JOURNAL OF APPLIED PHYSICS 102, 123302 !2007"
Measurement of gas temperature and convection velocity profiles
in a dc atmospheric glow discharge
Vadim P. Stepaniuk
Lenterra Inc., 7 Tenney Road, West Orange, New Jersey 07052, USA
Tindaro Ioppolo
Polytechnic University, 6 Metrotech Center, Brooklyn, New York 11201, USA
M. Volkan Ötügen
Mechanical Engineering Department, Southern Methodist University, P. O. Box 750337,
Dallas, Texas 75275, USA
Valery A. Shevereva!
Polytechnic University, 6 Metrotech Center, Brooklyn, New York 11201, USA
!Received 8 June 2007; accepted 20 October 2007; published online 19 December 2007"
Gas temperature and convective velocity distributions are presented for an unconfined glow
discharge in air at atmospheric pressure, with electric currents ranging between 30 and 92 mA. The
vertically oriented discharge was formed between a pin anode !top" and an extended cathode. The
temperature and velocity profiles were measured using laser-induced Rayleigh scattering and laser
Doppler anemometry techniques, respectively. The temperature field exhibited a conical shape with
the radius of hot temperature zone increasing toward the anode. A maximum temperature of 2470
K was observed on the discharge axis with the discharge current of 92 mA. Air velocity
measurements around the discharge demonstrated that the shape and magnitude of the temperature
field are strongly affected by natural convection. Estimates indicate that convective losses may
account for more than 50% of the power input into the positive column of the discharge. The
measured temperature fields and convective velocity profiles provide a set of data that is important
for the evaluation of dc atmospheric glow discharges in various applications such as sound
manipulation and acoustic noise mitigation. © 2007 American Institute of Physics.
#DOI: 10.1063/1.2822338$
I. INTRODUCTION
Glow discharges at atmospheric pressure have become
increasingly important for a variety of industrial and technical applications, including plasma processing, gas decontamination, and chemical detection.1,2 Atmospheric glow discharges received attention within the aerospace community
as well for their potential in several aerodynamic applications. Earlier experiments3,4 demonstrated that shock waves
undergo structural changes while passing through glow discharge plasma, raising the possibility of shock manipulation
by plasma. These findings led to new studies exploring several additional applications. For example, a number of researchers reported successful application of various types of
atmospheric glow discharges for near-surface flow control,5–7
drag reduction,8,9 and aeroacoustics. In this last application,
plasma formed in atmospheric air serves as a sound barrier.
A recent experiment10 demonstrated pure tone sound attenuation in excess of 20 dB by a row of unconfined pin-to-plate
discharges at one atmosphere. The dominant mechanism of
sound attenuation in this experiment was determined to be
reflection and scattering of the sound wave through sharp
gradients of gas temperature at the boundaries between the
excited !plasma" and undisturbed air. One- and twodimensional computational models of sound propagation
a"
Electronic mail: [email protected].
0021-8979/2007/102"12!/123302/5/$23.00
through a high-temperature barrier11–13 have also shown that
sound reflection occurs at the cold-hot gas interface, although with significantly smaller levels of attenuation than
those observed in the experiments. In order to accurately
assess the influence of gas temperature gradients on sound
attenuation by glow discharge, computational models need to
be three-dimensional. Also, the actual spatial distributions of
the gas temperature in and around the discharge have to be
incorporated in these computational models for appropriate
interpretation of the experimental data.
Measurements of gas temperature profiles are known for
low-pressure discharge tubes.14 While these results indicate
the existence of temperature gradients at the plasma boundary, they cannot provide quantitative information on dc unconfined atmospheric plasma. In the present paper, measurements of temperature distribution in and around a pin-toplate stationary dc discharge in atmospheric air are reported.
II. TEMPERATURE MEASUREMENTS
A. Experimental setup and procedures
While a number of well-established techniques have
been used in the past for the measurement of the neutral
component temperature in plasma, the laser-induced Rayleigh scattering technique is the method of choice for nonequilibrium plasma.14 It provides spatially resolved information on the density of the neutral gas component that can be
102, 123302-1
© 2007 American Institute of Physics
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123302-2
J. Appl. Phys. 102, 123302 "2007!
Stepaniuk et al.
