Low-Voltage Heated-Cathode Discharge Device with Controllable Plasma
Properties
V. I. Demidov1, A.S. Mustafaev2, S.F. Adams3, I. Kaganovich4, Y. Raitses4, I. Schweigert5
1
West Virginia University, Morgantown, WV 26506, USA
5
Sankt-Petersburg State Mining Technical University, Sankt-Petersburg 199106, Russia
3
Air Force Research Laboratory, WPAFB, OH 45433, USA
4
Princeton Plasma Physics Laboratory, Princeton, NJ 08543, USA
5
Institute of Theoretical and Applied Mechanics, Novosibirsk 630090, Russia
Abstract: A low-voltage gas discharge device with heated cathode is presented
which demonstrates control of the plasma properties by means of regulation of
nonlocal fast electrons. The discharge is formed between a heated cathode and an
anode. A special molybdenum diaphragm, the control electrode, is placed
between cathode and anode. A conical electrode (screen) restricts the discharge
plasma in the radial direction. Experiments and modeling of the device suggest
the presence of two dramatically different modes, which are dependent on the
diaphragm voltage. In the first mode the diaphragm current weakly depends on
voltage, but in the second mode the situation is opposite. This characteristic
behavior indicates that this device could be potentially used in a power
application requiring low-voltage current or voltage stabilization.
Keywords: plasma, discharge, heated cathode
1. Introduction
The goal of this work is developing new
methods of controlling plasma parameters based
on the nonlocal nature of the electron energy
distribution function (EEDF) [1] in a plasma in
the presence of so-called “energetic” electrons.
To demonstrate the possibility of the control a
short (without positive column) dc discharge
with a heated cathode has been used.
molybdenum anode is 3.0 cm. A special
molybdenum diaphragm (the control electrode)
2. A short dc discharges with a heated
cathode
A drawing of the discharge device is shown in
Fig. 1 and a photograph in Fig. 2. The discharge
takes place between a grounded cathode and a
positively biased anode. The indirectly heated
cathode is a disk with a diameter of 1.0 cm of
porous tungsten impregnated with bariumpotassium aluminates. The diameter of the
Figure 1. Schematic diagram of the experimental device.
with an external diameter of 3.0 cm, internal
diameter of 0.2 cm, and thickness of 0.2 cm is
placed between cathode and anode. The distance
between cathode and diaphragm is 0.8 cm and
the distance between the diaphragm and anode
is 0.1 cm. A conical electrode (screen) restricts
the discharge plasma in the radial direction.
the device is nonlocal and electrons can travel
from one wall to another without significant loss
of energy from elastic collisions. As a result, the
EEDF in the gap between the cathode and
diaphragm consist of two distinct groups of
electrons. Slow electrons which have been
created by inelastic processes in the volume
have a near-Maxwellian EEDF with the
temperature of a few eV and energetic electrons
originating from the cathode and accelerated by
the cathode fall voltage. The energetic electrons
have energy corresponding to the cathode fall
(15–30 V) and therefore much greater than the
energy of the slow electrons. At the same time,
the average electron energy is close to the
average energy of the slow electrons and
therefore energetic electrons are not affected by
the ambipolar field and follow essentially free
diffusion in the plasma volume.
Figure 2. A photo of a short discharge (heated cathode is
absent).
Experiments demonstrate that the floating
potential of the diaphragm is only slightly
higher (a fraction of voltage) than the cathode
potential, but much lower than the plasma
potential inside the device. As a result, for the
case of the floating diaphragm, most of the
energetic electrons are reflected from the
diaphragm (only energetic electrons flying
almost perpendicular to the surface have enough
energy to reach the diaphragm).
In all experiments presented here, the conical
screen was electrically connected to the cathode
and therefore also grounded. Note that this
electrode could also be used as an additional
control electrode and this could give more
flexibility for changing plasma properties. We
hope to investigate this in the future. A
cylindrical movable tantalum probe (not shown
in Figs. 1 and 2) with a diameter of 0.07 mm
and a length of 1 mm was introduced into the
plasma, perpendicular to the axis of the device,
for making measurements in the interelectrode
gap. The measurements were conducted in
spectrally pure helium at pressures from 0.5 to 5
Torr with a discharge current of 0.02 to 2 A.
