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Electric field measurement in the dielectric tube of heliumatmospheric pressure plasma jetCitation for published version (APA):Sretenović, G. B., Guaitella, O., Sobota, A., Krstić, I. B., Kovačević, V. V., Obradović, B. M., & Kuraica, M. M.(2017). Electric field measurement in the dielectric tube of helium atmospheric pressure plasma jet. Journal ofApplied Physics, 121(12), [123304]. https://doi.org/10.1063/1.4979310
DOI:10.1063/1.4979310
Document status and date:Published: 28/03/2017
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Electric field measurement in the dielectric tube of helium atmospheric pressureplasma jetGoran B. Sretenović, Olivier Guaitella, Ana Sobota, Ivan B. Krstić, Vesna V. Kovačević, Bratislav M. Obradović, andMilorad M. Kuraica
Citation: Journal of Applied Physics 121, 123304 (2017); doi: 10.1063/1.4979310View online: http://dx.doi.org/10.1063/1.4979310View Table of Contents: http://aip.scitation.org/toc/jap/121/12Published by the American Institute of Physics
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Electric field measurement in the dielectric tube of helium atmosphericpressure plasma jet
Goran B. Sretenovic,1,a) Olivier Guaitella,2 Ana Sobota,3 Ivan B. Krstic,1
Vesna V. Kovacevic,1 Bratislav M. Obradovic,1 and Milorad M. Kuraica1
1Faculty of Physics, University of Belgrade, P.O. Box 44, 11001 Belgrade, Serbia2LPP, Ecole Polytechnique, Route de Saclay, 91128 Palaiseau, France3Eindhoven University of Technology, EPG, Postbus 513, 5600MB Eindhoven, The Netherlands
(Received 22 December 2016; accepted 15 March 2017; published online 28 March 2017)
The results of the electric field measurements in the capillary of the helium plasma jet are presented
in this article. Distributions of the electric field for the streamers are determined for different gas
flow rates. It is found that electric field strength in front of the ionization wave decreases as it
approaches to the exit of the tube. The values obtained under presented experimental conditions are
in the range of 5–11 kV/cm. It was found that the increase in gas flow above 1500 SCCM could
induce substantial changes in the discharge operation. This is reflected through the formation of the
brighter discharge region and appearance of the electric field maxima. Furthermore, using the
measured values of the electric field strength in the streamer head, it was possible to estimate
electron densities in the streamer channel. Maximal density of 4� 1011 cm�3 is obtained in the
vicinity of the grounded ring electrode. Similar behaviors of the electron density distributions to the
distributions of the electric field strength are found under the studied experimental conditions.
Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4979310]
I. INTRODUCTION
Non-thermal atmospheric pressure plasma jets occupy
the attention of low temperature plasma (LTP) community
for the last two decades, due to various physical phenomena
interfering in its nature and its application in the field of
medicine.1–4 Numerous theoretical and experimental studies
revealed streamer properties of these discharges, and most of
these researches are listed in several reviews.3,5,6 Low tem-
perature plasma (LTP) jets are usually initiated by noble gas
flow through inter-electrode space while electrodes are sup-
plied by AC or pulse high voltage. The usual geometry of
LTP jet is coaxial, with one powered, needle or ring, elec-
trode and one grounded electrode around a dielectric tube
with typical diameters in the range of 1–5 mm. Thus, plasma
jet discharge could be divided into three parts: (1) the dis-
charge in the inter-electrode space—usually dielectric barrier
discharge; (2) discharge in the dielectric tube; and (3) dis-
charge in the plume of the jet. Until this moment, most of
the studies were focused on the guided ionization wave—
plasma bullet investigation in the plume of the jet.7–11
Plasma bullets move along the predominant path, defined by
the noble gas, usually helium or argon, mixing with ambient
air. Plasma jet plume exists in the limits determined by exact
maximal molar fraction of air in helium.12,13
The previous work of authors was also focused on the
plume of the plasma jet. Ionization wave propagation and
interaction with targets were investigated by means of fast