FIG. 1. Schematic of the Rayleigh scattering setup for temperature
measurements.
related to temperature using the state equation if the gas pressure is known. In the glow discharge, the ionization level is
typically !10−5, i.e., the number density of the electrons or
ions is much smaller than that of neutral particles, therefore
the Rayleigh scattering cross section in the plasma is basically equal to that of the unexcited gas. Hence, the technique
essentially measures the neutral gas density.
Rayleigh scattering is the elastic interaction between
photons and particles that are much smaller than the incident
light wavelength. In the present case, the particles are air
molecules. The scattered light is proportional to the Rayleigh
scattering cross section, the incident laser light energy, and
the gas number density as follows:15
E = E LnL"#$ =
E LL " # $ p
.
kT
!1"
Here, EL is the incident laser energy, n is the molecular number density of the gas, L is the length over which the scattered light is collected, # is the solid angle of the collecting
lens, " is the Rayleigh scattering differential cross section, $
is a factor that takes into account the efficiency of the collecting optics, p is the gas pressure, and k is the Boltzmann
constant. If the pressure is constant, the intensity of scattered
light is simply inversely proportional to the local gas temperature, T, and the proportionality constant !or calibration
constant" can be determined by measuring the scattered energy E0 at a known temperature T0. Hence, Eq. !1" becomes
E = E0
T0
.
T
!2"
The experimental layout is given in Fig. 1. The second
harmonic !532 nm wavelength" of a Nd:YAG pulsed laser
!Continuum, model PL9012" was used as the interrogating
light source. The laser beam was focused at the measurement
location using a 0.5 m focal length lens. The beam waist of
the optical probe was %100 %m. To avoid photoionization
of air in the focal area, the laser light intensity was reduced
to a sufficiently low level. The light that passed through the
discharge was captured in a trap. The scattered light from the
FIG. 2. Photograph of discharge !electrode gap is 15 mm, current is 50 mA,
voltage is 1.2 kV". Line ABCD indicates the domain of velocity
measurements.
probe location was collected at right angles to both the beam
propagation and light polarization directions using a lens 5
cm in diameter and 10 cm in focal length. Together with the
collecting lens, a 0.8 mm pinhole defined a 0.8-mm-long and
0.1-mm-diam cylindrical probe volume. The detection system also included a collimating lens, an interference filter,
and a photomultiplier tube. The interference filter with a central transmission wavelength of 532 nm and a total bandwidth of 1 nm suppressed the broadband background light
and emission from the plasma. The output signal was processed by a gated integrator !Stanford Research System, SRS
250". A small fraction of the incident laser beam was deflected to a photodiode that was connected to a second gated
integrator.
The laser provided a pulse width of %8 ns with a repetition rate of 10 Hz. The gated integrators were triggered by
the Q-switch output of the laser with appropriate delays. The
gate widths of the integrators were adjusted such that the
photodiode and photomultiplier outputs were integrated for a
period of 100 ns centered around the laser pulse. A second
integration took place approximately 1 ms after each pulse,
in order to establish a baseline for each measurement. The
outputs of the gated integrator were equal to the difference
between these two results, thereby removing any additional
background. The resulting signal from the photomultiplier
channel was normalized by that from the photodiode in order
to remove the pulse-to-pulse energy variations of the laser
output. The temperature was calculated using Eq. !2", where
E0 was measured at room temperature.
Before attempting the plasma temperature measurements, the experimental system was qualified in a hot air jet
facility. The jet was generated by passing air through an externally heated 6.5-mm-diam stainless steel tube. Temperature was measured on the jet axis at a distance of 3 mm from
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123302-3
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J. Appl. Phys. 102, 123302 "2007!
FIG. 3. Normalized temperature profiles measured 6 mm above the cathode.
The electrode gap is 12 mm.
FIG. 5. Electrical field !left" and Joule power input per unit length !right" as
a function of discharge current.
the nozzle using both a calibrated thermocouple and the Rayleigh scattering setup. The results were in good agreement
over the total range of measurements !300–520 K" with a
maximum deviation of 3%. Using a set of neutral density
filters placed in between the Rayleigh scattering probe and
the collecting lens, the system response was also found to be
linear for approximately two orders of magnitude.
