Experimental approaches for studying non-equilibrium atmospheric plasma jetsA. Shashurin and M. Keidar Citation: Physics of Plasmas 22, 122002 (2015); doi: 10.1063/1.4933365 View online: http://dx.doi.org/10.1063/1.4933365 View Table of Contents: http://scitation.aip.org/content/aip/journal/pop/22/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Role of ambient dielectric in propagation of Ar atmospheric pressure nonequilibrium plasma jets Phys. Plasmas 22, 050703 (2015); 10.1063/1.4921216 Influence of Penning effect on the plasma features in a non-equilibrium atmospheric pressure plasma jet J. Appl. Phys. 115, 103301 (2014); 10.1063/1.4868223 Characteristics of atmospheric-pressure non-thermal N2 and N2/O2 gas mixture plasma jet J. Appl. Phys. 115, 033303 (2014); 10.1063/1.4862304 Non-equilibrium atmospheric pressure microplasma jet: An approach to endoscopic therapies Phys. Plasmas 20, 083507 (2013); 10.1063/1.4817958 Optical emission spectroscopic diagnostics of a non-thermal atmospheric pressure helium-oxygen plasma jet forbiomedical applications J. Appl. Phys. 113, 233302 (2013); 10.1063/1.4811339

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Experimental approaches for studying non-equilibrium atmosphericplasma jets

A. Shashurin1,a) and M. Keidar21School of Aeronautics & Astronautics, Purdue University, West Lafayette, Indiana 47907, USA2Department of Mechanical and Aerospace Engineering, The George Washington University, Washington,District of Columbia 20052, USA

(Received 28 April 2015; accepted 10 May 2015; published online 20 October 2015)

This work reviews recent research efforts undertaken in the area non-equilibrium atmospheric

plasma jets with special focus on experimental approaches. Physics of small non-equilibrium

atmospheric plasma jets operating in kHz frequency range at powers around few Watts will be ana-

lyzed, including mechanism of breakdown, process of ionization front propagation, electrical cou-

pling of the ionization front with the discharge electrodes, distributions of excited and ionized

species, discharge current spreading, transient dynamics of various plasma parameters, etc.

Experimental diagnostic approaches utilized in the field will be considered, including Rayleigh

microwave scattering, Thomson laser scattering, electrostatic streamer scatterers, optical emission

spectroscopy, fast photographing, etc. VC 2015 AIP Publishing LLC.

[http://dx.doi.org/10.1063/1.4933365]

I. INTRODUCTION

Serious attention towards studying miniature non-

equilibrium atmospheric plasmas for treatment of biological

systems has been initiated about two decades in connection

with its efficiency for biological sterilization.1 In contrast

with thermal plasmas, low power non-equilibrium atmos-

pheric plasmas operate under the threshold of thermal dam-

age to the tissue (eliminating tissue burn) and induce specific

chemical responses on the cellular level and can offer mini-

mum invasive surgery technique. Currently, non-equilibrium

atmospheric plasmas found wide application in the areas of

sterilization and disinfection.2–4 More exotic utilization

includes cancer treatment, skin dentistry, drug delivery, der-

matology, cosmetics, wound healing, cellular modifications,

etc.1–7

One particularly important configuration of the non-

equilibrium atmospheric plasmas is jet. Non-equilibrium

atmospheric plasmas jets (NEAPJ) are typically formed in

the helium flow ejecting from a discharge tube to an open air

and characterized by high variance of temperatures of vari-

ous species.3,4,8–11 The NEAPJ diameter is typically in the

millimeter range, while the length is about a few centimeters.

Typically, NEAPJ are driven by the AC high voltage sources

varying from several kV up to 20–40 kV and consume aver-

age power within few Watts. Most common driving frequen-

cies of the driving high voltage are the kHz range, while few

works utilize higher frequencies up to MHz and GHz

range.1,4,7,8,11,12 The lower end of that frequency range is

associated with the situation when discharge decay times are

short compared to high voltage cycle and the discharge

decays every oscillation of the high voltage and re-ignites

over again every next cycle. Indeed, the plasma decay times

can be estimated as sr � 1bne

, where sr the plasma

recombination time, b the characteristic recombination coef-

ficient, and ne the plasma density. For atmospheric pressure,

the characteristic recombination coefficient b � 10�6

�10�7 cm3/s and for typical plasma densities in NEAPJ,

ne� 1012 – 1013 cm�3, the recombination time is about sev-

eral ls.13,14 Thus, NEAPJ are associated with discharge

decay and following re-ignition every cycle of the driving

high voltage for discharge driving frequencies up to few hun-

dred kHz range.

