approaching characteristics similar to the parallel plate geometry. Likewdse, spreading
the wires apart decreases the imiformity of charge accumulation across the array leading
to a less uniform discharge very close to the groimd plane, keeping all other conditions
constant. A wdre spacing of 0.25 inches is used during experiments between June 15,
2001 and December 14, 2001. Experiments run after December 14, 2001 are performed
using a wire spacing of 0.125 inches. The wire spacing has been decreased from 0.25
inches to 0.125 inches in order to attempt a more uniform discharge. However, no
significant improvement has been experienced since reducing the wire spacing.
Changing the wdre diameter of the wires is another factor that affects the
imiformity of the PCD very close to the ground plane. One may refer to Equation 2.1 to
see that reducing the wdre diameter (or radius) will increase the field enhancement factor.
Increased field enhancement generates a higher electric field between an individual wire
and the ground plane. In the current application, this is favorable only to a certain extent.
Too much field enhancement will provide for a concentrated discharge between each
wire and the ground plane limiting the ability to generate a uniform pulsed corona
discharge and quickly leading to voltage breakdown conditions (arcing).
13
It is desirable to create conditions such that the effective voltage at each wire is
just below the breakdown voltage in air at atmospheric pressure. The level of field
enhancement increases with decreasing wire diameter which in turn decreases the
breakdown voltage as seen in Equation 2.2 [3]. In Equation 2.2 Vb is the breakdown
voltage of a gas in a non-uniform electric field, E^ is the breakdown field of the gas, r is
the radius of the wire, and d is the distance between the vvdre and the ground plane.
Based on Equation 2.2, varying the wire diameter (or radius) directly changes the
(r+d) ' Vb= Lll-Em-r-h (2.2)
performance of the wire array with respect to the uniformity of the PCD. Equation 2.2
demonstrates a concept relating the field enhancement of a certain geometry to the
voltage at which the gas between the elecfrodes wdll break down and form a conducting
path. While the concept is the same. Equation 2.2 is meant for the static voltage case and
cannot be direcfiy applied to the case for this project where the voltage is pulsed at 100
Hz. The breakdown voltage in air will be higher for a pulsed voltage as compared to a
static voltage as seen when using Equation 2.3 [3]. This equation is valid for relatively
large gaps with divergent fields where Vb is the breakdown voltage in kV, p is the
pressure in atmospheres, d is the gap separation in cm, and t is the time in ]xs at which the
voltage exceeds 89% of the breakdown voltage, and K_ is equal to 22 and n is equal to
0.6 and both are constants dependent on the gas and the polarity of the voltage pulse.
Plugging in the appropriate values, a rough estimate for the breakdown voltage of air
under the operating conditions used on the project is about 100 kV/cm.
r 5 - l A
Vb = K VP -d -t J (2.3)
Twenty-two AWG wire has been used during all exposures of the specimens. 26
AWG wire has been used when attempting to find a combination of variables that
generate a more unifonn discharge. The effects of smaller diameter are known, however
the ease with which it can be handled limits its desirability when stnnging the wires of
the an-ay. Smaller v\dre is much easier to kink leaving points along the line with even
14
higher field enhancement leading to certain spots along the wires that consistently
generate voltage breakdown.
Despite the field enhancement of each wire, the wires in the array are spaced
close enough together that they still approximate a parallel plate geometry at a distance
between 0.8 inches and 1 inch from the ground plane leading to a higher electric field at
the edges of the anay. Because of this, if the distance from each wire to the plate is the
same, the discharge will be more concentrated along the outer wires of the anay. To
resolve this issue, the wires toward the outer edges of the anay curve away from the
ground plane. It is desirable to anange the wdres in a manner such that the electidc field
very close to the ground plane is uniform. Electric field is defined as voltage per
distance. Realizing that the elecfric field is stronger at the edges of the anay, one may
increase the distance between the vsdres at the edges anay and the ground plane in order
to counter the effects of field enhancement on the uniformity of the electric field close to
the ground plane.
A considerable source of breakdown that can occur is not between the wire anay
and the ground plane, but between the high voltage connector and the grounding ring at
the face of the pulser. The high voltage connector is a 6/32 machine screw protmding
from the center of a circular region on the front face. There are six grounding bolts
which sunoimd the high voltage connector at a radius of 49 millimeters (mm) spaced
every 45 degrees starting at the 0 degree point as viewed on a polar graph. These six
bolts are connected by an aluminum ring. Because of the location of the aluminum ring,
considerable care has to be taken to prevent breakdown at the pulser face when using the
wire anay. Breakdown at the pulser face is undesirable because it results in wasted
energy that is not applied to the intended specimen.
The geometry between the high voltage post and the grounding ring may be
approximated as coaxial. The inner conductor of this coaxial design is the high voltage
post to which a 0.75 inch (1.905 cm) piece of solid aluminum cylindrical rod is fastened.
The outer conductor diameter is the inner diameter of the aluminum ring that is fastened
to the face of the pulser connecting the grounding studs. The inner diameter of the outer
conductor is 7.7 cm. Choosing appropriate diameters for the inner and outer conductors
15
can maximize the breakdown voltage between the conductors at the face of the PCU.
The conductor radius ratio that yields a maximum breakdown voltage in a coaxial
geometry is provided by Equation 2.4 [4]. The variable Douter represents the inner
diameter of the outer conductor (the aluminum ring). The variable du er represents the
diameter of the inner conductor (the aluminum cylindrical bar). The ratio between the
outer and inner conductors is equal to 4.042. A figure presented by Smith in his article
shows that a ratio of 4.042 will present a breakdown voltage that is about 95% of
the maximum possible breakdown voltage. The maximum possible breakdown voltage is
attained with a diameter ratio of 2.718. Assuming the diameter of the outer conductor
cannot change, the diameter of the inner conductor would have to equal 2.833cm (1.115
inches) to attain the maximum breakdown voltage. However, given the performance of
the existing inner conductor and the availability of materials, no changes have been made
to the 0.75-inch aluminum rod, which delivers the high voltage pulse to the wire anay.
- 2.718 Ideal Diameter Ratio (2.4) dinner
Additional steps have been taken to insulate the high voltage rod from the
grounding posts at the face of the PCU. A lexan mounting bracket is fastened to the face
of the PCU by screwing it to the grounding studs. The mounting bracket extends inward
toward the high voltage post replacing the volume between the two conductors that is
filled with air wdth lexan, which has a higher relative permittivity, thereby increasing the
breakdown voltage. Silicon is used to seal any gaps that may exist between the lexan
mounting bracket and the face of the PCU. By appropriately sizing the diameter of the
inner conductor and insulating between the two conductors, energy loss due to corona
discharge or voltage breakdown at the face of the PCU has been minimized. Additional
steps could be taken to minimize any corona discharge between the two conductors at the
face of the PCU by filling the volume with a breakdown suppressing gas such as SFe
Considering the adequate performance of the existing layout, these additional steps have
not been taken to allow more time to be spent performing tests on the biological and
chemical simulants.
16
The wire anay has been designed with the intention of delivering all the energy
output by the PCU to the test specimen by means of a PCD. However, there are some
specific locations on the wire anay where it is virtually impossible to escape charge
accumulation resulting in energy loss. One of the main locations that unwanted
breakdown could occur is at the face of the pulser, which has already been addressed.
The other main locations for charge accumulation are at the ends of the copper rod
around which the wires of the anay are wrapped and at the ends of the wdres, which are
wrapped around the nylon tightening bohs. The ends of the cylindrical, 0.5 inch copper
rod have been rounded and sanded as smooth as possible to minimize field enhancement
at the ends. However, one cannot escape the fact that charge wdll accumulate at the ends
of a metalhc rod. The ends of the wdre are also an inescapable source of energy loss in
the system since each wire comes to a sharp point, which also tends to accumulate
charge. There is no arcing from these sources to the ground plate during operation of the
system because these sources of energy loss are located far from the ground plate. The
energy loss is assumed to be minimal compared to the energy delivered to the specimens
since there is no direct arcing or discharge to the ground plate from these locations.
2.1.2 Heat
When exposed to the arc jet plume, the test specimens are exposed to very high
temperatures by way of natural and forced convection of the air. It is of great interest to
know how much effect the heat has on the biological specimens during exposure. Is
heating the specimens a major force in killing the organisms? Does heating the simulants
tend to weaken them thereby making them more susceptible to destmction the killing
mechanisms of the PCD? These are questions that have been addressed in the
experiments performed. Tests have been performed that heat the biological specimens,
that expose the simulants to only the PCD, and that expose the simulants to a
combination of heating and PCD. The tests involving a combination of the two killing
mechanisms are performed as a cumulative process. The organisms are heated to certain
temperature for varying durations, then removed from the heat source and exposed to the
PCD for varying durations.
17
The biological specimens have been heated for increasing durations at different
temperatures. The chemical specimens have not been heated because the slides used to
plate the chemical compounds cannot withstand the temperatiires to which the specimens
would be exposed. In general, there is little interest in the effects of heat on the chemical
compounds. Refening to the biological specimens, there is a large amount of support and
data for the effects of temperature on living organisms. The spores must be exposed to a
higher temperature and longer duration in order to be killed. The E. coli, however, is
more sensitive to heat.
Studies have been conducted by Holwitt, Kiel, et al. conceming the thermal
sensitivity of biological warfare agents. The three biological warfare simulants studied
by this group for their susceptibility to heat were Bacillus anthracis var Sterne, Bacillus
thuringiensis var Kurstald, and Bacillus globigii var niger. "The survival of the spores of
all three organisms was studied at 130°C. This experiment... reaffirmed that BG
[Bacillus globigii] is perhaps the most heat resistant of the three types of spores studied.
Also, BA and BT spores are inactivated within 20 minutes at this temperature" [5].
Results wdll be presented in a later chapter that will closely agree wdth the work done by
Holwitt and his associates conceming the thermal sensitivity of spores. Based on
previous research such as that described above, it is necessary to explore the individual
and combined contributions of heat and the PCB in killing the biological simulants.
The heat source being used is a hot plate powered by 120 VAC. A layer of silicon
heat sink compound is placed on top of the hot plate and a thin sheet of copper that has
slightly larger dimension (length and width) than the top of the hot plate is placed on top
of the heat sink compound. The hot plate surface is not completely flat because of a lip
that extends around the edge. The heat sink compound is used to provide a good
conducting medium in the low regions of the hot plate where the copper plate may not
normally touch. On top of the thin copper plate is the grounding plate on which the test
samples wdll be placed. The mechanisms for heat transfer to the test specimens include
conduction and convection of the air directly above the hot plate. The purpose of the
above setup is to improve the uniformity of the temperature distribution across the
surface of the plate on which the test samples are placed. After taking temperature
18
readings with an infrared temperature probe at the four comers and the center of the top
copper plate (refened to from now on as just the copper plate), it has been detemiined
that the temperature across the surface of the plate does not vary by more than 1 degree
Celsius CC).
The copper plate has been divided into four quadrants in which each set of test
samples is placed. Figure 2.3 shows the layout and location of the test samples, IR
temperatiire probe, and surface temperattire probe. A piece of masking tape is placed
beneath the IR probe to ensure the most accurate temperatiire reading. Masking tape has
a known and constant emissivity. The emissivity is set in the IR probe confrol module
and then fine-tuned wdth the help of the surface probe. The surface temperattire probe is
a type K thermocouple made by Omega Engineering hic. The emissivity of copper can
be detemiined, but the emissivity changes as the surface of the copper oxidizes. Using
the masking tape minimizes enors in temperature reading that may occur as the surface
of the copper plate changes over time and in response to the increasing temperatures.
Copper Plate
IR Temp. Probe
Masking Tape
•-0
i r
lOmin exposure
Smin. exposure
Surface Probe taped down
5min. exposure
1 min. exposure
(Overhead View of Test Slide Location)
Figure 2.3: Layout for Heat Exposure Tests
Because the test sample lies on a fiberglass coupon, it is important to determine
how well the fiberglass conducts heat from the copper plate. Experiments have been
19
perforaied to detennine how long it takes the top surface of the fiberglass coupon to heat
to the same temperature as the copper plate. The infrared (IR) temperattire probe is used
to measure the temperatures of the hot plate and the top of the coupons. The maximum
time period of exposure is 10 minutes so this is used as the maximum time the coupon
has to heat up to the same temperature as the copper plate. The coupon is heated to
wdthin 5 degrees Celsius of the hot plate in approximately 1 minute. After 10 minutes,
the temperature of the surface of the fiberglass coupon remains 5 degrees cooler than the
copper plate.