The plasma was formed by applying a high voltage between a pin anode !a 1-mm-diam stainless steel rod" and an
extended cathode !a 12-mm-diam stainless steel cylinder
with rounded top" as shown in Fig. 2. Both electrodes were
placed on a translation stage to move the plasma relative to
the Rayleigh scattering optical system. For electrode gaps
between 3 and 25 mm, stable discharges were generated that
lasted for several minutes.
temperature distribution; the full width at half-maximum
!FWHM" of the profiles changes from approximately 7 to 8
mm when the discharge current is increased threefold. Figure
4 shows the effect of discharge current on the on-axis gas
temperature, Tc, measured, again, at the midpoint of the 12
mm electrode gap. A moderate increase of temperature, from
about 2000 to 2400 K, is observed as the discharge current is
increased from 30 to 92 mA.
In evaluating the temperature, it is important to determine the Joule heating in the plasma. The power input per
unit length of plasma is ' = iE, where i is the discharge current and E is the electric field. The electric field in the positive column can be determined by measuring the voltage
drop across the electrodes as a function of electrode gap and
calculating the slope.16 Measured this way, the electric field
and the corresponding power density ' are presented in Fig.
5. Tripling the discharge current results in an increase of
power input into the positive column of about 50%. At the
same time, the increase in the discharge radius over the same
current range is insignificant, as shown in Fig. 3. Therefore,
heat removal at high currents has to be more efficient to
explain the rather insignificant temperature increases seen in
Fig. 4.
Discharge axis temperature, Tc, measured at the midpoint between the electrodes for several different electrode
gaps and discharge currents is plotted in Fig. 6. Since the
power per unit length is constant over the length of the electrode gap !excluding the narrow cathode region", as expected, the temperature remains nearly constant as the elec-
III. RESULTS AND DISCUSSION
Normalized temperature profiles, &!r" = #T!r" − Ta$ / !Tc
− Ta", measured at the three different discharge currents of
30, 50, and 92 mA, are shown in Fig. 3. Here, r is the radial
distance measured from the discharge axis, Ta = 295 K is the
ambient temperature, and Tc is the temperature on discharge
axis, &T!r"&r=0. Each data point was calculated by averaging
500 individual temperature measurements and the standard
deviations are given by the error bars. These profiles were
obtained 6 mm above the cathode for a discharge with electrode gap of 12 mm. The profiles show that the discharge
current has a small but noticeable effect on the width of the
FIG. 4. Temperature on discharge axis as a function of current. The electrode gap is 12 mm.
FIG. 6. Maximum temperature as a function of discharge gap.
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J. Appl. Phys. 102, 123302 "2007!
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FIG. 7. Normalized temperature profiles at different axial positions. Discharge current is 40 mA and electrode gap is 25 mm.
trode gap is varied. Normalized temperature profiles at three
different axial positions between the electrodes are shown in
Fig. 7 for the discharge with current and electrode gap of 40
mA and 25 mm, respectively. The three profiles were taken at
the axial positions of z / d = 0.84, 0.5, and 0.08 !z and d refer
to the distance from the bottom electrode and the electrode
gap, respectively". The profiles indicate that the discharge
becomes wider toward the top electrode. This result is in
qualitative agreement with the emission distribution observed in Fig. 2. A typical distribution of temperature along
the discharge axis is presented in Fig. 8. In this figure, i
= 50 mA while d = 12 mm. Overall, the results presented in
Figs. 7 and 8 demonstrate that the temperature field in the
discharge has a conical shape, expanding toward the top.
Temperature fields in a cylindrical unconfined glow discharge in air were calculated in Ref. 17 for low-pressure
discharges !p ! 100 Torr" with current densities j
! 100 mA cm−2, where convective heat losses were neglected. For the maximum considered local energy addition
rate !realized at j = 25 mA cm−2 and p = 100 Torr", the predicted temperature at the discharge axis was Tc = 3800 K.