This paper will review research efforts in the field of

non-equilibrium atmospheric plasma jets (also known as

non-thermal plasmas or cold atmospheric plasmas) with spe-

cial focus on experimental approaches. The review will

cover NEAPJ driven by AC high voltage sources operating

in the tens of kHz frequency range and utilizing helium (He)

as a working gas. We will consider and analyze physical

processes involved in the discharge, including breakdown

conditions, ionization front propagation dynamics, electrical

coupling of the streamer tip with discharge electrodes, dis-

charge current spreading, transient dynamics of plasma, and

discharge parameters in NEAPJ, factors determining NEAPJ

length, diameter, and propagation path, etc.

A. Types of NEAPJ

Typical NEAPJ source uses tubular discharge tube made

of dielectric material and having few millimeters in diameter

through which working gas (helium) is supplied. The dis-

charge tube is usually equipped with pair of high-voltage

electrodes. There are two most common configurations of

the discharge electrodes as following. In one configuration,

central electrode is mounted on the axis of discharge tube

and the second electrode is mounted outside the discharge

tube [type A in Fig. 1]. The central electrode can be insulated

along its entire length or can have non-insulted tip directly

contacting the plasma.11,33,34 Other configuration uses twoa)Email: ashashur@purdue.edu

1070-664X/2015/22(12)/122002/10/$30.00 VC 2015 AIP Publishing LLC22, 122002-1

PHYSICS OF PLASMAS 22, 122002 (2015)

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ring electrodes mounted outside the discharge tube [type B

in Fig. 1].15,29

There are two types of the high voltage waveforms uti-

lized for initiation and supporting the NEAPJ, namely, sine-

wave AC high voltage with typical frequencies of several

tens kHz and rectangular high voltage pulses with typical

pulse durations in ls and sub-ls range and repletion rate fre-

quency in kHz range. Amplitude of high voltage varies in

relatively wide range from 3 to 40 kV.3,4,6,9,11

Typically, the discharge has two distinct spatial regions,

namely, main discharge and plasma jet. The main discharge

is sustained between the discharge electrodes and located

entirely inside the discharge tube (see Fig. 1). NEAPJ are

formed by the part of the discharge extending outside the dis-

charge tube having shape of well-collimated plasma jet with

typical length of approximately several centimeters and few

millimeters in diameter.5,11,15,20,27,29,30

II. EXPERIMENTAL STUDIES OF NEAPJ

A. NEAPJ as a sequence of “bullet-like” streamerbreakdowns

Intensified charge-coupled device (ICCD) cameras are

probably the first and the most common tool for NEAPJ

diagnostics. First studies of NEAPJ conducted by fast ICCD

cameras revealed non-continuous nature of the jets.15,16,27

Bright “bullet-like” ionization front extending from the dis-

charge and moving along the helium flow was detected in

these works (see Fig. 2). Velocities of ionization front propa-

gation were found to be in the range from 106 to 108 cm/s

depending on discharge parameters.10,15,27,29 The ionization

front typically slows down along the propagation path as

shown in Fig. 2.

Development of rapidly moving ionization front (or

spark) at breakdown is associated with phenomenon of

streamer.17 Classical definition of streamer is ionized chan-

nel rapidly growing between the breakdown electrodes sup-

ported by photoionization of gas in vicinity of the channel

tip and following production of secondary avalanches.17,18

Streamer type of the breakdown is typical for relatively large

gaps. For example, for air gaps created by two plane electro-

des, Townsend breakdown mechanism of avalanches multipli-

cation is employed than parameter p�d< 200 Torr�cm, while

streamer breakdown for p�d> 5000 Torr�cm, where p the gas

pressure, d the distance between the electrodes.17 Streamer

breakdown is characterized by significantly faster development

times compared to Townsend avalanche multiplication, since

it develops on the time of single avalanche flight between the

discharge electrodes, rather than Townsend breakdown which

is associated with development of multiple avalanches.

B. Air streamer fundamentals

Let us now present a very brief qualitative description of

streamer fundamentals using the example of cathode-

directed air streamer between two plane electrodes. More

FIG. 1. Schematics of NEAPJ sources.

Type A: with central electrode at the

discharge tube axis and outside ring

electrode; type B: with two ring elec-

trodes outside the discharge tube.

FIG. 2. Fast photography of the NEAPJ ionization front development and its velocity for different amplitudes of the applied HV pulse. Reproduced with per-

mission from N. Mericam-Bourdet et al., J. Phys. D: Appl. Phys. 42, 055207 (2009). Copyright 2009 IOP Publishing.