Based on the experiments described above, the low temperature alarm set in the
data acquisition unit is 5 degrees warmer than the desired test temperature and a 1 minute
"heating" time is added to all duration times. This ensures that the top surface of the
coupon, where the specunen is located, is at least the target temperature for the entire
specified duration. Due to the nature of the heating coil within the hot plate, there are 5
degrees of hysteresis wdth the temperature confroller. If the temperature drops below the
set threshold, the controller applies power to the hot plate until the temperature rises
above the threshold. The previously applied energy continues to heat the hot plate by 3-5
degrees Celsius, and then the plate begins to cool. The slow response of the hot plate
wdth respect to the power applied to it is due to the slow speed wdth which the electrical
energy is converted to thermal energy and then conducted from the heating coil, through
the surface of the hot plate, through the silicon heat sink compound, through the 3/8 inch
copper grounding plate, and finally through the fiberglass coupon. Figure 2.4 displays a
graph plotting the variation of the temperature during exposure. The temperature
threshold is set to 80 °C using an Agilent data acquisition unit.
Exposures of different durations can be run at the same time by timing when the
specimens are placed on the hot copper plate and removing them after the desired
duration. The 10-minute samples are laid down first, then the 5,3, and 1-minute duration
samples. There are three specimens per test duration. The time of exposure is started and
stopped as the second specimen (of 3) is placed down on the copper plate and removed
from the copper plate, respectively. It takes about 5 seconds for a set of specimens to be
either placed on the copper plate or removed from the copper plate.
20
Temperature Variation at 80C
15
Time (minutes)
20 25 30
Figure 2.4: Temperature Variation of Hot Plate at 80°C
Voltage and cunent data has been collected for each of the temperature settings.
The voltage recorded is the RMS voltage of the AC input voltage and the conesponding
cunent. The RMS voltage is measured directly by the HP data acquisition unit. The
RMS cunent delivered to the hot plate is measured with a Pearson cunent monitor,
model number 4997. This cunent monitor outputs 0.01 volts per 1 Ampere. A droop of
0.3 % per millisecond (ms) is well within acceptable levels for use with the 60 Hz signal
from a 120 volt outlet. A 50 Q matching resistor is placed across the conductors of the
coax cable connected to the cunent monitor. A gain of 2 is set in the data acquisition
unit on the channel monitoring the hot plate cunent to account for the voltage division
that occurs between the 50 Q coax cable from the cunent monitor and the matching
resistor. Because of the inexact nature of the temperature and its fluctuation, it is not
necessary to log the voltage and cunent data during tests. One may refer to Figure 2.5 to
view the layout of the data acquisition system showdng connections for the hot plate.
2.1.3 Hydroxide (OH) Radicals
A pulsed corona discharge in atmospheric air will generate, among other things, a
certain number of OH" radicals by splitting water molecules apart. The OH" radicals are
very unstable and exist for only a few microseconds. Because of their reactivity, it is
believed that OH" radicals may play a role in the deactivation of both biological and
chemical simulants. By increasing the amount of water vapor in the air between the wdre
21
anay and the ground plane, it is believed that a significant increase in the number of OIT
radicals directly around the specimens wdll occur. A fairiy complete set of data has been
accumulated on the effects that the increased levels of OH" radicals have on the spores.
Experiments exposing the chemical simulants to the increased levels of OH" radicals were
not performed because the main interests of the group concerned the viability of the
Bacillus spores when exposed to increased levels of OH" radicals.
An ultrasonic humidifier is used to generate the water vapor that is blown over the
wdre anay to increase the probability of OH" radical generation. The Sunbeam humidifier
(Model 696) has a 1.7 gallon capacity and is powered by 120 volts AC. The humidifier is
swdtched on and off through the relay formerly used to switch the hot plate and is
confroUed by the trigger pulse that triggers the function generator which in tum triggers
the PCD. The humidifier is turned on only during the PCD exposure. All of the
specimens to be exposed, regardless of intended exposure duration, are started at the
same time and as the appropriate time of exposure is accumulated, the conesponding
coupons are removed from underneath the wire anay and placed in sterile tubes. The
estimated volume of water vaporized by the humidifier over 10 minutes is 33.382 mL.
The estimated volume of water vapor applied to the volume directiy sunounding the wire
anay, ground plane, and simulants is calculated by measuring the amount of water
depleted from the humidifier's storage tank over a 10-minute period. Because the
ulfrasonic humidifier begins producing water vapor only 2 to 3 seconds after power is
applied, it is assumed that the amount of water vapor directed toward the wire anay
during the 1 minute exposure durations is immediate, consistent, and uniform. The water
vapor is directed to the volume of air between the wire anay and the ground plane using a
plastic snout approximately 3 inches in diameter. The snout increases the concenfration
of water vapor directed toward the desired volume directly sunounding the simulants.
2.2 Data Acquisition
The data acquisition system consists of all equipment needed to trigger the PCU
and record the cunent and voltage waveforais, tidgger voltages, trigger frequency, and
temperature. The cunent and voltage waveforms output from the PCU front face panel
22
are recorded using a Hewlett Packard Infinium oscilloscope. A computer mnning
Windows 98 and the Agilent 34970A Data Acquisition/Switch Unit are used to record
and monitor the temperatiire as recorded by the infrared temperature sensor, the tidgger
pulse voltage output by the Global Specialties function generator, the tidgger pulse
frequency delivered to the PCU, and the AC voltage and cunent applied to the hot plate.
As a Senior project Mike Hoffman was responsible for the initial set up and trouble
shooting of the data acquisition unit and the opto-isolation circuits. Mike also aided in
general trouble shooting of the data acquisition system until graduating December 2001.
Because of the high levels of electromagnetic interference and noise generated by
the PCD, considerable steps have been taken to filter and minimize noise on the signal
and power lines. Techniques used to minimize noise include opto-isolation, the use of
low pass fihers, the use of fenite toroids to increase in-line inductance, and the use of
screen rooms around both the data acquisition equipment and the PCU. The main
purposes for acquiring the data specified above are to enable the group to calculate the
electrical energy delivered to the test specimens and to monitor the performance of the
system during exposures to ensure consistency and repeatability or make note of the lack
thereof Figure 2.5 shows a schematic of the data acquisition system.
2.2.1 Oscilloscope Settings
The Hewlett Packard Infmium 500MHz, 2 giga-samples per second
(GSa/s) oscilloscope is used to monitor and record the voltage and cunent waveforms
output by the PCU which generate the pulsed corona discharge. The oscilloscope is set to
sample at a rate of 1 GSa/s for the tests using the nanosecond desktop SOS-based pulser.
The voltage waveform measured is a scaled version of the voltage delivered to the load.
The scaling of the voltage is performed by a resistive voltage divider within unit A2 of
the PCU circuitry. The cunent waveform measured is a measurement of the cunent
passing through the solid state opening switches. The cunent is measured with the use of
a cunent viewing resistor having a resistance of about 0.51 Q. The oscilloscope actually
measures the voltage drop across this cunent viewdng resistor. One must appropriately
set the gain for the probes of the respective channels to accurately display the cunent and
23
voltage delivered to the load. Coax connectors labeled "U" and "Us" located at the front
face of the PCU enable one to connect the measurement points to the oscilloscope using
50 Q coax cable.
Programmable F u n c t i o n G e n e r a t o r
.jvoc „a Trifln., HP P u l s e
Sci reen Room
G e n e r a t o r lOOH; I3.3y>
Dscilloscope
•DptoIsolator ~ircuit
Agilent DAQ Unit
DC Power Supply
<S5>
T .:. Ul
<S22> <22>
<S&
12 0VAC Isolated Power Supply
IDOHz 13.3-Nt
^ 1 ^ [Relay
Fer r i t e toroid around ohich the l ine is crrapped t:a miiDliiize noise on the l i ne .
S c r e e n Box
t ^
<s>
IR C o n t r o l l o d u l e
<2&
PI Network
SOS-Based P u l s e r
C,f2 ., 85uf "
85uF
IprobeL AC Cunent lAagijrimtrft
/iC ^Atoflft MeasvjrwTvent
Power S t r i p
X
120 vac
Hot P l a t e
H u m i d i f i e r
IR P robe
Figure 2.5: Schematic of the Data Acquisition System
The cunent waveform is displayed on channel 1 of the oscilloscope. The cunent
conesponding to the measured voltage can be calculated using Ohm's law shown in the
equation below where V is the voltage, I is the cunent, and R is the resistance.
1 = V R
(2.5)
Refening to the circuit in Figure 2.6, one can see that the value of the cunent viewdng
resistor is 0.51 Q. This resistor is also in parallel wdth an equivalent resistance of 50 Q
because of the 50 Q termination at the oscilloscope; however, because 0.51 Q is much
smaller than 50 Q, this parallel combination has been neglected. Plugging the value for
the cunent viewing resistor into Ohm's law and reananging the fonnula to show volts per
24
ampere, one sees that the necessary gain is the magnitiide of the cunent viewdng resistor.
Explicitly, the relationship between the voltage measured and the cunent flowdng through
the switch is such that 0.51 volts is equal to 1 ampere. The external gain for Channel 1 is
set to 0.51. A 20-decibel (dB) attenuator is attached at the input to Channel 1 of the
oscilloscope to fiirther reduce the magnitiide of the signal from the PCU so that the
voltage maximum input for the oscilloscope is not exceeded. The additional 20 dB of
attenuation are accounted for in the oscilloscope channel properties by setting the
attenuation of the channel to 20 dB. By choosing the appropriate gain and attenuation
settings, the voltage measured across the cunent viewdng resistor is viewed and recorded
at channel 1 of the oscilloscope as the cunent passing through the solid state opening
switch in unit A2 of the PCU.
L1
(Magnetic Switch section of unit A2)
025... 040
206.25 pF
V S0S1
V S0S2
R16...R31 75.2 kohm
R6...R15 0.51 ohm
RIoad
R32 10 ohm
'Is'(Current Monitor)
R33 10 ohm
'Us'(Voltage Honitor)
Figure 2.6: Location of Cunent and Voltage Monitors in A2 Unit of PCU
The voltage waveform is viewed on Channel 2 of the oscilloscope. The voltage
waveform recorded by the oscilloscope is a scaled version of the voltage pulse delivered
to the load. The amount of scaling is determined using the formula for a resistive voltage
divider. The values of the resistors may be seen in the circuit of Figure 2.6. Resistors
R32 and R33 are also in parallel wdth an equivalent resistance of 50 Q because of the
50 Q. termination at the oscilloscope yielding an equivalent resistance of 4.545 Q for the
low voltage leg of the voltage divider denoted as RLV in the voltage divider equation
below. The equivalent resistance of the series connection of resistors R16 through R31,
25
denoted as Ri6 31 in the equation below, is 75.2 Ul. (Notice that this is sixteen 4.7 kD
resistors in series). In Equation 2.6, V„,easured is the voltage measured by the oscilloscope
and Vioad is the voltage delivered to the load. Using these values for the resistors of the
voltage divider yields a voltage divider ratio of 6.0 x 10 ^ However, the minimum
external gain that can be set on the oscilloscope is 1 x 10" To overcome this limitation,
the external gain of the oscilloscope is set to 6.0 x 10" , a 20 dB attenuator is connected to
the Channel 2 input, and the attenuation is set to 20 dB in the probe settings menu.
RLV ^measured - Vioad'T T (2.6)
R L V + R l 6.31
2.2.2 Agilent Data Acquisition/Switch Unit
The Agilent 34970A Data Acquisition/Switch Unit is used wdth a computer
running Windows 98 to record the additional data required to monitor the performance of
the system, control the temperature of the hot plate when desired, and record the data
necessary to calculate the energy applied by the heat tests using the hot plate. The data
acquisition unit communicates wdth the computer by way of an RS-232 communication
port. The data acquisition unit comes wdth software that enables communication with the
computer and provides a graphical user interface for the real time display of data. From
the computer's user interface, one can easily export the data recorded during a test run to
a '.CSV' file for later use with a spreadsheet or other appropriate software tool.
