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)sretenovic@ff.bg.ac.rs

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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