122002-2 A. Shashurin and M. Keidar Phys. Plasmas 22, 122002 (2015)

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details can be found elsewhere.17,18 If the electric field

between the discharge electrodes (E0) is strong enough, the

electrons may start producing avalanches by ionizing the sur-

rounding gas atoms. The avalanches are amplified when the

ionization coefficient (a) exceeds the attachment coefficient

(a). The avalanches start from the seed electrons, grow, and

propagate towards the positive anode in accordance to the

law ne ¼ eða�aÞx, where x is the distance from the avalanche

stat point (see Fig. 3(a)). Due to much higher mobility of

electrons compared to ions, electrons are located in the lead-

ing part of growing avalanche, while uncompensated posi-

tive charge is located at the avalanche’s tail. This is

schematically illustrated in Fig. 3(b). Avalanches reach max-

imal amplification just prior the anode and then electrons are

absorbed by the anode surface, while positive ion tail is

remaining in the gap and drifting back towards the cathode

(see Fig. 3(c)). The ion tail produces its own electric field

(Eh). If avalanche prior to contact with the anode was strong

enough, so that Eh�E0, the ion tail can produce field suffi-

cient to trigger its own secondary electron avalanches (see

Fig. 3(d)). Reaching Eh�E0 when secondary avalanches are

initiated in vicinity of the ion tail indicates transformation of

avalanche into a streamer. Ion tail serves now as a head of

the streamer channel, propagating back towards the cathode

as follows. The streamer head field photoionizes electrons

around it and starts secondary avalanches which are pulled

into the positive streamer head. Electrons from the fronts of

the secondary avalanches are drawn into the streamer head

and compensate it, while ion tails of the secondary ava-

lanches form new streamer head slightly shifted towards the

cathode (see Fig. 3(e)). This process continues multiple times

and leads to formation of elongated streamer channel connected

with anode and growing towards the cathode as illustrated in

Fig. 3(f). Streamer breakdown develops in air when electric

field reaches about 30 kV/cm for air gaps >3 cm. This corre-

sponds to a� a¼ 6.5 cm�1 and number of electrons in ava-

lanche of about 108–109.17 The electric field in the air streamer

channel varies in relatively narrow range 4.5–5 kV/cm.18

Typically, streamer channel has relatively high electrical

conductivity providing good electrical contact between the

streamer tip and the electrode on which it is growing.17 In

approximation of ideally conducting streamer channel, the

electric field in the streamer tip vicinity can be determined

from the streamer tip potential (Uh) and potential of this

point of space (U0) in absence of the streamer (potential cre-

ated by the sources other than the streamer) as follows:18,19

E � Uh � U0

2rs; (1)

where rs is streamer channel radius.

It has to be noted that low-frequency NEAPJ operating

in He/Air mixture considered in this work differ to large

extent from the classical case of air streamer between two

plane electrodes. The similarities and differences of these

two objects will be discussed below.

C. Electrical coupling between NEAPJ streamer headand discharge electrodes. Electrostatic streamerscatterers and plasma potential measurements

Presence of strong electrical coupling between the

NEAPJ streamer tip and the discharge electrodes was dem-

onstrated by Shashurin et al.20 The electric potential of the

NEAPJ streamer tip was measured using external electrical

potential created on the streamer’s propagation path. The

idea of the method consists of stopping the streamer propa-

gation by means of externally created electric potential pro-

duced by the metal ring surrounding the NEAPJ as

illustrated in Fig. 4. An application of positive DC potential

to the ring electrode (Ur) of about 2.8 kV led to abrupt arrest-

ing the further streamer propagation beyond the plane of the

ring as shown in Fig. 4. This effect is caused by the reduction

of the local electric field around the streamer tip (Eh). The

notable reduction of the Eh is possible when the space poten-

tial at the streamer tip location (�Ur) reaches values close to

the streamer tip potential (Uh). In the zero order approxima-

tion, the simplified criteria that the streamer potential is

equal to the ring’s potential required to stop the streamer

propagation exactly at the ring’s plane can be used. More

details on the method can be found in Ref. 20.

Temporal evolution of the NEAPJ streamer tip potential

is shown in Fig. 5 for three amplitudes of driving high vol-

tages 2.6, 3.1, and 3.8 kV (type A configuration with bare

central electrode was used).20 One can see that the central

electrode potential in all cases was transferred along the

main discharge column and the streamer channel to the

streamer tip without significant voltage drops (less than

10–15%). The value of the electric field in the streamer chan-

nel can be estimated to be about 100 V/cm and electric field

in the tip vicinity Eh¼ 80–100 kV/cm.20

Experiments conducted with type B discharge configura-

tion with insulated discharge electrodes also support the idea

of presence of strong electrical coupling between the

streamer tip and the discharge electrodes.15,58 Indeed, these

works demonstrated an instant interruption and inability of

any further propagation of the streamer momentarily when

HV pulse applied to the electrode was turned off. It has to be

noted that these observations refute applicability of the

model of completely isolated streamer tip proposed in early

works for interpretation of NEAPJ physics.15

FIG. 3. Illustration of cathode-directed

streamer development between two

plane electrodes.