3. Controlling the plasma properties
A goal of this work is demonstration of strong
modification of plasma properties with
application of additional voltage to boundaries
in short dc discharges with heated cathode. For
the conditions investigated, the plasma EEDF in
Figure 3. IV traces of the cathode-diaphragm gap. Anode
current (electron current) is 0.1 A (1a and 1b), 0.2 A (2a and
2b), 0.3 A (3a and 3b), and 0.4 A (4a and 4b). Gas pressure is 1
Torr. The floating potential is indicated by the arrow.
Figure 4. Axial behavior of the plasma potential. Curves shown
are for case 2a of Fig. 3 with the diaphragm voltage at 13 V and
case 2b of Fig. 3 with the diaphragm voltage at 18 V. Arrows
indicate diaphragm potentials. The diaphragm is located 8 mm
from the cathode. The anode is not shown in the figure but is
located 11 mm from the cathode. Anode potentials are 32 V for
case 2a and 48 V for case 2b. Gas pressure is 1 Torr.
This situation arises because flux of energetic
electrons Γef is more than flux of ions Γi at the
diaphragm surface. Application of an additional
voltage to the diaphragm creates a current Ie to
the diaphragm and therefore changes the ratio
between Γef − Ie and Γi, causing nonambipolar
flow of the charged particles. Corresponding IV
traces of cathode-diaphragm gap for constant
discharge current are shown in Figure 3 and
axial behavior of the plasma potential is shown
in Figure 4. It is seen that increasing the
diaphragm potential leads to increasing electron
current through the diaphragm circuit (curves
with index “a”). This is due to the decreasing
potential drop between the diaphragm and
plasma and reduced reflection of energetic
electrons by the diaphragm. Curves with index
“a” have a specific saturation region which is a
clear indication of the freely diffusive character
of the energetic electron movement. This
situation is similar to the collection of electrons
by a Langmuir probe in a diffusive regime. In
this case, the probe electron current Ie for a δfunction EEDF with energy εef is equal to
Here, Nef is the density of energetic electrons, V
is a probe potential, e is the electron charge, and
A is a factor which depends on geometry and
electron mean free path. In principal, this
equation allows us to estimate the density of
energetic electrons. While increasing the
diaphragm voltage from zero to positive values
within curves “a”, the plasma parameters
(electron temperature, density, and ratio
between densities of slow and energetic
electrons) are changed gradually as energetic
electrons are absorbed at the diaphragm.
Eventually, at some voltage the loss of energetic
electrons leads to a situation where the plasma
changes abruptly to a completely different set of
conditions. As an example, in Figure 5 the
change in plasma glow is illustrated.
Figure 5. Two figures at the left: Schematic diagram of the
experimental device of dc discharge with hot cathode
demonstrating modification of plasma glow during the abrupt
transition between two regimes show at the right figure
(connection between regimes shown by arrows).
Thus, the above simple discharge device can
work in two regimes. The sharp transition
between regimes with and without self-trapping
of energetic electrons is also demonstrated.
Smooth control of the plasma parameters is also
possible.
4. Modeling: demonstration of the
existence of two modes of operation
Simulations show that if the diaphragm voltage
is low, the plasma density becomes low and the
sheath thickness can become larger than the
distance between diaphragm and anode. This
phenomenon was observed in particle-in-cell
simulations and shown in Fig. 6. Fig. 7 shows
electron current on the diaphragm as a function
of a diaphragm voltage. Those figures clearly
show the transition from plasma filling the
entire gap then receding as the diaphragm
voltage drops. At this time the electric field
increases in the diaphragm hole which then
causes the direct electron impact ionization and
generates dense plasma.
Figure 6. Particle-in-cell simulation results for 2D profiles of
the plasma density (ions), for different diaphragm voltages.
U=38V, P=1 Torr
Janode
0.24 A
0.29 A
0.36 A
0.35
0.30
0.25
D
0.20
jdiaph, A
Though we were able to obtain two regimes in
simulations, a quantitative description and
analytical study of two regimes are still not
accomplished at present time and need to be
further advanced in future.
C
A
0.15
0.10
B
0.05
0.00
0
5
10
15
20
25
Udiaph
Figure 7. Electron current on the diaphragm as function of
diaphragm voltage at three anode currents j_anode=0.24 A,
0.29 A and 0.36 A. Uanode=38V and P=1 Torr..
This work was supported by the DOE OFES
(Contract
No.
DE-SC0001939),
GK
14.740.11.0893 and AFOSR.
References
[1] L.D. Tsendin, Plasma Sources Sci. Technol.,
4, 200, 1995.
[2] V.I. Demidov, C.A. DeJoseph, Jr. and V.Ya.
Simonov, Appl. Phys. Lett. 91, 201503, 2007.