Download Plasma nonequilibrium in self-sustained normal DC atmospheric

Plasma nonequilibrium in self-sustained normal DC atmospheric pressure
glow discharges in noble and molecular gases
V.I. Arkhipenko, A.A. Kirillov, Ya.A. Safronau, L.V. Simonchik, S.М. Zgirouski
Stepanov Institute of Physics of the NAS of Belarus, Ave. Nezavisimosti 68, 220072 Minsk, Belarus
Abstract: It has been shown that self-sustained normal DC atmospheric pressure glow discharges in helium, argon and nitrogen exist in a large range of current values from hundred
microamperes to ten amperes. The plasma of these discharges is weakly ionized (one thousandth part of a per cent) and nonequilibrium. The plasma nonequilibrium degree in the positive column depends on the discharge current values. Parameters of the cathode region stay
constant over the whole current range of normal atmospheric pressure glow discharge.
Keywords: atmospheric pressure glow discharge, nonequilibrium plasma.
1. Introduction
At present time such kinds of discharges as corona one,
micro hollow cathode discharge, discharge with dielectric
capillary electrode and, of course, different kinds of dielectric barrier discharge are widely used for producing
atmospheric pressure plasma [1,2]. But volumetric power
density of these discharges doesn’t exceed several tens
watt per cubic centimeter. Meanwhile discharge volume
consists only of a few cubic centimeters. The creation of
stationary, nonequilibrium, homogeneous, large volume
(for example, of several tens of cubic centimeters, not
saying about nonequilibrium plasma volume comparable
with the volume of low pressure plasma) plasma at atmospheric pressure turned out to be a difficult engineering challenge even taking into account high level of technique development and the number of total knowledge in
the area of gas discharge physics. All these keep to look
for other sources.
DC Atmospheric Pressure Glow Discharge (APGD) is
the simplest source for producing nonequilibrium plasma.
The objective of this work is to determine the parameters
of nonequilibrium plasma in different regions of
self-sustained normal APGD in helium, argon and nitrogen
in the range of discharge currents from hundreds of microamperes to several amperes.
2. Experimental setup
Glow discharge is ignited between two electrodes in
air-locked chamber with quartz glass windows used for
discharge observation [3]. The discharge chamber dimensions are 100×100×70 mm3. Weakly rounded tungsten
anode is 6 mm in diameter and flat cooled cathode, made
from copper, is 36 mm in diameter. The cooled cathode
is a cylindrical cup whose outer bottom surface acts as a
cathode working surface. Interelectrode gap varied from
0.5 mm to 10 mm. The working gas (helium, nitrogen or
argon) flow of about 1–2 liter per minute was provided
through the discharge chamber.
A working gas (helium, nitrogen or argon) flow of about
1–2 liter per minute was provided through the discharge
chamber. Depending on the experimental situation, two
power supplies (PS in figure 1(a) were used. A power
supply with output voltages up to 4 kV and variable resistance in the range of a few kΩ op to several tens of
MΩ was used at low discharge currents (less than 100
mA); the other supply with output voltages of up to 600 V
and a ballast resistor of about 100 Ω was used at high
discharge currents (more than 100 mA).
3. Results and discussion
Images of the discharges in helium, argon and nitrogen
at different discharge currents and the same interelectrode
gap of 1 cm are shown in Fig.1. Excluding helium dis-
Fig.1 Images of the dc APGD in helium, argon and nitrogen at discharge gap of 1 cm and at different currents.
charge at 20 μA, the APGDs presented in Fig.1 have the
following structure: a thin layer (less than 0.5 mm in
thickness) of negative glow is located right above the
cathode surface; positive column extends to the anode.
There is Faraday dark space between these two luminous
regions. The anode end is covered by the anode glowing
layer. Negative glow diameter is determined by discharge
current values. At the same time the colour of negative
the gap increase is more significant at low currents less
than 100 mA. It is greater for nitrogen APGD and small
for argon one. This voltage increase with the current decrease results in the electric field increase in positive column.
Depending on discharge current and gas temperature
Fig. 2 V-I characteristics of the helium, argon and nitrogen APGDs at two gaps 1 and 10mm.
glow practically does not change and it is relatively homogeneous along a radius. In the case of helium discharge
at small current, as it is shown in the photo taken at current of 20 μA (Fig.1 (a)), negative glow is divided into
several spots (3 spots in this case). The positive column
appearance essentially depends on gas sort and current
value. In case of helium and nitrogen, the discharge transforms from diffuse mode into contracted one at the current increase. In argon the discharge is contracted in the
investigated range of current.
The voltage-current (V-I) characteristics (dependence of
the interelectrode gap voltage on discharge current) of the
APGDs in helium, argon and nitrogen at two interelectrode gaps (0.1 and 1 cm) are shown in Fig.2. As it follows from Fig.2, the V-I characteristics at the gap of 1
mm are relatively “flat” in the wide current range excluding small currents of values order or less than 1 mA.
Cathode falls in the APGD in helium, argon and nitrogen
with copper cathode are 145 V, 190 V and 202 V correspondingly [4]. The exceeding of these values is due to
other regions of discharge (negative glow, Faraday dark
space, anode region).
At the current decreases less than 0.3 mA for helium,
0.5 mA for argon discharge and 1.5 mA for nitrogen one,
the APGDs lose the structure characterized for normal
glow discharge. Oscillations of the discharges are observed. The negative glow area widens which denotes a
decrease of current density and its luminosity becomes
weaker. A voltage increase accompanies these changes
which is clearly seen for argon and nitrogen (Fig. 2b, 2c).