122002-3 A. Shashurin and M. Keidar Phys. Plasmas 22, 122002 (2015)

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D. NEAPJ streamer initiation requirements andpropagation speed

Streamer initiation is associated with reaching the

threshold value by the electric field sufficient for its repro-

duction at the next position along the propagation path.17

Due to the good electric coupling between the streamer tip

and discharge electrode, the electric field can be simply esti-

mated as Eh � Ud

2rsaccording to Eq. (1) (in absence of addi-

tional potentials created by other sources). Therefore,

initiation of streamer breakdown is governed by the magni-

tude of the discharge voltage.

In case low-frequency (�tens of kHz) AC high voltage

is utilized for the NEAPJ production, the streamer develop-

ment (typical times <2–3 ls) can be considered to be instant

on the time scale of the discharge-driving voltage oscilla-

tions. Therefore, streamer breakdown is governed by the

instant value of the discharge voltage and occurs exactly at

the moment when minimal voltage required for the break-

down is reached. The streamer can be initiated at different

phases of the time-dependent discharge-driving voltage

depending on when the threshold electric field is reached

(see Fig. 5). Experiments indicate that for type A discharge

configuration with the bare discharge electrode, this thresh-

old voltage value required was in the range Umin¼ 2.5–3 kV.

The corresponding minimal electric field can be estimated

from Eh � Umin

2rsyielding electric field around the tip

Eh¼ 80–100 kV/cm (using streamer channel radius

rs¼ 0.15 mm, see below).20

Similar minimal breakdown voltages are also common

for NEAPJ produced by the rectangle HV pulse, namely,

�4 kV.15,27 Numerical simulations conducted by Naidis pre-

dicted development of streamer down to 3 kV amplitude of

exciting HV pulse.56 Utilization of HV pulses with ampli-

tudes higher than the minimal voltage required for the break-

down is associated with so-called overvoltage conditions.

Overvoltage (above the minimal voltage required for the

streamer initiation) leads to development and propagation of

the streamer under higher instant electrode voltage and,

therefore, under higher electric fields in the streamer tip

vicinity.

The propagation speed of the streamer ionization front

is heavily dependent on the instant value of electric potential

on the discharge electrode at the moment of streamer devel-

opment. Indeed, experiments indicate that propagation

velocity increased from ts¼ 2� 106 cm/s for about 2.5–3 kV

AC voltage (Refs. 11 and 20) to (1–2)� 107 cm/s for

4–5.5 kV HV pulses (Refs. 15 and 27) and �(4–8)� 107 cm/s

for 10–13 kV HV pulses (Ref. 29). Minimal measured ts of

2� 106 cm/s is associated with the case of low-frequency

(�tens of kHz) AC high voltage sources utilization for the

NEAPJ creation, when streamer initiation occurs exactly at

the moment when minimal voltage required for the break-

down is reached (no overvoltage).11,20 In contrast, utilization

of HV pulses for the NEAPJ excitation readily creates over-

voltage conditions, if HV amplitudes higher than minimal

2.5–3 kV required for the breakdown are used. Larger over-

voltage causes higher electric field near the streamer tip Eh.

This in turn accelerates development of elementary ava-

lanches developing in front of the streamer tip, electron drift

towards the tip, and finally accelerates the ionization front

propagation. Numerical simulations providing good agree-

ment with the experimental data were conducted by Naidis

and shown in Fig. 6.56

E. NEAPJ streamer length, shape, diameterand propagation path

One of the major factors governing streamer length is

level of oxygen admixture in the helium flow. Attachment of

electrons to oxygen molecules quenches development of

avalanches and further propagation of the streamer.13

Obviously, helium flow ejected to the air from the discharge

tube has increased air content along the propagation path due

to interdiffusion of both gases. This leads to streamer stop-

ping at some particular distance from the discharge tube

when air content exceeds critical value and limits maximal

NEAPJ length. Another factor limiting the streamer length is

operating frequency of the device. Since typical streamer de-

velopment times are about few ls, driving frequencies of dis-

charge cannot be increased above the MHz frequencies since

otherwise streamer will be unable to fully grow during the

voltage cycle. Third factor limiting the streamer length is

plasma decay in the streamer channel behind the ionization

front which causes weakening of electrical contact between

streamer head and electrode, and thus reduces the electric

field around the streamer tip necessary for streamer

propagation.

The dominant factor that governs the NEAPJ streamer

length should be determined specifically in each experiment.