imaging, electric field, and charge measurement using Stark
polarization spectroscopy and Pockels effect.13–19 This arti-
cle is devoted to discharge development in a dielectric capil-
lary of the coaxial 30 kHz helium plasma jet with the
emphasis on the electric field strength dependence on the gas
flow rate. Discharge ignition and dynamics in dielectric tubes
are also studied by several groups of authors.20–31 Ionization
wave in dielectric tube travels in almost uniform gas mixture
and in a well-defined space, but with the additional influence
of dielectric tube on its propagation through the charge accu-
mulation on the walls. Behind the ionization front, a conduc-
tive, quasi-neutral plasma column forms, while the positive
charge deposits on the tube walls.27 The material of dielec-
tric tube impacts the propagation properties of ionization
waves. Higher permittivity increases the capacity of the tube
walls and thus the charging time. As the consequence, peak
velocity and peak electric field strength decrease with higher
permittivity.26
II. EXPERIMENTAL SET-UP
Atmospheric pressure helium plasma jet of coaxial
geometry described in the previous articles is also used in
this study.13–16 It consists of Pyrex tube with inner diameter
of 2.5 mm and outer diameter of 4 mm and two metallic elec-
trodes. Powered electrode is stainless steel pipe with inner
and outer diameters of 0.8 and 1.6 mm, respectively. It is
centered inside the dielectric tube, while the grounded elec-
trode, 3 mm wide ring, is placed around dielectric tube 5 mm
downstream the powered electrode, see Figure 1. Helium
flow (Messer, 99.996% purity) through metallic pipe is con-
trolled by means of mass flow controller BronkhorstVR
in the
range of 700–2000 standard cubic centimeters per minute
(SCCM). Metallic pipe is supplied by sinusoidal high voltage
with amplitude of 2 kV and frequency of 30 kHz. Voltage is
measured using Tektronix P6015A high voltage probe, and
current is determined as a voltage drop over 100 X non-
inductive resistor. For the spectroscopic measurement, Solara)[email protected]
0021-8979/2017/121(12)/123304/6/$30.00 Published by AIP Publishing.121, 123304-1
JOURNAL OF APPLIED PHYSICS 121, 123304 (2017)
MSDD 1000 spectrometer with the focal length of 1 mm and
the grating of 1200 grooves/mm is used. The light emitted
from the capillary tube is projected on the slit of spectrome-
ter by achromatic lens with a focal length of 150 mm.
Magnification ratio was 1:3. Due to the low intensity of the
observed light, slit width was set to 70 lm, which resulted
with the instrumental half width of 0.051 nm. The ICCD was
triggered using a time-delayed pulse signal from a non-
inductive resistor inserted between the grounded electrode
and the grounding point.
The aim of this work was the measurement of the electric
field strength along the capillary tube for different gas flows.
Electric field strength was measured by Stark polarization
spectroscopy method. More details about electric field mea-
surement using this method in low pressure glow discharge,
DBD, plasma jet, and radiofrequency atmospheric pressure
discharge in helium can be found in our previous publica-
tions.17–19,32–39 The method is based on the measurements of
the distance between two helium lines, forbidden and allowed,
which are shifting depending on the value of the external elec-
tric field. In order to separate two polarized components of
the emitted light, plastic polarizer was mounted in front of the
slit, and only p-polarized light was recorded. In this work, we
used the He (21P-41D) allowed line at 492.19 nm and the clos-
est forbidden He (21P-41F) line.
Discharge under investigation emits one guided ioniza-
tion wave during the positive half cycle.15 Therefore, electric
field is measured for the positive current signal, i.e., during
the propagation of the positive streamer. Gate width of typi-
cal 5 ls captured entire ionization wave propagation, from
the inter-electrode space until the plume end. Streamer
passes the length of the tube for about 2 ls, which corre-
sponds to the average velocity of about 10 km/s. Ionization
wave emits light only from the ionization front—streamer
head. Consequently, spatially resolved measurement also
reveals a temporal development of the streamer as it was
argued by Sobota et al.,13 see Fig. 2(b) in this reference.