Refer to Figure 2.5 for a schematic representation of the explanations provided in
this paragraph. The temperature is measured at Channel 1 using an infrared probe that
outputs a voltage between 0 and 5 volts conesponding to the temperature in degrees
Celsius. The duration of the trigger pulse output by the Global Speciahies function
generator is monitored at Channel 2. The trigger pulse is passed through an opto-isolator
before entering the unit so the voltage recorded is the voltage output by the opto-isolator,
not the voltage output by the Global Specialties function generator. This function
generator is programmed to output a positive voltage of about 10 volts for a duration of 1
minute, then an output of 0 volts for a duration of 5 minutes. The output of the Global
Speciahies function generator is sent to the external tidgger connection of the Hewlett
Packard pulse/function generator; the low voltage terminals of the relay which switches
26
the 120 VAC power sfrip in the PCU screen box on or off; and the input to channel 2 of
the data acquisition unit via an opto-isolator circuit. The cycle of 1 minute on and 5
minutes off is used to control the duration of exposure of the specimens and is necessary
to prevent over heating of the PCU as mentioned previously in the chapter. Channel 3 of
the data acquisition unit measures the frequency of the trigger pulse delivered to the PCU
by the Hewlett Packard 811 lA pulse/function generator. The output from the Hewlett
Packard function generator is set to a 100 Hz square wave wdth amplitude of 13.3 volts
and an offset of 7.2 volts. The resulting waveform delivered to the PCU is a 100 Hz
square wave with a peak voltage of 10 vohs and a minimum voltage of 0 volts which
meets the requirements by the PCU to have a drive pulse of 10 to 15 volts. The voltage
drop is assumed to be due to loading by the PCU. By operating this function generator in
gated mode, it wdll only apply the set output when it receives the 10 volt trigger pulse
output from the Global specialties function generator. Channels 4 and 5 are reserved for
use when running experiments wdth the hot plate. Channel 4 directly measures the AC
voltage applied to the hot plate. Channel 5 measures the voltage output of a Pearson
cunent monitor that is positioned around the conductor of interest. The Pearson cunent
monitor outputs 1 mV per ampere. The gain for Channel 5 is set to a value of 2 to
account for the 50 Q terminating resistor added to the line which matches the line
impedances, but also creates a voltage divider circuit that cuts the desired voltage in half
The only output from the data acquisition unit is an alarm that is only used during
the experiments with the hot plate. When the temperature of the hot plate falls below the
threshold set using the alarm, the data acquisition unit sends a +5 voh signal to the relay
which switches the AC power strip inside the PCU screen box on and off. The
temperature sensed by the infrared sensor, which is recorded at Channel 1, is the
temperature monitored by the alarm. This allows automation of the temperature control
and increased consistency conceming the temperature variation for a given temperature
during tests.
27
2.2.3 Solutions to Electromagnetic Interference (EMI)
The pulsed corona discharge radiates elecfromagnetic interference in all
directions. The signal lines and power lines leading to and from the PCU are injected
wdth noise because of this EMI. The Agilent data acquisition unit, the Global Specialties
synthesized function generator, and the HP Infinium oscilloscope are the most affected
by increased levels of noise on the lines. The four main techniques that have been
employed to try to minimize the presence of the noise on the lines include a PI network to
filter the DC supply line to the PCU, opto-isolation to filter the trigger duration and
frequency inputs to the data acquisition unit, fenite toroids used to increase in-line
inductance presenting a higher impedance to high frequency signals, and physical
enclosure of the PCU wdth a copper screen box and enclosure of all instrumentation wdth
a different screen room.
When refening to the data acquisition system schematic shown in Figure 2.5 one
wdll notice a block labeled PI network. The PI network is a low pass filter used to filter
the DC input power to the PCU (Figure 2.7). Because the power being filtered is DC,
the values of the capacitors and inductors merely need to be sufficient in size. No tuning
is necessary as one may find the need to do wdth other applications. The circuit takes
advantage of the impedance characteristics of the circuit components. The impedance of
a capacitor is equal to 1/jcoC and the impedance of an inductor is jcoL; co being the
frequency of the signal [6]. By inspection, one can see that the impedance of a capacitor
is very low for a high frequency signal and decreases with increasing signal frequency
and capacitance. The impedance of an inductor is relatively high and increasing wdth an
increase in the frequency of the signal and the inductance. To the noise on the DC power
fine, the capacitors act like shorts and the inductor acts like a high impedance, while the
DC power passes through the circuit with little hindrance because of its zero frequency
content. The PI network is encased in a metal box and placed very close to the PCU.
28
T C1
L1 C2 T
Figure 2.7: PI Network
A simple circuit employing the 5601 opto-isolator is used to minimize the effects
of noise on channels 2 and 3 of the data acquisition unit (Figure 2.8). Channels 2 and 3
record the trigger durafion output by the Global Specialties function generator and the
frequency of the trigger pulse delivered to the PCU, respectively. The opto-isolator
physically separates the function generators from tiie data acquisition along with any
noise that may be induced on the lines. Notice that the voltage of the signal measured by
the data acquisition unit is the voltage from the opto-isolator circuit, not the voltage
output by the function/pulse generators. Mike Hoffman designed and built the opto-
isolator used for this project.
From G e n e r a t o r
R1 ^AA vvv 330
1
7
^Q>
Nb(. PS2
t 501
ro
VCC = +9VDC 4 V
< R2 > 331k
To DAQ
Figure 2.8: Opto-isolator Circuit
Ferrite toroids are used at multiple places in the signal and power lines to increase
the in-line inductance which tends to inhibit the passage of high frequency signals such as
the noise induced on the lines from the EMI. By wrapping the signal line around the
ferrite toroid, one is essentially creating an inductor with an inductance based on the
number of turns. As mentioned during the explanation of the PI network, the impedance
of the element increases as both the inductance and frequency of the signal increases.
29
One may refer to the data acquisition system schematic shown in Figure 2.5 to note the
locations of the ferrite toroids, which are designated as an oval filled wdth hash marks.
The inductance of a closely wound rectangular toroidal coil can be found using Equation
2.7. In the equation, N is the number of turns, 'a' is the inner radius of the toroidal core,
b is the outer radius of the toroidal core, h is the height or thickness of the toroidal core,
and ^ is the permeability of the toroidal core [7]. Assuming that the dimensions of the
toroidal core are constant, notice that the inductance changes wdth the square of the
number of turns (N). Increasing the number of turns around the fenite core increases the
inductance, which increases the impedance seen by the high frequency noise signals.
f -N -h 2.U ru\ L= In -
2n
h
va; (2.7)
The last method used to minimize noise on the lines induced by EMI is the use of
a copper mesh screen room that sunounds all data acquisition equipment and a copper
mesh screen box that sunounds the PCU and all equipment needed to change
environmental conditions during the exposure process. The screen room limits the EMI
that radiates into the volume sunounding the data acquisition equipment. In addition, the
power strips provided inside the screen room are isolated from outside power sources by
way of an isolating transformer. The screen box performs the opposite objective as the
screen room; that being to keep as much of the EMI inside the box as possible. The
screen room and screen box have BNC connector panels that allow one to pass the signal
lines through the box with minimal gaps in the panel. There are also holes in the panel
that permit one to run lines directiy into the screen room/box. These holes are patched
with metal tape after all desired lines are run through them.
2.2.4 Power Supplies
There are two DC power supplies used in this data acquisition system. One of the
power supplies is used to power the infrared temperature sensor control module. The
second power supply is a 600 volt, 1.7 ampere DC power supply made by Xantiex and
used to supply the main power to the PCU. While the ratings of the power supply exceed
the power required by the PCU, the Xantrex power supply cannot recharge the primary
30
storage capacitors of the PCU fast enough to maintain the desired voltage at a pulse
repetition rate of 100 Hz. This inability to recharge the primary storage capacitors causes
the pulsed corona discharge at the load to cut out sporadically. In order to help the power
supply maintain the necessary charging voltage, three 85 |xF capacitors have been added
in parallel to the power supply. One may view the locations of these capacitors in the
data acquisition system schematic shown in Figure 2.5. The capacitors are an unlytic
type UL30 lOOOV DC capacitor manufactured by Electronic Concepts wdth a tolerance of
plus or minus 10%. These capacitors are initially charged by the power supply and
during the firing of the PCU, act to provide quick bursts of charge as needed whenever
the power supply cannot keep up with the repetition rate.
31
CHAPTER III
CHEMICAL AND BIOLOGICAL AGENTS
3.1 Biological Agents
3.1.1 Background
The microorganisms that were used in the pulsed power experiments were
selected after consideration of their sensitivity to the separate elements of the
experiments, and the simplicity in analyzing them in the resuhs of the experiment.
Escherichia coli strain 802 was chosen for its tetracycline marker, which was used to
screen against contaminants that may have been inttoduced in tiansport and
experimentation of the microorganism. The tefracycline marker would allow the E. coli
to grow on tetracycline culture medium while preventing growth of any contaminants.
The E. coli used in pulsed power experiments is a general bacterial paradigm, and was
used preliminarily to check for difficulties or variations in the field. E. coli also tested for
the effectiveness of the pulsed power machine in reducing cell number. Experimental
analysis of E. coli was mostly uncomplicated because it is one of the most researched and
documented microorganisms, and therefore planning for preparation and analysis could
not be more uncomplicated.
Bacillus stearothermophilus is a spore forming microorganism, generating a
microorganism that is more thermally and chemically resistant than many other known
Bacillus. This microorganism was chosen because it is documented to be less thermally
and chemically sensitive to spore inactivation than bacillus anthracis and because it
could be purchased from NAMSA containing the desired number of cells per milliliter
for experimentation. Also, the germination temperature of the bacillus was 55 degrees
Celsius, which is higher than virttially any other possible contaminant, thereby almost
eliminating contaminants from analysis.
3.1.2 Preparation
The fiberglass coupons that carried the microorganisms for ttansport and
experimentation were obtained from the elecfrical engineering departinent and sterilized.
32
The sterilization process included first washing the coupons with biological detergent,
and then multiple times wdth DI water. The coupons were then placed inside a sterile
glass petri dish and sterilized in an autoclave for twenty minutes and 18 to 20 psi. The
coupons were then removed and dried in a sterilization oven at 136 °C as a final
safeguard.
E. coli was prepared for experunentation by first isolating a colony to be cultured
then aseptically placing it in two milliliters of sterile, liquid cultiire medium containing
tetracycline for ovemight incubation at °C. This temperature is ideal for maximum cell
growth. After ovemight incubation the broth culture typically contained 10 cells per
milliliter. Twenty microliters of broth culture were then ttansfened aseptically to sterile
fiberglass coupons, and placed in sterile glass petid dishes for ti-ansport. The cell
concentration per coupon is approximately 10 per milliliter.
Bacillus spores were prepared for experimentation by aseptically removing them
from a spore suspension with a sterile syringe and placing them in a sterile container.
Twenty microliters were removed for each individual coupon and these were then placed
in a sterile glass petri dish for fransport.
There were many controls used in transport and experimentation of both types of
microorganisms to assure the most accurate results for analysis. The coupons containing
the microorganisms were placed in a Styrofoam container during transport to attempt to
control temperature equilibrium, and therefore try to reduce cell death. Also, there were
coupons that remained in the microbiology laboratory to determine if transport was a
factor in cell loss for every experiment, and these coupons were used in analysis of the
results of the experiment. Coupons that were fransported to the pulsed power laboratory
but not exposed to any tests were used as a confrol to try to determine any possible cell
loss that might have occuned. These coupons were also used in analysis of coupons
exposed to any temperature or electrical field.
3.1.3 Coupon Analysis
After coupons were exposed and transported back to the microbiology lab they
were analyzed. The exposed coupons were placed in sterile plastic tubes along wdth two
33
milliliters of buffer salts, and then vortexed to remove cells from the coupon. The
microorganisms were then assayed using a serial dilution technique for each individual
coupon, which was suspended in Phosphate Buffered Saline solution and placed in sterile
test tubes. After serially diluting them, the bacterial cells were plated on nutrient agar
plates, incubated ovemight (12-14 hrs) at 37°C for E.coli 802 and at 55°C for Bacillus
stearothermophilus. The percent survival was determined and compared wdth all the
confrolled samples.
The explanation for the use, source, preparation, and evaluation of the biological
agents was provided by Kathryn Royce and verified by Dr. Joe Fralick in the Texas Tech
University Health Sciences Center Microbiology department. Kathryn Royce prepared
and evaluated all biological agents that were used for this project. Kathryn also helped
handle the test specimens during the exposure process. Dr. Fralick has been the main
source for expertise and advice on any issues conceming the biological specimens.
