22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Near cathode processes in stationary vacuum arcs with distributed cathode spot
R.H. Amirov1, N.N.Antonov1,2, N.A.Vorona1,2, A.V. Gavrikov1,2, G.D. Liziakin1, V.P. Polistchook1, I.S. Samoylov1,
V.P. Smirnov1, R.A. Usmanov1,2 and I.M. Yartsev1
1
Joint Institute for High Temperatures of Russian Academy of Sciences, Moscow, Russia
Moscow Institute of Physics and Technology (State University), Dolgoprudny, Russia
2
Abstract: Characteristics of distributed vacuum arc on hot cathodes with significantly
different emission properties (gadolinium, plumbum and chromium) are presented. This
discharge is characterised by absence of random voltage fluctuations; micro particles of
cathode erosion products are absent too. Features of processes in cathode layer on different
metals are discussed.
Keywords: vacuum arc, emission processes, electron temperature, charge transfer
1. Introduction
It is assumed that significant difference between arc and
low-current discharge properties is determined by
significant role of emitted electrons in near-cathode
processes [1]. Therefore questions about electron
emission mechanism and relative ion proportion in charge
transfer on cathode are raised. In paper [2] it was
proposed to characterize vacuum arc cathodes by flows
ratio of thermal evaporated atoms to electrons S ae . This
ratio may be used for estimation of relative contribution
of ions and thermionic electrons to the charge transfer on
cathode. All metals may be divided into two large groups
by value of S ae . For some metals (mercury, copper, alkali
metals and others) S ae ratio is much greater than unit.
Therefore for these metals thermionic contribution in
charge transfer on cathode has to be small. For other
metals (tungsten, molybdenum, rare-earth metals and
others) S ae ratio is vice versa much less than unit and part
of ion current on cathode is also small. In this case
mechanism of cathode heating is not clear [2].
In this paper the investigations of vacuum arc with
cathode distributed spot (CDS) on gadolinium
(S ae ≈ 0.07), chromium (S ae ≈ 104) and plumbum
(S ae ≈ 108) are presented. Some characteristics of this
discharge on gadolinium (Gd) and chromium (Cr) had
been submitted in papers [2-4]. For the first time similar
discharge was studied on chromium cathode in paper [5].
Vacuum arc with CDS is distinguished by low current
density on cathode less than 102 A/cm2. This discharge is
characterised by absence of random voltage fluctuations;
micro particles of cathode erosion products are absent too.
It is all very attractive for different plasma technologies.
2. Experimental setup and diagnostics
The discharge was initiated in vacuum chamber with
residual gas pressures less than 10 mPa. Studied metal
was placed in molybdenum heat-insulated crucible
(cathode) with external and internal diameters of 25 and
19 mm respectively. Outlet of the crucible had diameter
of14 mm. In experiments with plumbum the cathode
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crucible was covered by molybdenum cap that has 6mm
hole. It allowed us to decrease Pb evaporation rate
approximately in five times. Electron-beam heater (EBH)
with power N up to 1 kW was situated under the crucible.
EBH allowed changing of cathode temperature at fixed
arc current. The water-cooled 20 mm wide steel disc with
central hole of diameter 15 mm was used as the anode.
The distance between electrodes was about 30 mm. Arc
current was regulated by rheostat.
The crucible temperature T c was measured by
brightness pyrometer. By estimations the difference
between measured temperature and mean temperature of
cathode surface due to temperature drop in crucible wall
was less than 3%. By method from [2] volt equivalent
(VE) V c of heat flow from plasma to cathode Q c was
measured (V c = Q c /I, where I is an arc current). Spectra
of plasma radiation were recorded. Electron temperature
T e of plasma behind the anode (Gd, Pb) and into
discharge gap (Gd) was measured by electrical probe.
Condensation probe situated on the distance of 27 mm
behind the anode was used for determination of average
charge Z a of heavy particles (Gd, Pb). Average charge Z a
was calculated from ratio of transferred charge to mass
increase by the exposure time of 5-10 minutes.