FIG. 4. Interaction of the NEAPJ

streamer with the DC potential created

by the ring located at z¼ 3 cm and de-

pendence of streamer length L versus

ring potential Ur (amplitude of the dis-

charge driving voltage amplitude

¼3.1 kV). The plasma jet is ejected

from discharge tube located on top

towards down. Reproduced with per-

mission from A. Shashurin et al.,Plasme Sources Sci. Technol. 21,

04006 (2012). Copyright 2012 IOP

Publishing.

122002-4 A. Shashurin and M. Keidar Phys. Plasmas 22, 122002 (2015)

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For type A NEAPJ with bare electrode in direct contact with

plasma and 4 mm exit diameter of the discharge tube,

streamer growth had interrupted at z¼ 4 cm from the dis-

charge tube when head potential was about 4 kV (even

higher than at streamer initiation) and channel plasma den-

sity is high.20 This suggests that most likely, the dominant

limiting mechanism is quenching of avalanches due to ele-

vated air contain in the helium flow. Other recent work

shows that absence of oxygen content in the He flow can

remove the above limitation and significantly elongate the

streamer length. The experiments conducted at 75 Torr nitro-

gen pressure yielded extremely long 28 cm length NEAPJ.21

Oppositely, admixture of the oxygen directly in He flow at

levels of about fractions of percent is sufficient to signifi-

cantly reduce NEAPJ length.22,23

The NEAPJ propagation path coincides with the direc-

tion of the He flow. This is caused by higher presence of the

ambient air and elevated attachment oxygen in direction per-

pendicular to the helium flow. This causes weaker ava-

lanches developing perpendicular to the jet compared to that

growing along the axis and sets preferential direction of the

streamer propagation along the He flow.24 Direction of the

streamer propagation can be controlled by creating more

complicated paths of the He flow as shown in Ref. 10.

Additional way to perturb the streamer propagation path

is to create external electrical potential. It was already shown

above that streamer can be stopped by using the ring elec-

trode with applied electric potential (see Fig. 4 and Ref. 20).

Another interesting effect is attraction of streamer to positive

electrode,24,27 which may look contradictory from the first

glance since streamer tip carries positive space charge as

well. The interpretation of this result is related to the fact

that streamer propagation is associated with motion of elec-

trons rather than heavy particles. Indeed, the electrons of the

avalanches developing in front of the streamer tip are

attracted simultaneously by the positive streamer tip and by

the positive capacitor’s plate, and the resultant influence of

both objects may cause streamer deflection towards the posi-

tive capacitor plate.24

Let us now consider cross-sectional shape of NEAPJ.

Spectroscopic studies of NEAPJ indicate that radiation is

dominated by helium, nitrogen, oxygen, and hydroxyl radi-

cals’ spectral lines.31–33 Cross-sectional distribution of spec-

tral intensities of different species follows density

distributions of the corresponding species. Indeed, helium

radiation has maximum on the axis due to maximum of the

He density along the axis, while radiation produced by the

air-contained species is shifted from the axis since their den-

sity is minimal on the axis and increases in radial direction.25

For example, peak of nitrogen radiation is shifted off the

axis about 0.7 mm at 20 mm away from the discharge tube

exit (3 mm exit diameter of the discharge tube was used).25

Due to diffusion of air nitrogen into the helium, the off-

centered shift decreased to about 0.3 mm at 40 mm away

from the discharge tube.25 Relative magnitudes of powers

radiated by helium to that by air-contained species may vary.

Some cross-sectional photographs indicate donut cross-

sectional shape of the NEAPJ, indicating dominant radiation

from the air species,25–27 while others indicate presence of

bright core in the center due to radiation from helium.11,28

Fig. 7 presents high-magnification images with bright core

on the axis taken for NEAPJ shown in Figs. 4 and 8(a) and

utilized in Refs. 6, 11, 20, and 24. The diameter of the bright

streamer channel was found to be about 0.3 mm for high

voltage amplitudes in the range 2.5–4 kV.

Numerical simulations of radial electron density distri-

butions also predict two types of distributions, namely, with

maximum on the axis and annular with off-centered maxi-

mum. Transformation from annual to axial shape is predicted

when molar fraction of the air in helium flow reaches about

10�2.55,56

F. Dynamics and temperature of heavy particles

It is important to understand that breakdown of the inter-

electrode gap and following development of NEAPJ

streamer are happening in virtually immovable gas. Indeed,

typical gas speeds are about 10 m/s.27,56 So, for the entire

FIG. 5. Temporal evolution of streamer tip potential and discharge electrode

potential for three amplitudes of driving high voltages 2.6, 3.1, and 3.8 kV.