3.2 Chemical Agents
The chemical agents chosen for use in this project are Malathion and parathion.
The agents have been chosen based on their relatively low vapor pressure, comparatively
low human toxicity, and availability. Dr. Karthikeyan Sathmgnan, a post-doctoral
researcher in the Texas Tech University Chemistry department, under the supervision of
Dr. S. Dasgupta, has prepared the chemical simulants for exposure and has evaluated the
test slides after exposure. Preparation of the slides consists of plating 1-inch square glass
slides wdth 20 nanomoles (nmole) each of Malathion or parathion. Two hundred fifty
nmoles conesponds to 82.5 jug of Malathion and 72.75 j^g of parathion. A normal test
group consisted of around fifteen slides providing three different exposure durations and
one control set wdth three test samples in each set. After exposure, the slides are washed
with 0.5 mL of acetonitrile and the washes are analyzed by liquid chromatography for
their parent compound and metabolites. There have not been any significant problems
with the preparation or evaluation of the chemical compounds. Dr. Dasgupta has been
the main source for expertise and advice on any issues conceming the chemical
specimens.
34
CHAPTER IV
CHEMICAL AND E. COLI TEST RESULTS
4.1 Method for Calculating Results
Before presenting the specific resuhs for each simulant under different exposure
conditions, the method by which the results have been calculated wdll be explained.
Because different energy sources, such as the PCD and heat, are being used as killing
mechanisms, a common method to compare the various killing schemes has to be
established. The total energy applied to the specimen has been chosen to be this point of
comparison. The energy applied will be presented on two different scales. One scale
describes the total energy, in Joules (J), applied per specimen area. The specimen area is
the area of the slide on which the coupon is placed. The specimen area for the chemical
slides is 6.25 cm^ and the specimen area for the biological slides is 1.21 cml The second
scale describes the equivalent total energy, in kilo-Joules (kJ), that it is estimated would
be necessary to provide the same results on a 1 square meter area of surface. The main
assumption made when calculating the energy is that the electric field of the PCD to
which the simulants are exposed is relatively uniform.
In order to calculate the energy of the PCD, the voltage and cunent applied to the
load are recorded by the oscilloscope and saved as data files. Voltage and cunent
waveforms are saved for the different durations of exposure during a given test. For
example, if a test consists of 1-minute, 3-minute, and 5-minute exposure durations, there
will be three sets of voltage and cunent waveforms saved. The oscilloscope is also set up
to record the mean peak voltage and cunent and the standard deviation of these mean
values. The voltage and cunent waveforms cannot be multiplied together, because they
are not recorded at the same point in the circuit within the PCU. However, the cunent is
assumed to be at its maximum value and fairly constant during the peak of the voltage
pulse. The mean current value may come from one of two different sources. The
prefened source is the mean peak cunent value recorded by the oscilloscope. If this
value is not available, the mean peak cunent value is calculated from the cunent pulses
recorded for a given test. The voltage waveforms for a given test are averaged together.
35
The power delivered to the load is then calculated by multiplying the averaged voltage
waveform by the mean peak cunent value. The energy of a single pulse is then found by
integrating the power pulse over the duration defined by the Full Width at Half Maxunum
(FWHM) of the power pulse. The total energy applied over a certain duration is
obtained by multiplying the energy for one pulse by the repetition frequency of the PCU
and the total number of seconds of exposure. This total amount of energy is estimated to
be the energy applied over the area directly beneath the wire anay. This energy is
appropriately scaled down to obtain the energy applied to a single specimen and
appropriately scaled up to obtain the energy per square meter.
The energy applied to the hot plate in order to heat the plate and maintain the
given temperature is calculated using the electidcal energy applied to the hot plate.
Voltage and cunent data has been collected for each of the temperature levels used for
exposure during separate tests mn for the purpose of estimating the energy applied to the
simulants. The voltage and cunent recorded are multiplied together and then integrated
over the duration of exposure in order to obtain the energy applied to the entire surface of
the hot plate. As with the PCD, the energy applied to the area of the hot plate is
appropriately scaled to give the energy applied to a single specimen and the energy
needed to be equally effective across a square meter surface. The characteristics of the
heating and cooling of the hot plate have been observed to be very consistent, so the
energy calculations are assumed to be sufficient and representative of any test at a given
temperature.
The level of destmction will be presented in terms of either percent survival of the
simulant or the number of cells or spores killed (labeled "Cell Count Reduction" or
"Spore Count Reduction"). The resultant percent survival, or percent growth for
chemical degradation products, is calculated as a percentage of the mass of the control for
the chemical compounds and a percentage of the cell or spore count of the confrol for the
biological agents. Except for being exposed, the control specimens are submitted to the
same conditions as the exposed specimens. This minunizes inconsistencies in results due
to preparation, storing, transporting, or evaluation of the specimens. The other main
factor that may cause variations in results for individual specimens is the location of the
36
specimen under the wire anay because the PCD generated by the wire anay is not
entirely uniform. The test specimens are more quickly destroyed by the PCD when they
are placed directly beneath a wire of the wire anay. For the biological simulants, the
most consistent tenninology lies with the percent survival because not all biological tests
are conducted wdth the same concentration of simulant. The final results are averages
wdth a given standard deviation and are shovm in graphs and tables that are provided
within the text of this paper or in Appendix A. Layouts for test run have also been
included in the Appendix C.
4.2 Chemical Experimental Results
4.2.1 Exposure of Malathion to the Pulsed Corona Discharge
Malathion has been exposed to the PCD for durations ranging from 2 seconds to 5
minutes. Repetition frequencies of 10 Hz, 100 Hz, and 200 Hz are also tried. Based on
the PCU performance, a pulse repetition frequency of 100 Hz is used for the tests that
yield noticeable results. No noticeable reduction in the amount of Malathion begins until
an exposure duration of 1 minute is reached. The height of the wdre anay above the
ground plane is 2.5 cm and the wdre anay spacing for these tests is 0.25 inches. The first
wdre anay stmcture has been used for these tests. In addition to the reduction of
Malathion, the generation of its degradation product, DDTP (Dimethyldithiophosphate),
can be seen in the chromatograms which show the Malathion before exposure to the PCD
and after 3 minutes of exposure (Figure 4.1). Remember from the previous chapter that
the concentration of Malathion on each slide is 250 nmole, which conesponds to 82.5 ^g
of Malathion. The chromatograms have been provided by the Chemistry department at
TTU.
Graphs showing the full scope of the results obtained from this set of tests may be
seen in the Figures 4.2 and 4.3. The energies along the x-axis of the graphs conespond to
0, 1, 2, 3, and 5 minutes of exposure. Each minute of exposure is separated by 5 minutes
of rest time that allows the PCU to cool. Notice that after five minutes of exposure, about
15% of the Malathion still remains and the DDTP has not begun to diminish. Increasing
the exposure time to durations greater than five minutes would obviously results in
37
complete destmction of the Malathion. At some point past the five-minute mark, the
generation of the DDTP would be expected to reach a maximum and proceed to be
eliminated by the PCD.
Malathion control Malathion after 3 min expsoure
0.12 1 £ 0.1 -S 0.08 -T- 0,06 -d 0.04 -= 0,02 -
0
Unkmow f
Mala&ion
/-"-
i^ ^ ^
Time in seconds
0.08 -,
•S 0.08 •
g 0.04 •
S 0,02 •
0 J
DDTP
> '
h - T^ T - 00 U l CN O) <D (O CN CN (O •«• ^ Wl CD
Time in seconds
o
Figure 4.1: Destmction Process of Malathion
The data points in Figures 4.2 and 4.3 indicate the average percent survival after
exposure to the given energy levels. The bars through the data points on the graphs
indicate the standard deviation of these averages. The inconsistencies in the percent
survival of the Malathion are atfributed to the non-uniformity of the PCD and preparation
and evaluation enor. The test specimens are placed at different locations undemeath the
wdre anay in order to obtain a complete survey of the killing effectiveness of the wire
anay. During the experiments, it has been found that the Parathion is a more robust
compound so further tests wdth Malathion were not performed in an effort to concenfrate
on the destmction of Parathion.
4.2.2 Exposure of Parathion to the Pulsed Corona Discharge
Parathion has been exposed to the same initial conditions to which the Malathion
has been exposed. These tests are also performed with the same experimental setiip as
the tests perforaied on Malathion. Like Malathion, Parathion also has a degradation
product, Paraoxon, which is of interest and that must be broken down to completely
eliminate the threat of the compound. The initial concenfration of tiie Parathion is 250
38
nmole conesponding to 72.5 |Lig of mass. The Parathion has been exposed to durations
that exceed 5-minutes in order to determine the energy necessary for a complete kill.
100%
3 75%
e ^ 50%
1 0) (J
I 25% Q.
Malathion Survival (PCD Only)
0% -+- -+•
470 850
Energgf(kjy»n' 2)
1340 2230
Figure 4.2: Malathion Survivability When Exposed to the PCD
DDTP Present (PCD Only)
10%
470 850
Energy (kJ/m*2)
1340 2230
Figure 4.3: Percentage of DDTP Present When Exposed to the PCD
Figures 4.5 and 4.6 show the average percent survival of the Parathion and the
generation of Paraoxon as a fimction of the total applied average energy representing
exposure durations of 0,3, 5,10, and 15 minutes. After 15 minutes of exposure to the
PCD, the Parathion is completely desfroyed and the Paraoxon is reduced to about 18
percent of the initial mass of tiie Parathion Because it has been observed tiiat the
39
Malathion is more easily killed by the PCD, it is believed that the Malathion would also
be completely destroyed after a 15-minute exposure to the PCD.
Paratliion Survival (PCD Only) 100%
s e O
a.
1300 1990
Energy (kJ/ni 2)
3980 5B40
Figure 4.4: Parathion Survivability When Exposed to the PCD
100% Paraoxon Present (PCD Only)
1990
Energy (lcj/m*2)
Figure 4.5: Paraoxon Present When Exposed to the PCD
Chromatograms showing the degradation of the Parathion and the growth of the
Paraoxon may be seen in Figure 4.6. The first chromatogram (on tiie left) shows the
amount of Parathion present before exposure to the PCD. The second chromatogram (on
the right) shows the amount of Parathion and Paraoxon present after five minutes of
exposure to the PCD. The first step in neufralizing the Parathion is breaking the
Parathion down into its degradation product, Paraoxon. The Paraoxon is then broken
down into a harailess compound. This is why one notices that the Paraoxon first grows
in percent concentration, reaches a maximum value, and then begms to degrade toward
zero percent. The chromatograms have been provided by the Chemistty department at
TTU.
40
Parathion Control
0,3
02 5 O-is •
S. 0,1 • 3 0,05
Patathion
Time in seconds
Parathion after 6 min exp.
0.3
ii 0.1
0
PNP
A Paraoxon
y FaratMon
g ' 67 133 199 265 331 397 483 529 595 661 72;j|
-0.2
Time h seconds
Figure 4.6: Parathion Chromatograms
4.3 E. coli Experimental Results
While E. coli is a harmful microorganism that should be taken very seriously, the
main focus of the project was not aimed toward finding methods that effectively kill E.
coli. There are already many methods for killmg E. coh which work very well and are
very convenient. When compared to a Bacillus spore, E. coli is relatively easy to kill.
The purpose for including E. coli in the experiments was partly as a pre-test. If a certain
exposure method cannot kill E. coli, it wdll certainly not be able to kill Bacillus spores.
Because of the expense of the Bacillus spores, it has been the practice of this group to
validate the effectiveness of a given exposure method by first killing E. coli and then
attempting to kill Bacillus spores. In addition, the data taken while running the tests with
E. coli may be a very good source for later research. The results and any considerations
regarding specific test conditions are therefore included in the following sections.
The exposure methods that have been used on the E. coli include exposure to:
only the PCD, only heat, heat and the PCD, and PCD wdth water vapor. One may notice
in some of the graphs that the standard deviation of the percent survival is quite large.
Keep in mind that E. coli is relatively easily killed and tiie task of maintaining a secure,
controlled environment while fransporting these microorganisms between two labs is not
a trivial task. The specimens are prepared and later evaluated in the Micro-Biology Lab
in the Health Sciences Center (HSC). This requires fransporting the specimens across
campus (3 miles each way). The number of cells on the conti-ol samples (Control 1)
taken to the Electrical Engineering building have always been less than the number of
41
cells on the control samples (Control 2) that are left at the HSC. If E. coli
decontamination experiments should contmue, improvements for preparing and handling
the specimens will need to be made. A new source for obtaining the E. coli specimens
should also be considered.