For the pin-to-plate atmospheric discharge in our experiments, the on-axis temperature did not exceed 2500 K, for
current densities up to 0.6 A cm−2. The present experimental
data, along with available measurements of gas temperature
in the atmospheric glow discharges in air #%2000 K in a
microhollow discharge at j = 3.8 A cm−2 !Ref. 18" and
%1800 K in an ac glow discharge with a water cathode at
FIG. 8. Temperature distribution along discharge axis. Discharge current is
50 mA and electrode gap is 12 mm.
FIG. 9. Vertical component of velocity measured along lines AB and DC.
j = 5.2 A cm−2 !Ref. 19"$ suggest a dominant effect of convection as a heat removal mechanism in the atmospheric
plasma. In order to assess this assumption, we carried out a
set of gas velocity measurements around the plasma.
A. Velocity measurements
Gas velocity around the plasma caused by the natural
convection currents was measured using a single-component
laser Doppler velocimeter !LDV". Air around the discharge
was seeded with 5-%m-diam TiO2 particles and the BSA-50
LDV system by Dantec Dynamics was used for particle velocity measurements. The light source for the LDV was a 10
mW He-Ne laser. The laser output was split into two equal
intensity beams and focused to the probe location by a lens
with 50 mm diameter and 400 mm focal length. The cylindrical probe volume was 230 %m in diameter and 1.1 mm in
length. LDV Doppler bursts were analyzed using a frequency
domain processor.
It was not possible to measure the velocity inside the
plasma since TiO2 seed particles were charged and were
pulled by the electric field toward the cathode. In other
words, the discharge efficiently cleaned itself from the TiO2
dust preventing velocity measurements inside the discharge
core. Figure 2 shows the domain of velocity measurements.
The z component of velocity, vz, was measured along lines
AB and DC, and the radial velocity, vr, was measured along
the CB line. The results are shown in Figs. 9 and 10, respec-
FIG. 10. Radial component of velocity measured along line CB.
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123302-5
J. Appl. Phys. 102, 123302 "2007!
Stepaniuk et al.
tively. Each data point corresponds to the average of 3000
individual velocity measurements, and standard deviations
are plotted as error bars.
These results clearly indicate a strong convective flow
generated by the discharge. Ambient air was entrained into
the discharge area from the bottom and the sides and exited
at the top of the plasma. The lower temperature region near
the cathode observed in Fig. 8 is caused by the cold air
entrained from the bottom. To estimate the effect of this flow
on the temperature inside the plasma, we consider a cylinder
of rotation of the rectangle ABCD around the discharge axis
as a control volume. The net heat power loss, Pc, from the
discharge region due to convection can be estimated using
the energy conservation equation
Pc =
'(
'(
AB
' '(
'
(vzc pTdA −
−
CD
BC
(vrc pTdA
'
(AB =
BC
!3"
(vzc pTdA .
' '( '
'( '
(vrdA +
AB
CD
Current !mA"
Convective
heat loss !W"
Total power input
in discharge !W"
Power input into
positive column !W"
30
50
92
18
21
33
46
64
89
35
45
53
charge currents along with total discharge power input and
power input into the positive column. The values for the
power input into the positive column are taken from Fig. 5.
The results demonstrate that convection losses can be as high
as %50% of the power input into the positive column or
%30% of the total discharge input.
1
Here AB, BC, and CD indicate integration over the top, side,
and bottom surfaces of the cylinder, respectively, ( is the air
density, and c p is the specific heat at constant pressure. The
values of the vertical and radial velocities are those shown in
Figs. 9 and 10. Surfaces BC and CD are far enough from the
discharge for gas temperature to be essentially equal to the
ambient, Ta !see Fig. 3". For the top surface, AB, this is not
the case. The Rayleigh scattering measurements could not be
performed in that region of the discharge due to strong reflection from the anode surface. The average temperature on
surface AB was estimated from the mass conservation instead. The average !bulk" density on surface AB was calculated from:
'(
TABLE I. Comparison of convective heat losses with total power inputs
into discharge.
(vzdA
!4"
vzdA
and the temperature TAB was determined using the equation
of state TAB = p / (ABR, where R is the gas constant for air.
Based on these assumptions, the net power loss, Pc, was
calculated and presented in Table I for three different dis-
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