For example, in the mentioned article where the same dis-
charge is investigated, spatio-temporally resolved electric
field measurements were compared with the spatially
resolved measurements. The overlap of the data initiated fur-
ther integrated measurements in order to reduce the duration
of time consuming measurements anyhow. The obtained
electric field strength values are close to the maximal values
of the electric field in the streamer head. Figure 2 shows the
representative line profile emitted from the capillary.
Spectrum consists of the forbidden and the allowed counter-
part. Allowed line has a Lorentzian half width of 0.084 nm.
This half width represents the influence of pressure broaden-
ing which consists of van der Waals and resonance broaden-
ing and corresponds to gas temperature of about 300 K at
atmospheric pressure.18 This means that the observed line is
not significantly broadened by the influence of the emission
from the tail of the streamer, which is the case for higher
electric fields in the plume, especially for the plasma jets
supplied with fast rising high-voltage pulses.40 The observed
helium line is weak and visible only in the regions with sig-
nificant electric field strength.19 Electric filed along the
whole capillary appears to be not so high, and consequently,
the electric field behind the streamer head is lower and in
this case insufficient to excite enough helium atoms to emit
He 492.19 nm line in that region.
III. RESULTS AND DISCUSSION
A. Visualization of the discharge in the tube
Differences in the operation properties of plasma jet for
different helium flows are visible by naked eye. Beside the
length of the plume, the changes in the discharge in the tube
with the increase in gas flow could be also observed, see
Figure 3. While the intensity of the emission from the capil-
lary for 700 SCCM almost homogeneously decreases along
the tube, the intensity maxima in the second part of the capil-
lary is observed for the flow of 2000 SCCM. One can con-
clude that the gas flow rate greatly influences the discharge
appearance and that flows above a certain value cause a local
maximum in the intensity of the emitted light between
10 and 15 mm away from the end of the charged electrode,
FIG. 1. Schematic overview of the plasma jet construction and spectroscopic
measurement system.13
FIG. 2. Example of the observed helium line profile recorded for the flow of
1000 SCCM.
123304-2 Sretenovic et al. J. Appl. Phys. 121, 123304 (2017)
which is clearly demonstrated in the right part of Figure 3.
Such an occurrence is further investigated by spectroscopic
measurements of the electric field distribution in the glass
tube for different gas flow rates.
B. Electric field strength distribution
The results of the electric filed measurements in the
dielectric tube for different gas flows are presented in Figure 4.
The result errors, estimated from the quality of the fit, are in
the range of 7%–15%. For all gas flows, electric field has a
maximum just after the grounded electrode and its value
decreases monotonically along the tube in its first third. For
lower gas flows, 700–1500 SCCM, electric field reaches a
plateau, which at the end of the profile has a slight drop to
the values of 5–7 kV/cm. Decrease in the electric field
strength in the streamer head along the jet tube is also per-
ceived in the electric field measurement experiments carried
out in the plasma gun by Pockels-effect based electro-optic
sensor (Kapteos company).29 Such a tendency could be
found in a recently published study of the plasma bullet
propagation in long dielectric tubes, where the influence
of the tube diameter on the discharge appearance was
investigated.31
For higher gas flow rates, electric field strength first
decreases, but instead of plateau, its value has another rise;
it goes through second maximum and then decreases as the
ionization wave approaches to the end of the capillary.
Such a behavior is in consistence with the appearance of the
discharge region with the bright core observed in Figure 3.
Observing this phenomenon by the naked eye, one could
see the constricted hairline plasma structure in this region,
which is much brighter than the expanded surrounding.