4.3.1 E. coli Exposed to the Pulsed Corona Discharge
The durations for which E. coli has been exposed to the PCD include 2 seconds, 5
seconds, 10 seconds, 30 seconds, 1 minute, 3 minutes, 5 minutes, and 10 minutes.
Different pulse repetition frequencies have also been tried, which include 10 Hz, 100 Hz,
and 200 Hz. The DC voltage input to the PCU was varied between 450 V and 600 V.
While all of these conditions were tried at one time or another, the optimum operating
conditions based both on the PCU performance and the results were found to be exposure
durations between 3 and 10 minutes, a pulse repetition frequency of 100 Hz, and a DC
voltage input to the PCU of 600 V. The other main variable present during the tests is the
location of the specimen under the wire anay. Better kill rates are obtained when the
specimen is placed directly beneath a wire of the wire anay because the PCD is not
completely uniform. For shorter exposure durations, the percent survival is half as high
directiy beneath the wdre. For the peak exposure duration in these experiments, there is
littie or no difference between results based on the location of the coupon. The first anay
stmcture is being used with a wdre spacing between the wdres of the anay of 0.25 inches
for these tests.
The graphical results presented in this paper are an average of all the results
regardless of the location of the specimen. This is a significant contribution to the
standard deviation of the averages. Even though the PCD is a cold plasma, one is conect
to assume that it does generate some thermal energy. However, the surface temperature
of the specimen coupons never exceeded more tiian about 10 °C above the ambient room
temperature and is assumed to not be a significant killing factor during the PCD
exposure. One may refer to the Heat Only section to see data showing temperatiires
necessary to facilitate a timely kill of the E. coli.
42
The process of exposing the specimen consisted of exposing four slides at a time
with two coupons directly beneath a wire and two coupons centered between two wires.
The four coupons are exposed to a certain duration. After the desired duration of
exposure has been reached four new coupons are placed in the same four locations under
the wire anay and a different duration of exposure is performed. The exact location of
the four slides did change between tests, but there were always two coupons directly
beneath a wdre and two coupons centered between wdres. Placing the coupons at different
locations under the wire anay helped to verify the relative uniformity of the PCD.
Figure 4.7 shows the survivability of the E. coli and one can see that the average
killing rate is relatively linear and decreasing to zero from 3 minutes to 10 minutes of
exposure. The data points in the graph conespond to 0, 3, 5, and 10 minutes of exposure
time, respectively. While the standard deviations for the 3 minute and 5 minute durations
are affected by the factors previously discussed, 10 minutes of exposure to the PCD
results in a virtually complete kill on a consistent and repeatable basis. The actual values
after 10 minutes of exposure reveal a 0.39% survival with a 0.47% standard deviation.
This standard deviation is attributed to the differences in specimen location.
E. Coll Survival (PCD Only) 100%
h H
S e (J
'S •£ w u 09
75%
6U%
25%
0% 1230 1880
Energy <kJrtii*2)
3580
Figure 4.7: E. coli Survivability When Exposed to the PCD
4.3.2 E. coli Exposed to Only Heat
It is believed that the heat generated by the PCD is not a significant factor in the
killing of the biological or chemical agents. However, in order to verify this assumption,
both E. coli and Bacillus spores are heated to increasing temperatures for varying
43
durations. A secondary and more important motive behind the investigation of this
environmental enhancement is to determine at what temperature the destiiiction of the E.
coli becomes significant. Because a hot plate is used as the thermal energy source, it is
assumed that the air immediately above the hot plate is the same temperature as the hot
plate itself However, even though the specimen is plated on a thin piece of fiberglass,
the fiberglass still has some degree of thermal insulation. Because of the thermal
properties of the fiberglass coupon, the tests are mn so that there is a one minute heating
period allowed at the beginning of each test to give the fiberglass coupon time to heat to
the desired temperature.
The tests are performed by placing the coupons on the hot plate in groups of three
in increments of 30 seconds per group of three. There are normally three or four different
exposure durations run per test. As a desired duration of exposure is reached, three or
four coupons are removed from the hot plate and placed in sterile tubes. The main
benefit of this process is that it saves tune by allowdng one to run different exposure
durations at the same time. Figure 4.8 displays the survivability of E. coli when exposed
to a temperature of 50°C.
E. coli Heat Only Test (COC) 100%
% 75% c o if 50% o
2 25%
0%
+
Exposure Duration (minutes)
Figure 4.8: E. coli Survivability When Exposed to 50°C
10
The data points in Figure 4.8 conespond to exposure durations of 0, 1,3, 5, and
10 minutes. Notice that there is still about a 25% survivability after 10 minutes of
exposure. This tends to confirm the assumption that the heat generated by the PCD,
which has a maximum temperature of about 35''C, is not a significant contributor to the
destiiiction of the E. coli during its exposure to the PCD. Pursuing the heat tests fiirtiier,
it can be seen in Figure 4.9 that significant destmction begins to occur at a temperature of
44
75°C. At 75°C there is less than a 25% survivability after only one minute of exposure.
The data points in Figure 4.9 conespond to 0 ,1 , 3, 5, and 10 minutes of exposure. After
five minutes of exposure, one can see that the E. coli is almost completely desfroyed.
The results shown below suggest that exposing E. coli to both heat and the PCD could
dramatically decrease the exposure time necessary to produce a complete kill.
E. coli Heat Only Test (75C) 100%
2 75% *i c o if 50%
c H « a.
26%
0%
\
\
\l 1
^ — 4——^ ^ T ~ ~ — * - 1 * 1
3 5 Exposure Duration (minutes)
10
Figure 4.9: E. coli Survivability When Exposed to 75°C
4.3.3 E. coh Exposed to Heat Then the PCD
Continuing with the train of thought begun at the end of the last section, a test
combining the killing mechanisms of the PCD and of increased temperature has been
conducted. The second wire anay stmcture is used for these tests and the spacing
between the wdres of the wire anay is 0.25 inches. The increased temperature is
considered to be an enhanced en-vironmental condition imposed before exposure to the
PCD. The E. coli is heated to 75^0 and exposed to this environment for the specified
duration and then the specimens are exposed to the same duration of the PCD. The heat
and the PCD have been applied consecutively and it is suggested that the effects of tiie
two killing mechanisms are cumulative. The energies given in Figure 4.10 represent the
energy applied by the pulsed corona discharge. The durations for which the spores were
exposed to heat is given in the parenthesis next to the conesponding data point. After five
minutes, there is about a 7% survival of the E. coli. Comparing this wdth the heat only
test, one can see that there is no great improvement over the individual exposures to tiie
PCD or to heat. While these resuhs may seem to be inconsistent wdth the previous heat
results, one must keep in mind that the data set from which Figure 4.10 is derived is
45
limited, and would need to be expanded for a more accurate representation of the process.
However, since no great improvement has been experienced, the use of heat as an
environmental enhancement to the PCD has been abandoned.
E. coli Exposed to Heat (75C) Then PCD
1250 Energy (kJ/m*2)
Figure 4.10: E. coli Survivability When Exposed to Heat Then the PCD
4.3.4 E. coli Exposed to the PCD With Water Vapor
The second and final environmental enhancement applied with the PCD is
increased humidity. The humidity in the air frnmediately above the specimens is
increased by using a water vapor spray from a small, room-sized humidifier. The major
impact on the survivability of the E. coli from the water vapor in the pulsed corona
discharge is the generation of OH" radicals which come in contact wdth the specimens.
No attempt has been made to measure or vary the density of the OH" radicals. As with
the tests conducted with heat and the PCD, the second wire anay stioicture is used for
these tests with a wdre spacing of 0.125 inches.
The resuhs for this test show a complete kill, destroying an average of 9.67 x 10*
cells after five minutes of exposure time, which conesponds to an average energy of
1177 kJ/m^ Based on the limited amount of data collected for this type of exposure
environment, it appears that increasing the humidity in the air directiy above tiie
specimens is a more effective environmental enhancement technique for increasing the
destmction capability of the PCD than is heat. The main question conceming these
results is whether the E. coli cells were "washed" off tiie coupons by the water vapor
46
during the exposure process. Tests were performed wdth only the water vapor in an effort
to clarify this question, but the results were inconclusive. Because E. coli is so easily
killed, it is unknown whether the percent survival recorded is a direct resuh of the water
vapor or whether the E. coli were killed by some other environmental factor. Further
investigation of this exposure method has been continued with the more robust Bacillus
spores. The survivability of the spores under these environmental conditions wdll be
presented in the next chapter.
47
CHAPTER V
SPECIFIC APPLICATION TO THE
ANTHRAX PROBLEM
5.1 Motives and a Re-Evaluation
In its conception, this project was scheduled as a three year exploration of
electrically driven energy sources which are capable of rapid biological and chemical
decontamination of surfaces without destroying the object that is contaminated. Because
of a lack of funding renewal, this project has become both the first and the last year of the
proposed three year program. With tiie use of biological and chemical weapons
becoming an increasing threat to the nation and the world, it has been concluded by the
faculty members of the research group that the last few months of research should be
focused on a specific problem area in order to complete the research efforts wdth the most
immediately practical solution. The specific problem that seems most appropriate for the
research infrastmcture that has been developed by this project is the contamination of
mail by Anthrax.
The background research for a realizable mail decontamination system has been
conducted over the last three years by the faculty and students working on this project
and projects such as the arc jet decontamination system presented m Chapter I.
Experiments evaluating the decontamination of Bacillus spores, which are shnulants for
Anthrax, using a pulsed corona discharge have been performed over the last quarter of
the first year of research in order to obtain specific information about the possibility of
using a PCD to decontaminate the mail. As with the previous focus of the research, it is
desirable to find the most energy efficient exposure method, or combination of methods
that produces the most rapid destmction of the agent.
While the arc jet was very effective in killing Bacillus spores very quickly (on the
order of micro-seconds) and would not even bum paper when passed by the arc jet
plume, the energy efficiency of this system is something left to be desired. The exposure
methods that have been explored for the application of decontaminating mail include:
exposure to only the pulsed corona discharge, exposure to only heat, exposure to heat and
48
then to the PCD, exposure to the PCD with water vapor, and exposure to the PCD wdth
water vapor while the coupons are covered by 20 lb. printer paper or placed inside
various types of envelopes. The different types of envelopes used to simulate the mail
include padded envelope mailers with shredded newspaper as the padding, a stiff
cardboard mailer, and a plastic mailer. The number of the sheets of paper placed above
and below the coupons is also varied from 1 sheet above and below the coupons to 3
sheets above and below the coupon. The realization of a PCD decontamination system
that could be used by the US Post Office may be the size of a 24 inch television or a
standard microwave and be powered by 220 VAC. The importance of the last few
months of this research project has been to define the energy level as a fimction of the
decontamination of Bacillus spores. Once the energy levels necessary for
decontamination are established, a suitable power conditioning unit could be chosen or
designed wdthin a reasonable amount of time and the application of the research could be
completed.
5.2 Bacillus Experimental Results
5.2.1 Bacillus Exposed to the Pulsed Corona Discharge
The first experiment conducted involves exposing the Bacillus spores to just the
pulsed corona discharge. The experimental setup of these tests is the same as the similar
tests performed on the E. coli and the chemical compounds. The first wdre anay structure
is used with a wire spacing of 0.25 inches and separated from the ground plane by 1 inch.
The coupons are placed mainly beneath the wdres of the wire anay. Tests were
performed using pulse repetition frequencies of 10 Hz, 100 Hz, and 200 Hz, and input
voltages of 450 volts and 600 volts, but due to initial problems with the spores, these tests
did not yield results. The tests that did yield results were performed wdth a pulse repetion
rate of 100 Hz and an input supply voltage of 600 volts.
One can see in the in Figure 5.1 that it took almost 1000 k j W to reduce the
number of spores to just under 50% of the confrol count and an energy of almost 2000
kJ/m^ is necessary to reduce the number of spores to about 13% of the conttol count. The
energies at the data points on the graph conespond to exposure durations of 0, 5, and 10
49
minutes. Based on previous experiments performed on the E. coli, it is believed that
using environment enhancement techniques such as heat or increased humidity may be
very effective in increasing the rate of kill while decreasing the necessary energy.
100% -r-
o 75%
o 'd 50% o 0)
S 25% Q.