One can see that streamer tip potential is close to the potential of the central

electrode and following its temporal behavior. Reproduced with permission

from A. Shashurin et al., Plasme Sources Sci. Technol. 21, 04006 (2012).

Copyright 2012 IOP Publishing.

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discharge duration (about 5 ls), gas column moves about

50 lm. Size of the discharge gap and NEAPJ length is signif-

icantly larger, typically several centimeters. Strictly speak-

ing, utilization of word “jet” in term NEAPJ is not accurate,

since “jet” reflects motion of plasma media as a whole,

which is not the case for the NEAPJ.

One of the most common tools for characterization of

NEAPJ is Optical Emission Spectroscopy (OES).15,27,29–31

OES was used in number of works to detect ionized and

excited species in NEAPJ, including oxygen, nitrogen, he-

lium, hydroxyl radicals, etc.31–34 In addition, OES can be used

to determine gas temperature in NEAPJ. Lu and Laroussi

measured rotational temperature of N2 based on fitting of

simulated emission spectra of nitrogen second positive system

of N2 to one measured experimentally for NEAPJ.32,33 The

rotational temperature was assumed to be equal to the gas

temperature due to fast rotational-translational relaxation

(<10�7 s).35–38 Finally, OES method demonstrated that gas

temperature of NEAPJ is close to the room temperature and

varies in the range of 300–350 K.22,32–34 It has to be noted that

significantly higher vibrational temperatures in NEAPJ,

namely (about 2000–3000 K) were determined using OES due

to relatively slow vibrational-translational relaxation

(�10–100 ls).33,34,37 Dominant portion of gas is in form of

neutrals (ionization degree �10�6–10�7).11

NEAPJ streamer produces extremely long-living meta-

stable He states.17 One of the mechanisms of relaxation of

these states is Penning ionization, when the energy of meta-

stable level (e.g., 19.8 eV for He 23 S, lifetime 6� 105 s) is

sufficient to ionize ambient gas which ionization potential is

lower (e.g., 14.5 eV for nitrogen). Penning ionization of the

ambient gas can serve as an effective source of seed elec-

trons required for initiation of avalanches in front of the

streamer and this can potentially cause reduction of the num-

ber of electron generations created in front of the streamer

tip compared to the air streamer.17

Recent numerical simulations pay great attention to clar-

ification of how various species excited in the NEAPJ inter-

act with biological tissue.39–47 These works simulate rich

plasma chemistry emerging in gas phase prior to delivery to

the tissue, including tens of species and hundreds/thousands

of various reaction and consider further effects they induce

in the tissue, including of electrical charging, action of UV

radiation, spreading, and penetration of the reactive species

into the tissue.

G. Plasma density measurements

Application of conventional plasma density diagnostics

for low frequency NEAPJ is problematic. Insertion of any

type of probes into the plasma jet (typically few millimeters

in diameter) is associated with strong perturbation of the

plasma due to change of capacitive coupling of plasmas to

ground leading to shorting the jet to the probe. Conventional

microwave interferometry fails due to the small size of the

plasma compared to the microwave wavelength, which leads

to diffraction and negligible phase change; electrostatic

probes introduce very strong perturbation and associated

with difficulties at application in strongly-collisional atmos-

pheric conditions.48

Recently, plasma density and electron temperature in for

RF microdischarges and DC microdischarges were measured

using Thomson scattering on free plasma electrons (laser fre-

quencies � xp, �m).8,49,50 These measurements are charac-

terized by outstanding spatial resolution down to 10–50 lm.

However, the method has limited sensitivity when plasma

ionization degree is low, namely, minimal detectable plasma

ionization degree of about 10�6 was indicated (�1013 cm�3

for atmospheric pressure discharges), due necessity to extract

signal related to Thomson scattering on free plasma electrons

from large amplitude background signal produced by

Rayleigh scattering on molecules.8,49–51 In addition, laser

FIG. 6. Dependence of the ionization front propagation speed along the

propagation path for different discharge voltages. Reproduced with permis-

sion from G. V. Naidis, J. Appl. Phys. 112, 103304 (2012). Copyright 2012

IOP Publishing.

FIG. 7. High magnification images of

the NEAPJ streamer. (a) Longitudinal

distribution of the radiation intensity

along the jet flow. (b) Cross-sectional

distributions of the radiation intensity.

The photographs are taken with low-

frequency He/air NEAPJ with expo-

sure time of 100 ns.