According to this, it is reasonable to conclude that this is a
region with higher space charge density. It is difficult to
determine the reason of such behavior, but it is clear that it
is the consequence of the gas flow rate and the mutual influ-
ences between gas flow and discharge itself. The gas flows
of 700, 1000, 1250, 1500, 1750, and 2000 SCCM corre-
spond to the Reynolds numbers of 54, 78, 97, 116, 136, and
155, respectively, and according to these values, all flows
are laminar. However, it was shown in our previous article
that discharge onset induces vortices in the gas flow for
higher gas flows.13 Satti and Agrawal investigated flow
structure of helium gas jets into air for Reynolds numbers
from 40 to 150.41 For low jet Reynolds numbers, the flow
was steady and back flow of air upstream of the tube exit
was recorded. At higher jet Reynolds numbers, 90 and 150,
buoyancy induced acceleration contracted the jet core to
form a toroidal vortex by entrainment of the ambient air.
This caused self-excited periodic oscillations at a unique
frequency at jet Reynolds numbers of 90 and 150.
Furthermore, vortices interacted with helium jet core could
also affect the laminarity of the flow inside the tube and be
a main cause of the formation of observed region with the
increased electric field.
Interestingly, the final electric field strengths at the exit
of the capillary are all in the same range of 5–7 kV/cm.
Accordingly, the electric field strength drops to about a
half, from 10–11 kV/cm in the vicinity of the grounded
electrode to 5–7 kV/cm at the tube exit. It was reported in
our previous articles that the electric field in the plume of
the jets starts to rise from the tube end along the jet
axis,13,19 see also Figure 7. Moreover, electric field in the
plume starts to rise from the same values (5–7 kV/cm) for
all gas flows, which demonstrates the consistence between
the results and the reproducibility of the used measurement
method.13 The probable reasons for the decrease in the elec-
tric field strength along the tube could be charging of the
inner dielectric walls inducing subsequent electric potential
shielding.26,30,42
FIG. 3. Photographs of the discharge for two different gas flows, 700 SCCM
and 2000 SCCM.
FIG. 4. Electric field strength distributions along the dielectric tube of
plasma jet for different gas flows. The grounded electrode is placed at z¼ 0,
while the end of the capillary is at z¼ 20 mm.
123304-3 Sretenovic et al. J. Appl. Phys. 121, 123304 (2017)
C. Estimations of the electron density in the streamerchannel
Directly obtained values of the electric field in the
streamer head enable the estimation of the electron density
in the streamer channel by equating the inverse of the
Maxwell relaxation time to the ionization frequency at the
maximum field in the streamer head ee0
lene ¼ vi; where e0 is
the vacuum permittivity, e is the elementary charge, ne is
the electron density, and �i is the ionization frequency.11 If
we recall the meaning of the Townsend ionization coeffi-
cient a ¼ �i
ue¼ �i
leE ; where ue is the electron drift velocity and
E is the electric field strength, we will obtain the formula
ne ¼ e0
e aE: Such an approach to the electron density estima-
tion in the streamer channel, i.e., plasma, is not new. It was
introduced by D’yakonov and Kachorovskii for streamers in
semiconductors,43 and later used by Kulikovsky for stream-
ers in air,44 by Boeuf et al. for streamers in helium plasma
jets,11 and lately by Babaeva and Naidis, also for streamers
in air.45 More details about this topic could also be found in
the book of Bazelyan and Raizer.46
Ionization coefficient a is also the function of the electric
field, and it can be obtained from Bolsigþ.47 Figure 5 shows
the dependence of the a coefficient on the electric field
strength for the range of the electric field of 4–18 kV/cm, i.e.,
the range of the measured values in the present experiment.
Townsend coefficient is calculated for five gas compositions:
pure He, He with the admixture of 100 ppm, 1000 ppm, 1%
and 2% of air. In the last four cases, the a coefficient repre-
sents the reduced Townsend coefficient. The admixture of
1000 ppm of air in He is estimated as the upper limit in the
dielectric tube where constant flow of helium with initial
40 ppm of impurities is present. Higher air fractions are
presented for the sake of comparison with the measure-
ments made in plume.13 It is clear from the graphs in
Figure 5 that the mentioned admixtures do not affect
strongly the values of the ionization coefficient; thus, one
could consider the discharge in the tube as the discharge in
pure helium.