0%
Bacillus Exposed to the PCD
990
Energy (kJ/m*2)
1980
Figure 5.1: Bacillus Survivability When Exposed to the PCD
5.2.2 Bacillus Exposed to Only Heat
Once again, the experimental setup established by the heat tests performed on
E. coli, is also used for the experiments wdth Bacillus spores. The spores have been
exposed to temperatiires of 50, 75, 100,125,130,135, and 150°C for durations of 1,3, 5,
and 10 minutes. As explained in the hot plate section of Chapter H, the temperatiire of
the hot plate is confrolled in such a way that it oscillates above and below the intended
exposure temperature. The results for these tests show that a temperature around 100°C
must be reached before the heat begins to kill any of the Bacillus spores at all. The most
significant results show a complete kill of the spores at around 130°C. This data supports
previous research performed by Holwitt, Kiel, et al. presented eariier in the paper that
sets a 130°C threshold for significant destmction of Bacillus spores.
Figures 5.2 and 5.3 show the destiiiction of tiie Bacillus spores at 100°C and
125°C, respectively, after exposure durations of 0,1, 3, 5, and 10 minutes. Notice tiiat at
100°C, the spore survival is significantly reduced after five minutes, but even after 10
minutes of exposure, there is not a complete kill. Referring now to Figure 5.3 showdng
the desti^ction of spores at 125°C at the same duration increments, one can see that after 50
five minutes, there is only a trace of the Bacillus spores surviving. Comparing the
gradual, but evident killing of the Bacillus spores occuning at 125°C to the complete kill
of spores at 1 minute, 130''C exposures supports previous research suggesting that 130°C
is a threshold temperatiire for this strain of Bacillus spores. Tables wdth all temperatiire
and duration results can be found in Appendix A. While these results demonstrate a
complete kill of the Bacillus spores, maintaining such a high temperature on a large open
surface for even tiie shortest exposure duration is not practical and is destioictive to tiie
contaminated surface. Because of this destiaictiveness, the concept of using heat as a
killing mechanism is not a desirable solution to the problem at hand.
Bacillus Heat Only Test (100C) 100%
1 3 5 Exposure Duration (minutes)
Figure 5.2: Bacillus Survivability When Exposed to 100°C
10
Bacillus Heat Only Test (125C)
c o o
c a> u
a.
100%
0 1 3 5
Exposure Duration (minutes)
Figure 5.3: Bacillus Survivability When Exposed to 125°C
51
5.2.3 Bacillus Exposed to Heat Then the PCD
While the sole use of heat has been eluninated as a feasible option for
decontamination, the possibility of using a lower temperature to pre-treat the spores
before exposure to the pulse corona discharge is an idea worth evaluating. The goal in
such a test is to attempt to use about the same amount of energy as that used in other
tests, but apply it using the two methods being discussed. These tests have been
conducted in the same way that the E. coli tests were perfonned. The second wire anay
stiiicture is used for these tests wdth a wire spacing of 0.25 inches and a separation
between the wire anay and the ground plane of 1 inch.
If one compares the energies of exposure for this method with the exposure
methods using only heat or the PCD, one can see that combining the two individual
energy sources provides no improvement over either method. The energies in Figure 5.4
conespond to a heat exposure of 0, 5, and 10 minutes followed by an exposure to the
PCD for 0, 5, and 10 minutes, respectively. The energies given in the graph represent the
energy applied by the pulsed corona discharge. The durations for which the spores were
exposed to heat is given in the parenthesis next to the conesponding data point. Given
that the test usmg only heat is more efficient than this combined method of exposure; h is
safe to say that this is not a more efficient method for decontamination.
Bacillus Exposed to Heat (75C} Then PCD 100%
is 75% s o U -- 50% S 0)
Q.
25%
0%
(0 min!r"'" -.v, ^^^
^^"---.v^ (5 min.)
" ^ " • - f - 1 ^ ^'""^^---^^..^.^(lO min.)
1 1 ^"^"^^^ 1
990 Energy (lcJ/m*2)
1980
Figure 5.4: Bacillus Survivability When Exposed to Heat Then the PCD
5.2.4 Bacillus Exposed to the PCD With Water Vapor
Increasing the percent kill of Bacillus using an environmental enhancement such
as water vapor is a very atfractive possibility. This is because it takes very little energy to
52
generate water vapor using very common, readily available equipment. A common
household humidifier has been used to fill the volume between the wdre anay and the
ground plane wdth water vapor for these experiments, just as was done with the E. coli
experiments. One may remember that tiie increased water vapor in the air is believed to
cause an increased number of hydroxide radicals to be produced, which is thought to be
the actual mechanism that improves the killing efficiency of the PCD exposure. The
second wdre anay stmcture is used for these tests and the wire spacing has been reduced
from 0.25 inches to 0.125 inches in an effort in increase the uniformity of the discharge.
The water vapor is directed toward the wire anay using a snout. A continuous
flow of water vapor is applied to the volume sunounding the Bacillus coupons during the
entire duration of the PCD exposure. The energies at the data points in Figure 5.5
conespond to exposure durations of 0, 1,3,4, 5, and 10 minutes. One wdll notice that
there is almost a complete kill at an energy of 1258 kJ/m . Comparing this wdth the other
exposure methods, one wdll notice that this is a significant improvement over just
exposing the Bacillus spores to the pulsed corona discharge and is comparable wdth the
heat test performed at 125°C (Figure 5.3). While it is comparable in energy to the heat
test, using the PCD with water vapor is a much more practical solution and is much less
likely to harm the surface of the object bemg decontaminated. One may argue that using
the heat test wdth 130°C requires only 266 kJ/m^ of energy (1 minute) for a complete kill,
but the fact remains that heating a surface to such high temperatures for the necessary
duration will most likely be destmctive to the surface as well as to the agent.
Bacillus PCD and Water Vapor Exposure
260 750 990
Energy (kJ/m*2)
1260 2540
Figure 5.5: Bacillus Survivability When Exposed to the PCD wdth Water Vapor
53
5.2.5 Bacillus Exposed to the PCD With Water Vapor when Covered
In continuing with the possibility that the pulsed corona discharge when
combined with water vapor could be a feasible method to decontaminate the mail,
experiments have been performed testing the effect that paper, plastic, and cardboard
barriers have on the survivability of the Bacillus spores. It is reasonable to believe that
there is a limitation to the effectiveness of the PCD through thick objects such as
cardboard boxes. The main purpose of these tests is to determine what common
materials and in what thickness of these materials does the PCD become ineffective. The
materials that have been chosen to be used include: up to 3 sheets of copy paper placed
above and below the coupons, a US Post Office padded mailer with shredded newspaper
as the padding, a cardboard document mailer, and a plastic mailer. When possible, the
coupons are slid inside the envelope being tested to most accurately simulate conditions
that would be encountered with mail. The same equipment setup has been used for this
test that was used for the previous PCD with water vapor tests.
Figure 5.6 shows the results after exposure to the PCD wdth the test specimens
sunounded by the US Post Office padded mailing envelope. Because the envelope would
not fit under the wire anay, large strips of the envelope were cut and placed above and
below the coupons plated wdth Bacillus spores. The envelope was cut so as to cover the
entire surface of the ground plane and positioned in a manner that prevented the coupons
from direct access to the water vapor. This was done in order to more closely simulate
the situation of Anthrax inside such an envelope. The energies at the data points in
Figure 5.6 conespond to exposure durations of 0,3,4, and 5 minutes. Comparing these
resuhs wdth those for the PCD with water vapor exposure without any barriers (Figure
5.5), it is clear that the presence of the envelope increases the survivability of the Bacillus
spores. The results for the plastic mailer, provided in Appendix A, tend to suggest that
the more effectively the banier prevents the water vapor from coming in close contact
with the coupons, the greater tiie survivability of the coupons. The plastic mailer is much
thinner than the padded mailer; however, tiie survivability of the coupons in the plastic
mailer is higher tiian tiiat for the padded mailer. This supports the idea that tiie water
vapor is a key factor in the effectiveness of the pulsed corona discharge.
54
The inconsistencies with some of the results are attributed to the probability that
some of the barriers being used are wiping spores off the coupons when they lay on top
of the coupons or when the baniers are removed from on top of the coupons. At this
point, it is not completely clear how much of a factor this wiping action is playing in the
survivability of the spores. However, it is clear that as the number of sheets of paper is
increased or as a thicker envelope is used, the survivability of the spores is increased.
These baniers are hindering the effectiveness of the pulsed corona discharge.
Bacillus Exposed to PCD & Water Vapor & Covered (Cushion Mailer) 100%
940 1250
Energy (kJ/m*2)
1560
Figure 5.6: Bacillus Survivability When Placed Inside the Cushion Mailer
55
CHAPTER VI
FUTURE RESEARCH
The objectives for the first year of work on this project have been met and great
strides toward the immediate application of this technology to the decontamination of
mail have been made. In addition to accumulating meaningful data, the infrastmcture for
further testing has been established. The lab space is now fully operational and is
ananged in such a manner that adapting to new pieces of equipment and new test setups
would be a sfraightforward task. The necessities such as available power and necessary
venting are now available and convenientiy accessed. While the goals for the first year
have been met, the continuance of the project would necessitate some basic
improvements.
The main improvements lay wdth the power conditioning unit (PCU) and the wdre
anay stmcture. It is desirable to be able to perform the tests in a continuous manner
wdthout the time gaps that are cunently allowed in order for the PCU to cool down. In
addition, it is believed that the duration of exposure could be reduced down to seconds or
micro-seconds by merely increasing the pulse repetition frequency into the hundreds of
kHz range. Shortening the duration to these time periods would demonstrate the
capabilities of the pulsed corona discharge on a more realistic time scale. This is an
assumption that cannot be tested with the cunent PCU.
The main limitation wdth the wire anay is the uniformity of the electric field that
it produces, which is directly related to the uniformity of the pulsed corona discharge.
Generating a more uniform pulsed corona discharge directly translates into more
consistent and repeatable results. Alternative architectures that have been considered
include the use of dielectric barriers between the wdre anay and the ground plane (a
barrier discharge) or completely abandoning the wire anay and moving to a pin anay. In
either case, the main point is to improve the uniformity of the pulsed corona discharge.
Anotiier source of inconsistency throughout the project has been the biological
specimens. As has already been explained, the methods for preparing, fransporting, and
evaluating the biological specimens must be improved. It is believed that a major source
56
of variation wdth the biological resuhs lies with the transporting of test specimens
between laboratories. No matter how many precautions one takes to ensure safe fransport
of these specimens, the 6 mile round trip voyage of the biological specimens consistentiy
results in the reduction of cells or spores whether the sample had been exposed or not.
This is a situation that must be addressed for further tests.
With respect to further experiments, one of the main sources that has not yet been
applied is the electron beam. This source has been studied by many other research
groups and is actually cunently being used by the US Post Office to decontaminate mail.
However, it is important to perform these tests using the same basic setup and test
specimen that have been used so far in order to establish a point of comparison with the
pulsed corona discharge. Tests evaluating the effectiveness of the elecfron beam as the
sole source and the elecfron beam combined wdth the pulsed corona discharge are of
particular interest. The survivability of the chemical compounds when exposed to the
electron beam source would also be of particular interest. Because of the concentration
on Bacillus spores, there has not been much data accumulated for the chemical
compounds. This database needs to be expanded and experunents wdth an elecfron beam
would be one source by which to expand the chemical database.
Other techniques that would help one to better understand which killing
mechanisms are most dominant within the pulsed corona discharge include tests wdtii UV
light, tests that limit the production of hydroxide radicals, such as using a Nifrogen
blanket, and tests that vary the amount of ozone to which the specimens are exposed. It
would also be of interest to use a spectrometer to determine the specfral elements of tiie
pulsed corona discharge. This would help in deteraiining how much UV or IR light is
actually being generated by the PCD. It would be interesting to plate the specimens on
different materials and evaluate thefr survivability based on the material to which the
specimen is attached. While the type of material is limited by the ability to recover the
specimens after exposure, this could be a very important factor which detennines the
survivability of the specimen.
57
CHAPTER VII
CONCLUSIONS
After multiple experiments testing the ability of the pulse corona discharge (PCD)
to destroy both biological and chemical agents, it has been verified that the PCD is an
effective destmction technique. As expected, the Bacillus spores were not as easily killed
by the PCD as the E. coli and the Parathion proved to be more robust than the Malathion.