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Thomson scattering method was only used with discharges

associated with time-independent plasma density since long

accumulation of signal (tens of milliseconds) is required in

order to achieve detectable signal level.8,49

1. Rayleigh microwave scattering

Plasma density in the low-frequency NEAPJ has been

measured using elastic scattering of microwaves on plas-

mas.11,52 Microwave radiation (linearly polarized along the

NEAPJ) is utilized to induce electron oscillations in plasma

channel, and in the Rayleigh regime (when the channel is

thin compared to the skin layer depth), the electric field am-

plitude of the scattered wave (Es) is proportional to the total

number of electrons in plasma volume Es / neV, where Vthe plasma volume, ne the plasma density. Absolute value of

plasma density can be determined from the measurement of

the scattered radiation signal and plasma volume using cali-

bration procedure and satisfying certain requirements for

detection system as described in detail in Refs. 11 and 53.

It has to be noted that Rayleigh Microwave Scattering

(RMS) technique uses significantly lower microwave fre-

quencies compared to that in laser Thomson scattering.

Electron motion in this case is not free, but instead heavily

affected by collisions with gas atoms and restoration force

from ions on electrons.52,53 In this case, Rayleigh scattering

on the plasma electrons becomes the dominant component in

the scattered signal (�Rayleigh scattering on molecules)

due to significantly stronger polarizability of plasmas

compared to that of molecules. This allows to significantly

improve method sensitivity with respect to low plasma den-

sities and obtain measurement in the single exposure in con-

trast with long accumulation of signal required for laser

Thomson scattering which might be problematic for time-

dependent ne in NEAPJ.

Fig. 8 presents temporal evolution of the electrical and

plasma parameters along with series of instantaneous ICCD

camera images of the GWU low frequency NEAPJ device.54

It was observed that the discharge consists of series of ele-

mentary breakdown events inside the discharge tube fol-

lowed by the development of streamer propagating outside

the tube in an open air shown in Fig. 8(b). Breakdown occurs

once per period of the AC high voltage during a positive half

wave at the central electrode. The breakdown of the intere-

lectrode gap is indicated by the peak of the discharge current

(Id) observed around t¼ 0 in Fig. 8(c). Id increases to about

6–8 mA at about 1 ls after the breakdown and then decays

with the characteristic times of about 3–5 ls. This stage of

the discharge is characterized by the presence of the dis-

charge in the interelectrode gap only (main discharge), while

no ionization wave outside the discharge tube is presented

[see t< 3 ls in Fig. 8(b)]. The main discharge stage typically

lasts about few ls after the breakdown and indicated by the

green bar in Fig. 8(c).

The next stage of the discharge starts at about 3 ls after

the breakdown when ionization front (streamer) comes out

the discharge tube [see Fig. 8(b)]. The streamer propagates

FIG. 8. (a) Photograph of the discharge

assembly and non-equilibrium atmos-

pheric plasma jet (NEAPJ) produced

by type A source with bare electrode

(working frequency—25 kHz, high-

voltage amplitude—3.6 kV, exit diam-

eter of the discharge tube—4 mm, and

He flow rate—11 l/min). (b) Series of

fast photographs (100 ns exposure

time, except for the right bottom image

taken with 5 ls exposure time) of the

NEAPJ indicating streamer propaga-

tion. (c) Temporal evolution of dis-

charge current, discharge voltage,

streamer tip voltage, and average den-

sity in the streamer channel. Green,

red, and brown bars indicate different

stage of the discharge, namely, break-

down of the interelectrode gap,

streamer developing stage, and post-

ionization wave decay of the streamer

channel.

122002-7 A. Shashurin and M. Keidar Phys. Plasmas 22, 122002 (2015)

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24.43.39.170 On: Thu, 12 Nov 2015 16:45:18

axially about 4 cm with speed of about 2� 106 cm/s along

the He flow ejected to an ambient air until it decays at

t� 5 ls as shown in series of instant photographs shown in

Fig. 8(b). This stage of the discharge is marked by the red

bar in Fig. 8(c). Measurements of plasma density (ne) in the

streamer channel conducted using RMS facility yield aver-

aged value of ne along the streamer channel of about

1013 cm�3.54 Numerical simulations provide same range of

ne values.14,55,56 Note the plasma ionization degree in the jet

is very low � 5� 10�7 (gas density at 1 atm and 300 K is

around 2� 1019 cm�3). Electrical potential of the streamer

tip measured during the streamer development is indicated

by red dots in Fig. 8(c). It is seen that the streamer tip carries

potential close to the potential of the central electrode as

described above. The ionization front propagation quenches

at t� 5 ls and remaining streamer channel decays for the

next 2–3 ls. The plasma decay is governed by dissociative

recombination of He2 ions with electrons (b � 10�7 cm3/

s),14,17 yielding the decay time �1 ls for measured plasma

density of 1013 cm�3. The afterglow stage is indicated by the

brown bar in Fig. 8(c).