For the calculation of the electron density, it is appropriate
to use the functional dependence of the Townsend ionization
coefficient a on the electric field strength. The usual general-
ized and simplified form deduced under several assumptions
by Townsend48,49 has an exponential form a ¼ ANe�BNE ; where
N is the number density of gas molecules in cm�3 and E is the
external electric field in kV/cm. Parameters A and B depend
on the gas type and electric field range, and they are derived
from the fit of data obtained from Bolsigþ.47 Sets of electron
impact cross sections were taken from the Biagi dataset.50
Values are calculated for the atmospheric pressure and the gas
temperature of 300 K. For the observed range of electric field,
obtained parameters are as follows: AN ¼ ð700610Þ cm�1
and BN ¼ ð26:060:3Þ kV/cm. Electron density calculated for
the observed range of the electric fields using the expression
ne ¼ e0
e ANe�BNE E is also presented in Figure 5. As it can be
concluded from the previous expression, electron density in
the streamer channel exponentially rises with the rise of the
electric field in the streamer head. For the lower electric field,
the value of 4 kV/cm electron density is about 4 � 109 cm�3,
while at 6 kV/cm, it has an order of magnitude higher val-
ue,and finally, for the maximal electric field in the capillary
of about 12 kV/cm, electron density reaches the value of 5
� 1011 cm�3. For the electric field strength in the streamer
head of 18 kV/cm, electron density in the streamer channel
exceeds 1012 cm�3.
Using the data for the electric field strength distribution
shown in Figure 4 and the previous analysis, it was possible
to estimate electron density distributions in the streamer
channel for different gas flows. The results are presented in
Figure 6.
Electron density has its maximum near the grounded
electrode for all flows, similar to the electric field distribu-
tion. Correspondingly, concentration of the electrons
decreases along the tube, but with the higher rate and the
drop could be described by the exponential function.
Electron density falls from 3–4 � 1011 cm�3, near the ring
electrode, to the values of 2–4 � 1010 cm�3 at the exit of the
capillary. The starting and the final values of the electron
densities are mostly the same, but for the higher gas flows,
FIG. 5. Dependence of the first Townsend coefficient for pure helium and
reduced first Townsend coefficient for helium with different admixtures of
air in the range of the electric fields measured in this work. Dependence of
the electron density in the streamer channel on the electric field in the
streamer head.
FIG. 6. Electron density distributions along the dielectric tube of plasma jet
for different gas flows.
123304-4 Sretenovic et al. J. Appl. Phys. 121, 123304 (2017)
due to the increase in the electric field, electron densities
attain the additional maxima. Electron densities estimated in
this work may look underestimated, but they are in line with
modeling results presented by Jansky et al. where the peak
density of 1012 cm�3 is obtained for slightly different experi-
mental and modeling conditions.28 The electron density in
helium plasma jet is measured by H€ubner et al. utilizing the
Thomson scattering technique.51 Measurements are per-
formed on a nanosecond pulsed high voltage dielectric bar-
rier discharge helium plasma jet flowing into the ambient air
surrounding. Results are presented for the effluent only. At
the distance of 2 mm from the nozzle, electron density at the
axis of the jet is below the detection limit, while towards the
rim it rises above detection limit reaching 3–4 � 1012 cm�3.
The detection limits are mostly determined by the laser stray
light and are in the order ne > 1012cm–3.52 The electron den-
sities estimated in this work are well below 1012 cm�3 just
before the exit of the tube. In order to make better compari-
son with the measurements of the electron density by
Thomson scattering and the estimation presented in this arti-
cle, we calculated electron densities in the tube and the efflu-
ent of the plasma jet operating with gas flow of 1000 SCCM.