The most promising combination of destmction mechanisms was found when combining
the pulsed corona discharge with water vapor. After exposing the Bacillus to just the
PCD, it was found that 1980 kjW of energy reduced the number of spores to around
13% of the confrol specimen count. When a constant flow of water vapor across the wdre
anay was added as an environmental enhancement, a 1258 kJ/m^ exposure yielded a 3%
survivability. Increasing the amount of water vapor in the air directly surroimding the
specimens proved to be an energy-efficient and non-destmctive additive that increased
the capability of the pulsed corona discharge to kill Bacillus spores and left no residue.
After further tests with the Bacillus spores, it has been verified that barriers such
as cushion mailers, cardboard document mailers, and plastic mailers inhibit the effective
ness of the PCD. When placed inside a cushioned mailing envelope and exposed to 1560
kJ/m^ of energy from the pulsed corona discharge with the water vapor, the percent
survival increased to about 30%. Using the same exposure method, the survivability of
the spores is further increased when placed inside a plastic mailing envelope resulting in
a 70% survival of the spores exposed to an energy of about 1580 kJ/m .
Based on the results, it is believed that the PCD combined wdth water vapor is an
energy efficient, non-destmctive technique that could be used to successfully destroy
biological agents such as Anthrax that may contaminate the mail. Continued research is
necessary to verify that the duration of exposure could be significantly reduced by
increasing the repetition rate of the pulsed corona discharge. A possible decontaminating
machine utilizing the pulsed corona discharge could be realized as an appliance the size
of a large microwave powered by 220 VAC into which a bag of questionable mail could
be placed for exposure durations of a few minutes or less.
58
REFERENCES
1. Schoenbach, K. H., R.P. Joshi, R.H. Stark, F.C. Dobbs, and S.J. Beebe, "Bacterial Decontamination of Liquids wdth Pulsed Electric Fields," IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 7 No. 5, pp. 637-645, October 2000.
2. Laroussi, Mounir, "Sterilization of Contaminated Matter with an Atmospheric Pressure Plasma," IEEE Transactions on Plasma Science, Vol. 24, No. 3, pp. 1188-1191, June 1996.
3. Pai, S.T. and Qi Zhang, Introduction to High Power Pulse Technology, River Edge, New Jersey: Worid Scientific, pp. 50-58, 1995.
4. Smtth, Phillip H., "Optimum Coax Diameter," Electronics for Communication Engineers, Ed. John Markus and Vin Zeluff New York: McGraw Hill, pp. 548-551, 1952.
5. Holwitt, Eric, Johnathan L. Kiel, John L. Alls, Pedro J. Morales, and Homer Gifford, "Thermal Sensitivity of biowarfare simulants," Proceedings ofSPIE Reprint, vol. 4036, pp. 31-39, 2000.
th
6. Nilsson, James W. and Susan A. Riedel, Electric Circuits, 5 Edition, New York: Addison-Wesley, p. 372, 1996.
7. Cheng, David K., Field and Wave Electromagnetics, 2°'' Edition, New York: Addison-Wesley, p. 270, 1989.
8. Electrophysics Institute, Ural Branch, Russian Academy of Sciences, Nanosecond Desk-Top SOS-Based Generator Operating Manual, Ekaterinburg, 1997.
59
APPENDIX A
RESULT TABLES FOR ALL TESTS
60
Chemical Test Results
Table A. 1: Malathion Exposed to the Pulsed Corona Discharge
Malathion Initial Concenfration:
Duration 1 min. 2 min. 3 min. 5 min.
Total Energy
(kJ/m^) 470 850
1340 2230
Std. Dev. (Energy)
0 0
63 104
82.5 mg (250 nmole)
% Survival 83.22% 50.33% 30.43% 14.64%
Std. Dev. (% Survival)
9.14% 6.21% 7.05% 5.73%
% DDTP 2.63% 2.60% 4.90% 7.18%
Std. Dev. (% DDTP)
1.86% 1.84% 0.92% 1.69%
Table A.2: Parathion Exposed to the Pulsed Corona Discharge
Parathion Initial Concentration:
Duration 3 min. 5 min. 10 min. 15 min.
Total Energy
(kJ/m^) 1300 1990 3980 5640
Std. Dev. (Energy)
0 130 270
0
72.75 mg (250 nmole)
% Survival 46.38% 26.91%
8.64% 0.00%
Std. Dev. (% Survival)
5.16% 13.66% 15.37% 0.00%
% DDTP 55.80% 71.57% 60.44% 17.14%
Std. Dev. (% DDTP)
4.51% 13.66% 19.88% 4.67%
61
E. coli Test Results
Table A.3: E. coli Exposed to the Pulsed Corona Discharge
E. coli Initial Concenfration:
Duration 3 min. 5 min. 10 min.
Total Energy
(kJ/m^) 1230 1880 3580
Std. Dev. (Energy)
8 270 600
10^ cells
% Survival 52.78% 15.78% 0.39%
Std. Dev. (% Survival)
31.29% 14.35% 0.47%
Table A.4: E. coli Exposed to Only Heat
E. coli 1
Temp.
CC) 50
75
100
nitial Concentiation:
Duration
1 min. 3 min. 5 min. 10 min. 1 min. 3 min. 5 min. 10 min. 1 min. 3 min. 5 min. 10 min.
Total Energy
(kJ/m^)
70 200 330 660 130 390 640
1400 180 540 900
1810
Std. Dev. (Energy)
0 0 0 0
20 10 2 2 0 0 0 0
10^ cells
%
Survival
81.16% 55.43% 42.12% 23.33% 21.79% 10.15% 2.23% 2.06% 0.33% 0.08% 0.01% 0.00%
Std. Dev. (% Survival)
18.42% 17.49% 29.64%
5.77% 13.90% 5.38% 2.68% 0.82% 0.48% 0.10% 0.01% 0.00%
Cell Count Reduction
1.30E+05 1.60E+05 1.28E+05 7.67E+04 5.12E+06 5.70E+06 5.66E+06 6.23E+06 7.47E+06 7.50E+06 7.51E+06 7.51E+06
Std. Dev. (Cell Count Reduction)
1.27E+05 1.28E+05 7.19E+04 5.77E+03 5.33E+06 6.04E+06 6.00E+06 6.72E+06 8.15E+06 8.19E+06 8.30E+06 8.20E+06
Table A.5: E. coli Exposed to Heat Then the Pulsed Corona Discharge
E. coli Initial Concenfration:
Temp.
CC) 75 75
Duration 5 min. 10 min.
Total Energy
(kJ/m^) 1870 3040
Std. Dev. (Energy)
0 0
4.67 X 10 cells
%
Survival 7.14% 0.25%
Std. Dev. (% Survival)
4.95% 0.03%
Cell Count Reduction
4.34E+06 4.66E+06
Std. Dev. (Cell Count Reduction)
2.33E+05 1.53E+03
62
E. coli Test Results
Table A.6: E, coli Exposed to the Pulsed Corona Discharge wdth Water Vapor
E. coli Initial Concenfration:
Duration 5 min. 10 min.
Total Energy (kJ/m^)
1180 2360
Std. Dev. (Energy)
0 0
% Survival 0.00% 0.00%
9.67 X 10 cells
Std. Dev. (% Survival)
0.00% 0.00%
Cell Count Reduction
9.67E+06 9.67E+06
Std. Dev. (Cell Count Reduction)
0 0
63
Bacillus Test Results
Table A.7: Bacillus Exposed to the Pulsed Corona Discharge
Bacillus Initial Concenfration: 2.00E+4 spores
Duration: 5 min. 10 min.
Total Energy
(kJlrn) 990
1980
Std. Dev. (Energy)
0 0
% Survival Bacillus
45.45% 12.73%
Std. Dev. (% Survival)
0.00% 11.53%
Spore Count Reduction:
1.20E+04 1.92E+04
Std. Dev. Spore Count Reduction
0.00 2535.74
Table A.8: Bacillus Exposed to Heat Then the Pulsed Corona Discharge
Initial Bacillus Concentration:
Temp.
CC) 75 75
Duration 5 min. 10 min.
Total Energy
(kJ/m^) 2020 4040
Std. Dev. (Energy)
0 0
2.20E+04
%
Survival 37.42%
0.00%
Std. Dev. (% Survival)
10.85% 0.00%
Spore Count Reduction
1.38E+04 2.20E+04
Std. Dev. Spore Cnt. Reduction
2.39E+03 O.OOE+00
Table A. 9: Bacillus Exposed to the Pulsed Corona Discharge wdth Water Vapor
Initial Bacillus Concenfration:
Duration 1 min. 3 min. 4 min. 5 min. 10 min.
Total Energy
(kjW) 260 750 990
1260 2540
Std. Dev. (Energy)
1 15 0
25 20
% Survival 39.00% 12.55% 14.14% 2.09% 0.00%
Average 1.58E+04
Std. Dev. (% Survival)
15.44% 14.71% 14.79% 2.53% 0.00%
Std. Dev. 2.62E+03
Spore Count Reduction
1.06E+04 1.30E+04 l.lOE+04 1.48E+04 1.73E+04
Std. Dev. Spore Cnt. Reduction
2668 2224 1893 2634
0
64
Bacillus Test Resuhs
Table A. 10: Bacillus Exposed to Heat Only
Bacillus Heat Only Tests (hiifial Concentration on order of 5E+4 spores)-
Temp.
CC) 75 C
100 C
125 C
130 C
135 C
150 C
Duration
1 min. 3 min. 5 min. 10 min. 1 min. 3 min. 5 min. 10 min. 1 min. 3 min. 5 min. 10 min. 1 min. 3 min. 5 min. 10 min. 1 min. 3 min. 5 min. 10 min. 1 min. 3 min. 5 min. 10 min.
Total Energy
(kJ/m^)
140 400 670
1350 150 440 730
1460 250 720
1200 2400
270 800
1330 2660
210 620
1400 2080
270 800
1330 2660
Std. Dev. (Energy)
0 0 0 0 0 5
10 20 10 5
10 15 0 0 0 0
40 120 600 500
10 30 60
110
%
Survival
73.91% 91.30% 86.96%
104.35% 100.00% 68.70% 39.22% 34.87% 23.67%
7.34% 0.50% 0.44% 0.00% 0.00% 0.00% 0.00% 0.00%) 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
Std. Dev. (%
Survival) 45.76% 15.06% 4.35% 3.83% 0.00%
22.24% 26.31%
0.00% 18.26%) 6.89% 0.55% 0.73%) 0.00% 0.00% 0.00%) 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%)
Spore Reduction
2.95E+05 1.20E+05 9.00E+04
-3.00E+04 O.OOE+00 2.26E+04 4.94E+04 5.04E+04 2.83E+04 3.59E+04 4.07E+04 4.06E+04 1.30E+04 1.30E+04 1.30E+04 1.30E+04 1.88E+04 1.88E+04 1.88E+04 1.88E+04 2.39E+04 2.39E+04 2.39E+04 2.39E+04
Std. Dev. Spore Reduction
3.46E+05 O.OOE+00 3.00E+04 2.65E+04 O.OOE+00 2.87E+04 4.01E+04 3.62E+04 2.95E+04 3.93E+04 4.49E+04 4.47E+04 O.OOE+00 O.OOE+00
• O.OOE+00 O.OOE+00 6.40E+03 O.OOE+00 O.OOE+00 O.OOE+00 1.20E+03 O.OOE+00 O.OOE+00 O.OOE+00
65
Bacillus Test Results
Table A. 11: Bacillus Exposed to the PCD with Water Vapor when Covered
Bacillus Efield & Paper & Humidity Tests: Average Bacillus Initial Concentration;
Duration
1 min. 3 min. 3 min. 3 min. 3 min. 3.5 min. 3.5 min.
4 min. 4 min. 4 min. 4 min.
4.5 min. 4.5 min. Smin. 5 min. 5 min. Smin. 5 min.
5.5 min. 5.5 min. 10 min. 10 min. 3 min. 4 min. Smin.
3 min. 4 min. S min. 5 min. 10 min.