At larger interelectrode gaps (more than 1 mm), V-I
characteristics have negative derivatives dV/dI (Fig.2,
curves for gaps of 1 cm). That corresponds to the negative
differential resistance. The interlectrode voltage rise with
the electric field in the positive column was determined
either from the probe measurements or from the formula
(U10 – U1)/0.9, where U10 and U1 – electrode voltages in
volts at 10 mm and 1 mm gaps (field is in V/cm). Dependences of electric field E in APGD positive column on
discharge current are shown in Fig.3. The maximal electric field is observed for nitrogen and it is about one order
of magnitude less for argon. One can see that electric field
in the positive column reduces with discharge current
increase for all gases.
The gas temperature Tg was determined by using the
Fig.3 Electric field strength vs discharge current in helium,
argon and nitrogen.
resolved rotational bands of nitrogen ions (0,1) N+2(B 2Σ+u
− X 2Σ+g) and hydroxyl radicals (0,0) OH(A2Σ+-X2Πi).
The methods of gas temperature determination are based
on the measurements of the relative rotational line intensities of corresponding bands of nitrogen ions and hydroxyl under the assumptions that the rotational level
populations of excited electronic states follow a Boltzmann distribution and that the rotational temperature Trot
corresponds to the gas temperature Tg.
tain reduced electric field as a function of discharge current for helium, argon and nitrogen. The dependences are
shown in Fig.5. One can see that the lowest reduced field
Fig.4 Gas temperature in the helium, argon and nitrogen APGDs vs a discharge current. The interelectrode gap is 1 cm.
Fig.4 shows the dependence of gas temperature in
cathode region and positive column on discharge current.
Gas temperature in negative glow weakly changes with
discharge current as expected for normal glow discharge
with cooled cathode. Temperature growth in cathode region at current more than 100 mA is probably due to insufficient cooling of cathode surface. Gas temperature
values as other positive column parameters were obtained
for the middle part of 1 cm gap. The temperature in positive column is much higher and it more significantly
grows with current than in cathode region. Gas temperature is appreciably higher in nitrogen than in helium or
argon in both cathode region and positive column.
In the absence of nonlocal effects many of gas dis-
Fig.5 Reduced electric field strength in positive column vs discharge current.
charge plasma characteristics are the functions of reduced
electric field E/N due to the scaling laws. Taking into account the dependences of electric field E (Fig.3) and gas
temperature Tg (Fig.4) on discharge current one can ob-
is observed in argon. The values E/N are about one order
of magnitude higher in helium and about 50 times higher
in nitrogen than in argon.. The decrease of reduced electric field is observed at the discharge current increase.
Average electron energy in positive column is uniquely
defined by the value of reduced electric field E/N. For
determination of average electron energy ε e in positive
column we used the dependences of reduced electric field
on discharge current (Fig.5) and the Townsend’s data
about electron energy at different value of electric field
for these gases [6]. The dependences of average electron
energy on discharge current obtained for helium, argon
and nitrogen are shown in Fig.6. The values of average
electron energy in these gases are comparable only for
Fig.6 Average electron energy in the middle of gap of 1 cm
vs discharge current.
large current region. At the discharge current decrease the
difference between the energy values increases. For low
current the maximal electron energy is observed in argon,
the minimal in nitrogen, in spite of the opposite for the
values of reduced electric field (Fig.5).
It is known that atomic gas plasma is nonequilibrium if
its electron temperature exceeds the gas temperature
(Te >> Tg). Nonequilibrium of the positive column plasma
is characterized by Fig.7. Here, both the electron and gas
temperatures are presented in one scale (Kelvin). The
at current increase.
Discharge parameters and plasma characteristics in
cathode region and positive column of APGD were obtained during investigation. Parameters of cathode region
are constant within the whole current range of a normal
APGD mode. The APGD plasma is weakly ionized. The
Fig.7 Electron and gas temperatures in the helium, argon and nitrogen APGDs vs a discharge current.
electron temperature Te was determined from average
electron energy ε e (Fig.6) according to the expression
ε e =3/2·kTe.
One can see that, at discharge current increase the electron energy reduces and the gas temperature increases and
consequently plasma nonequilibrium degree decreases.
Plasma nonequilibrium is much higher in atomic helium
and argon gases than in diatomic nitrogen which is due to
the presence of additional channels of electron energy
transfer into translational degrees of freedom in molecular
gases. In atomic gases the mechanism of energy transfer
from electrons to gas is virtually elastic energy loss. The
efficiency of this process is extremely low because of the
large difference in electron and atomic masses. In molecular gases electrons are additionally able to transfer
their energy to vibrational and rotational degrees of freedom with the following relaxation to translational degrees
of freedom which is significantly more effective mechanism. A weak increase of electron energy in nitrogen at
discharge current decrease can be explained by this reason
as well.
4. Conclusions
The self-sustained normal dc APGDs in helium, argon
and nitrogen operating in large current range are presented. The discharge appearance essentially depends on
gas sort and current value. In case of helium and nitrogen
the discharge transforms from diffusive to contracted
mode at the current increase. In argon the discharge is
contracted in the range of investigated current. A form of
the V-I characteristic strongly depends on an interelectrode gap. The V-I characteristic is “flat” only at a gap
less than 1 mm. At larger gaps the characteristic is falling
nonequilibrium degree of positive column plasma in
atomic gases helium and argon is essentially higher then
in molecular nitrogen. For all gases in positive column,
nonequilibrium degree decreases with the current increase.
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