Results are presented in Figure 7. Systematic measurements
of the electric field in the effluent of the plasma jet are pre-
sented in our previous article.13 It is useful to note that the
electric field strength starts growing as the ionization wave
exits the tube, and depending on the discharge conditions, it
reaches the maximal values of several millimeters or more
than a centimeter from the exit of the capillary. Furthermore,
electric field strength starts to decrease and the discharge
vanishes.19 The reasons for the increase in the electric field
is the increase in air admixture in the helium stream and a
consequent increase in the density of nitrogen ions, which
are main constituents of space charge layer in the streamer
head. Also, the contraction of the helium effluent in the flow
direction occurs,41 leading to the decrease in the streamer
radius and the increase in the space charge density. When air
entrainment exceeds certain limits, electric field strength
becomes insufficient for the direct ionization processes; that
is, the value of the Townsend ionization coefficient rapidly
decreases. The increase in the electric field strength in the
streamer head causes the increase in the electron density,
which is in consistence with the outcome of Thomson scat-
tering measurements.51 The values obtained with Thomson
scattering are higher, and H€ubner et al. obtained maximal
1.5� 1013 cm�3 approximately 25 mm from the tube exit. In
our case, a maximal density is below 2� 1012 cm�3 9 mm
from the end of the capillary. At the same position, Thomson
scattering gives below 5� 1012 cm�3.
From the presented analysis, it is reasonable to conclude
that the estimated electron density values seem right indicat-
ing the differences in the observed discharges. For example,
plasma jet investigated by Thomson scattering has a different
geometry, powered electrode is placed only 7 mm from the
tube exit, amplitude of the pulsed applied voltage was 7 kV
with the rise time of 150 ns and duration of 250 ns. Pulsed
voltage with short rise time results in the overvoltage break-
down, which could enhance ionization and excitation pro-
cesses and cause the increase in all discharge parameters
such as electron temperature, electric field, and electron
density.5
IV. CONCLUSIONS
This article is devoted to the measurements of the distri-
bution of the electric field in a streamer head in the capillary
of helium plasma jet. The values of the electric field are
obtained utilizing the Stark polarization spectroscopy
method. The experiments were performed for several gas
flows in the range of 700–2000 SCCM. Using the experi-
mentally obtained values of the electric field, electron den-
sity distributions are estimated on the basis of existing
streamer theory.
The following facts are revealed by this article:
(i) The electric field has its maximum in the vicinity of
the grounded ring electrode and decreases as the ioni-
zation wave moves toward the end of the dielectric
tube. It starts from 10–11 kV/cm in the vicinity of the
grounded electrode and ends with 5–7 kV/cm, which
is close to the lower limit for streamer propagation in
helium. The possible reason for the electric field
decrease is charging of the inner walls of the dielec-
tric tube, which leads to electric potential screening.
(ii) The gas flow rate influences the electric field distribu-
tion, but the values near the grounded ring and at the
exit of the tube remain similar for all gas flows.
Higher gas flow rates, 1500 and 2000 SCCM, cause
the appearance of the bright discharge region around
the center of the tube. Such phenomena cause local
maximum in the spatial distribution of the electric
field in the ionization wave front. Regardless of these
occurrences, electric field strength decreases again
after the mentioned peak and the electric field in the
streamer ends with similar values as in the case of
lower flow rate.
(iii) Electron density distribution in the streamer channel
can be estimated if the values of the electric field
strength in the streamer head are known. Existing
streamer theory connects the electron density in the
plasma with the electric field strength in the streamer
head and the electric field dependent Townsend
FIG. 7. Electric field strength and estimated electron density for the capillary
and plume of the plasma jet for the gas flow of 1000 SCCM.
123304-5 Sretenovic et al. J. Appl. Phys. 121, 123304 (2017)
ionization coefficient. In such a way, estimated elec-
tron concentration has a maximum of 4 � 1011 cm�3
near the grounded ring and decreases for an order of
magnitude as the ionization wave approaches to the
capillary nozzle. The dependence of the electron den-
sity on the gas flow rate has a similar trend as the
dependence of the electric field strength.
ACKNOWLEDGMENTS
A.S. would like to thank the European Cooperation in
Science and Technology Action COST TD1208 for the
financial support for a short-term scientific mission. G.S.,
I.K., V.K., B.O., and M.K. would like to thank the Ministry
of Education and Science of the Republic of Serbia for the
financial support through Project Nos. 171034 and 33022.
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