Sheets Above
1 1 2 3 3 1 2 1 2 3 3
1 2 1 1 2 3 3
1 2 1
1 1* 1* 1* ] * *
J** 2** J*** } * * *
Sheets Below
1 I 2 2 3 1 2
1 2 2 3
1 2 0 1 2 2 3
1 2 0
1
1* 1* 1* ] * *
1** J** J*** J***
Total Energy
(kJ/m^)
250 800 800 800 800 920 920
1070 1070 1070 1070 1190 1190
13S0 13S0 1350 1350 1350 1450 14S0 2720 2720 940
1250 1560
930 1240 1550 1580 3180
Std. Dev. (Energy)
0 95 95 95 95 0 0
135 13S 135 135
0 0
180 180 180 180 180
0 0
430 430
5 S S
40 50 65
20 40
Average: 1.33E+04
% Survival
11.66% 22.62% 19.27% 15.09% 28.86% 72.56% 39,07%
3.83% 10.71% 11.47% 9.73%
42.31% 33.01% 2.66% 0.50% 4.04% 6.69% 0.00%
66.25% 25.10%
1.03% 0.04%
51.94% 35.69% 30.67% 21.01% 11.92% 18.51% 68.33% 42.15%
Std. Dev. 5.46E+03
Std. Dev. (% Survival)
6.43% 40.94% 18.04% 3.34%
20.06% 0.00% 0.00% 4.03%
14.09% 3.83%
11.05% 8.97%
19.34% 0.58% 1.08% 6.11% 3.38% 0.00%
48.32% 43.48%
1.79% 0.06% 6.63% 6.70% 8.20%
3.23% 2.62% 8.19% 7.92% 5.37%
Spore Count Reduction
1.82E+04 1.23E+04 8.39E+03 1.09E+04 7.89E+03 1.97E+03 4.37E+03 8.17E+03 9.23E+03 1.13E+04 l.OOE+04 4.14E+03 4.80E+03 1.26E+04 1.45E+04 9.80E+03 1.19E+04 l.llE+04 2.42E+03 5.37E+03 1.28E+04 2.06E+04 4.72E+03 6.32E+03 6.81E+03 1.64E+04 1.83E+04 1.70E+04 6.S8E+03 1.20E+04
Std. Dev. (Spore Count Red)
1.32E+03 8.45E+03 2.90E+03 4.27E+02 2.22E+03 O.OOE+00 O.OOE+00 1.43E+03 2.72E+03 4.90E+02 1.23E+03 6.43E+02 1.39E+03 5.58E+01 6.42E+03 2.22E+03 4.33E+02 O.OOE+00 3.46E+03 3.12E+03 2.31E+02 1.29E+01 6.52E+02 6.57E+02 8.05E+02 6.71E+02 S.45E+02 1.70E+03 1.64E+03 1.12E+03
: US Post Office Cushion Mailer (Shredded Newspaper as Cushion) ** : Airborne Express Cardboard Mailer *** : Airbome Express Plastic Mailer
66
APPENDIX B
SLIDE LAYOUTS FOR
SELECTED TESTS
67
Ground Plane
1
4
7
10
(From Pulser)
2
5
8
11
3
6
9
12
Wire Array
(Overhead View of Test Slide Location)
Figure B. 1: Layout for Bacillus K, L, N, O Tests
Ground Plane
Wire Array
9
6
3
(F rom Pul<
8
5
2
>er)
7
4
1
(Overhead View of Test Slide Location)
Figure B.2: Layout for Bacillus X, Y, Z, AA, BB, CC, DD, FF, KK, LL, NN, 0 0 and Parathion M Tests
68
Ground Plane
Wire Array
1
4
(From Pulser)
2
5
3
6
(Overhead View of Test Slide Location)
Figure B.3: Layout for Bacillus J, M, P, S and E. coli Q Tests
Ground Plane
Wire Array
4
5
6
(F rom Puis >er)
1
2
3
(Overhead View of Test Slide Location)
Figure B.4; Layout for Bacillus V and W Tests
69
Ground Plane
ire Array
13
14
15
16
17
18
(From Pulser)
7
8
9
10
11
12
1
2
3
4
5
6
(Overhead View of Test Slide Location)
Figure B.S: Layout for Bacillus T and U Tests
Ground Plane
Wire Array
1
7
4
(F rom Puis
6
3
1
ser)
5
2
(Overhead View of Test Slide Location)
Figure B.6: Layout for Bacillus EE and GG Tests
70
Ground Plane
\
Wire Array
, 6
4
2
(From Pulser)
5
3
1
(Overliead View of Test Slide Location)
Figure B.7: Layout for Bacillus HH Test
Ground Plane
Wire Array
, 1,4
1.4
(F rom Pul!
2,5
2,5
ser)
3.6
3,6
Malathion H
Parathion K
(Overhead View of Test Slide Location)
Figure B.S: Layout for Parathion K, Malathion H Tests
71
Ground Plane
Wire Array
,
6,9
3, 12
(F rom Pul;
5,8
2, 11
ser)
4,7
1, 10
(Overhead View of Test Slide Location)
Figure B.9: Layout for Malathion I Test
(Overhead View of Test Slide Location)
Figure B. 10: Layout for Parathion L Test
(Empty)
Ground Plane
Wire Array
12
9
6
3
(F rom Pul!
11
8
5
2
5er)
10
7
4
1
20 min.
15 min.
10 min.
5 min.
72
APPENDIX C
POWER CONDITIONING UNIT (PCU)
73
In order to generate the PCD, a power source capable of delivering high voltage,
nanosecond pulses is necessary. A Russian built nanosecond desktop semiconductor
opening switch (SOS) based generator is used to deliver the necessary high vohage
pulses. The nanosecond desktop SOS-based generator is referred to as the power
conditioning unit (PCU) throughout this paper. The PCU is capable of delivering voltage
pulses with amplitude of up to 150 kV and 400 Amps at a pulse repetition frequency in
burst mode of up to 100 Hz. The maximum recommended operating time in burst mode
is one minute, but may also be operated in a continuous pulsing mode if the pulse
repetition frequency is reduced to 10 Hz. The typical pulse width of the delivered
waveform varies between 20 and 30 ns at Full Width Half Maximum (FWHM). The
PCU output pulses are controlled by input drive-pulses with voltage amplitude of 10-15
volts and a pulse length of 5-10 ^s at a 50 Q load. The main supply voltage to the PCU
is 600 Volts DC and the mains power consumption of the PCU should not exceed 1 kVA
at the maximum pulse repetition frequency of 100 Hz. The PCU is capable of delivering
pulses at the stated voltage levels and a pulse repetition frequency of 100 Hz for up to
two minutes, but one risks overheating the pulser under such operating conditions. A
diagram showing how the PCU is connected to the rest of the data acquisition and power
supply systems can be viewed in Figure 2.5.
The PCU consists of three stages of power conditioning units. The first unit is a
thyristor converter, referred to as the A1 unit, which converts the 600 volt DC input into a
microsecond (^s) long pulse using a thyristor switch. A schematic diagram of the Al
unit may be seen in Figure C. 1. The A2 unit is the high voltage nanosecond (ns) pulse
shaper, which compresses the pulse from unit Al to a 1 |us pulse with pulse amplitude of
50-60 kV using a magnetic switch and pulse transformers. The semiconductor opening
switch at the output of unit A2 converts the energy of the microsecond pulse to the
nanosecond range of tune simultaneously increasing the voltage and current by about 3
times. The load is connected to the output of unit A2. Figure C.2 provides a schematic
diagram of the A2 unh. The third unit (A3) of the PCU is the confrol unit and the circuit
diagram for this unit may be seen in Figure C.3. Umt A3 amplifies the input dnve pulses
74
from the external generator. Transformed pulses of the required amplitude and length are
sent to control the electrode of the thyristor in the Al unit [8].
The PCU operating specifications limit the ability to deliver pulses for continuous
exposure durations of more than 1 minute at a pulse repetition frequency of 100 Hz. The
components within the PCU that limit the output specifications due to overheating are the
charging resistors for the primary storage capacitors in the Al unit and the unknown
limitations of the semiconductor opening switch in the A2 unit. In order to limit power
dissipation by the charging resistors, it would not be difBcuh to replace these ceramic,
high power resistors with resistors of a higher power rating and higher resistance.
Replacing the resistors would lower the current flowing through the resistors thereby
lowering the amount of heating at the resistors assuming a constant input voltage. The
next major component of concern is the semiconductor opening switch which switches
the power in the final stage delivering the nanosecond high voltage pulse at the output.
More detailed specifications considering the switch's power rating and recovery times
would be preferred before operating the pulser for longer continuous durations or at
higher frequencies than 100 Hz. Because of limited information about the specifications
of individual components within the pulser, the replacement of the charging resistors has
not been pursued. Due to the limitations of the PCU, the experiments have been
restricted to applying the pulsed corona discharge exposures in one minute increments
wdth a five minute cooling period in between each exposure.
The output voltage and current through the SOS interrupter are measured via
BNC connectors labeled "Us" and "Is" that are located at the front face of the pulser. The
front face of the pulser is also the face from which the high voltage nanosecond pulse is
delivered to the desired load. The connector Us is intended to measure the output voltage
delivered to the load and the connector labeled Is is intended to measure the current
through the interrupter. The voltage and current waveforms are monitored using a
Hewlett-Packard Infinium Oscilloscope capable of measuring frequencies up to 500 MHz
at a 2 GSa/s resolution. The peaks of the voltage and current waveforms will not be
aligned when monitored with the oscilloscope because the voltage and current are not
measured at the same points in the circuit.
75
The connector Is measures the current passing through the SOS interrupters by
measuring the voltage across the parallel connection of resistors R6...R15 and the 50 Q
termination at the oscilloscope. The equivalent resistance of the parallel elements is
0.505 Q. This can be rounded to 0.51 Q. The current is calculated using Ohm's law. The
resistance is a constant value of 0.51 Q, neglecting any changes due to heating and the
oscilloscope records the voltage drop across the 0.51 Q resistance. The current may be
viewed on the oscilloscope by appropriately setting the external gain and the attenuation
in the channel properties for channel 1 of the oscilloscope. A more detailed explanation
for the setup of the oscilloscope is provided in the Data Acquisition section of Chapter II.
The current delivered to the load is approximately the same as the current measured by
the current monitor Is. During the reverse pumping of the SOS switches, the current
passing through the switches charges LI, an inductive storage component. The load
resistance is estimated to be much less than the resistance presented by the voltage
divider. Because of the low load resistance, virtually all of the energy stored in the
inductive storage component is transferred to the load. Maintaining a very low load
resistance allows one to assume that the current delivered to the load is approximately the
same as the current passing through the SOS switches a few nanoseconds beforehand as
measured by the current monitor.
The voltage (Us) delivered to the load is measured using a resistive voltage
divider. The series connection of resistors R16 through R31, summing to 75.2 kQ, make
up the high voltage leg of the divider and the parallel combination of R32, R33, and the
50 Q termination at the oscilloscope, equaling 4.545 Q, make up the low voltage leg of
the divider. The voltage across the low voltage leg of the divider is a known fraction of
the voltage delivered to the load. A mathematical explanation is provided in the Data
Acquisition section of Chapter II to explain the setup of the oscilloscope conceming the
proper gain settings for the signals used to measure the voltage and current delivered to
the load. Figure C.2 is a schematic diagram for part of the A2 unit showing the locations
in the circuit where the current and voltage are measured.
76
fll
+ o-
'POUER"
P.7M :3 . . ,C !2
- ^ ^ J L ^ 2] ^ fT8 H ijl
'5fl"
s <r''^^fc 0 to ! R2
R3,R4
Tl
"=600 U" ^ ' ^ tu
UDl RS
t o i n3
* 0 i
o to 2 fl2
Figure C. 1: Circuit Diagram of the Thyristor Charging Unit Al
ft2 C 1 3 . . . C 2 1
111 d
L ^ u^ 4- L.
n C ! . . . c i ; R3 1
1
2
— I I . '
il i l^o-vv-
"\k
p " T-XI
Ul- -u i -
HO) H(J)
R5...fll5
9i
U)" kD
Jfi lS. . .Rjl
I 4 i 1 ' •
R 3 2
' ^ '
fjBM....
Figure C.2: Circuit Diagram of the High Voltage Nanosecond Pulse Shaper A2
77
fl3
R I . . . R 3 R9. . .R11 to 1 fll 1 o—r—I 1—, 1 f ^
Ct
t o 2 fll 2 <y-^ I-
Cl C3
T2
3
: : osi
R7 ^1 R8 R12
"TRIG-T i f xl
UD2
x_3
. 1 R13 • RM
CZh'
-N- UD3
3
Figure C.3: Circuit Diagram of the Confrol Unit A3
-o to 3 fll Lo to 4 fll
78
PERMISSION TO COPY
In presentmg this thesis m partial frilfiUment of the requuements for a master's
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