The electrical current spreads in the discharge as fol-

lows. At the main discharge stage [green bar in Fig. 8(c)],

the discharge current (�5–10 mA) is circulated via the cir-

cuit: central electrode - main discharge - grounded ring elec-

trode. When the streamer is developing, a portion of the

discharge current (less than 1 mA according to Rogowski

coil measurements conducted by Shashurin et al.)57 flows

out of the discharge tube along the streamer and then closes

to the ring electrode by displacement current. The electrical

circuit in the latter case is as follows: central electrode -

main discharge - NEAPJ - ring electrode.

2. Other methods

Dielectric probe inserted in NEAPJ and OES can be

used to obtain an estimation of the plasma density.58,59

These methods can be used for rough estimations only due to

the assumptions made. For dielectric probe approach, jet cur-

rent is assumed to be equal to that collected by the probe

which is not accurate, since probe significantly perturbs the

current spreading. For OES method, certain air admixture in

helium gas has to be assumed along the jet propagation. The

plasma density in NEAPJ in these works was estimated to be

1011–1012 cm�3.58,59

H. Differences between NEAPJ streamer and airstreamer

One important difference between the NEAPJ and air

streamer is the seed media on which both streamers are

growing. Classical air streamer starts from the avalanche

developing at breakdown of the air gap. In contrast, NEAPJ

streamer appears after breakdown and establishment of dis-

charge in the interelectrode gap and growths directly on the

plasma column of the main discharge. In addition, air

streamer typically breakdowns the entire gap between the

electrodes. NEAPJ streamer develops through the hole of the

grounded electrode away from it and usually breakdowns

only part of the gap since it may exist only in vicinity of the

He flow ejecting from the discharge tube.

Another difference is that NEAPJ streamer in He-air

mixture has pre-determined straight path along the He flow

in contrast with the random-walk of the air streamer, due to

elevated attachment to oxygen perpendicular to the jet as

indicated above. In addition, the electric fields in the air

streamer channel differ significantly from that in NEAPJ he-

lium streamer, namely, �5 kV/cm in air compared to

�100 V/cm in NEAPJ streamer channel, respectively.18,20

This is consistent with the experiments conducted at differ-

ent gaseous atmospheres summarized in Ref. 18, where the

increase in the oxygen (or water vapor) contain was demon-

strated to lead to the growth of the electric field in the

streamer channel.

Typical parameters of the low-frequency NEAPJ uti-

lized at GWU excited by 25 kHz AC high voltage in He flow

ejected for the discharge tube to an open air are presented in

Table I.11,20,54

III. CONCLUSION

This paper reviews state-of-the art of the research con-

ducted in the field of non-equilibrium atmospheric plasma

jets operating in the kHz frequency range and using helium

as working gas. These NEAPJ are associated with highly

non-steady dynamics of the plasma parameters. Transient

dynamics of plasma and discharge parameters was consid-

ered and physical processes involved in the discharge were

analyzed. Good electric conductivity of the streamer channel

and transfer of the electrodes potential to the streamer tip

without significant potential drop is demonstrated. Presence

of the air content in the helium flow governs streamer propa-

gation along the straight path, its cross-sectional structure,

and maximal streamer length. Excitation of the streamer

with low frequency AC high voltage is associated with the

streamer initiation at the moment when minimal breakdown

conditions are reached. Plasma parameters of such streamer

include average plasma density in the streamer channel of

about �1013 cm�3, ionization front propagation speed of

TABLE I. Typical discharge and plasma parameters of low-frequency

NEAPJ produced by type A source with bare electrode with working fre-

quency 25 kHz, high-voltage amplitude 3.6 kV, exit diameter of the dis-

charge tube 4 mm, and He flow rate 11 l/min.11,20,54

Discharge voltage 2.5–5 kV

He flow 11–12 l/min

Discharge current <10 mA

Plasma jet current <1 mA

Main discharge stage duration �3 ls

Streamer stage duration �2 ls

Afterglow stage duration �2–3 ls

Streamer triggering voltage 2.5–3 kV

Electric field in the streamer tip vicinity 80–100 kV/cm

Field in the streamer channel 100 V/cm

Streamer length 4–5 cm

Streamer diameter 0.3 mm

Speed of ionization front 2� 106 cm/s

Average plasma density in the streamer channel �1013 cm�3

122002-8 A. Shashurin and M. Keidar Phys. Plasmas 22, 122002 (2015)

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24.43.39.170 On: Thu, 12 Nov 2015 16:45:18

about 2� 106 cm/s, and post streamer plasma decay gov-

erned by dissociative recombination He2 ions with electrons

of about 2–3 ls.

ACKNOWLEDGMENTS

We would like to thank Dr. Mikhail Shneider for very

fruitful discussions.

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