3. Experimental results
The experiments were carried out by following scheme.
With the help of EBH crucible was heated to necessary
temperature value, after that voltage from power supply
(380 V) was applied to discharge gap and vacuum arc
with CDS initiated. At work temperature Gd and Pb were
in molten state and Cr was solid. After arc initiation EBH
turned off in curtain regimes. On Fig. 1, Fig. 2 and Fig. 3
photos of discharge on Gd, Cr and Pb cathodes are shown.
In Table 1 main results of carried experiments
supplemented by results from works [2-4] are presented.
Ranges of arc current I, arc voltage V a and volt equivalent
V c belong to self-sustaining regimes (N = 0). Discharge
volt-current characteristic on Gd and on Cr cathodes is
decreasing. Voltage of discharge in Pb vapour had low
dependence on arc current and was about 15 V. Average
1
cathode current density J c changed in range from 10 to 30
A/cm2 depending on cathode metal and arc current.
Fig. 1. Vacuum arc with CDS on gadolinium cathode:
I = 52 A, U = 3.1 V, T c = 1.92 kK.
outlet and 70-80% of evaporated atoms return to the
cathode [4].
Fig. 3. Vacuum arc with CDS on plumbum cathode:
I = 20 A, U = 13.8 V, T c = 1.33 kK.
Characteristics of discharge on Cr and Pb are low
depended on power of EBH. Increase of EBH power N
leads to increase of cathode temperature T c , at these
conditions voltage of arc in Pb vapor has weak rising
trend and arc voltage on Cr cathode noticeably decreases.
For example, with increase of N from 0 to 500 W arc
voltage on Cr cathode decreases from 16 to 11 V
(I = 50A) and volt equivalent V c decreases from 8 to 5 V
[3]. The similar N increase at discharge on Gd cathode
leads to voltage reduction from 75 to 3.5 V (I= 50 A).
Volt equivalent V c drops from +10 to -3 V that means
cathode cooling in the arc. This is because the heat flow
from plasma to cathode is smaller than heat losses on
emission cooling [2].
Fig. 2. Vacuum arc with CDS on chromium cathode:
I = 52 A, U = 10.1 V, T c = 2.04 kK.
In Table 1 values of saturated vapor pressure P s of
studied metals at measured temperature T c are presented
[6].Values of cathode evaporation rateG c are also shown.
For Cr and Pb cathodes these data were obtained for selfsustained regimes (N = 0). It was experimentally shown
that Pb evaporation rate in arc conditions is approximately
two times less than at the same crucible temperature
without arc. Almost the same influence of arc processes
on Gd evaporation rate was established in paper [4].
Based on data from [4] it may be shown that in described
discharge in Pb vapor about 90% of evaporated atoms
return to the cathode surface as ions or atoms. In
discharge with Gd the crucible had other geometry of
2
Table 1.Characteristics of discharges.
Cathode
Parameter
Gd
Cr
Pb
4
S ae
0.07
10
108
T c , kK
1.9-2.2
1.9-2.1
1.2-1.5
P s , Pa [6]
1-10
60-700
50-2000
I, A
30-200
30-220
10-70
Va, V
95-20
22-11
15
Vc, V
17-4
9-4.5
4.5-9.5
G c , mg/s
1-2.5
3-6
25-50
T e , eV
0.5-8
0.4-0.7
Za, e
0.5-1.5
0.17-0.25
χ, a/e
0.01-0.015
0.08-0.12
0.6-0.9
Electron temperature in discharge gap for arc in Gd
vapour was measured at current I = 40 A and voltage
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range V a = 3-20 V. T e was measured by probe at 1451mm form discharge axis and 5-10 mm from cathode
surface, it increased from 0.8 to 8 eV with voltage
increase. Also T e was measured above the anode, probe
was situated at distance of 27 mm. Increase of arc voltage
from 5 to 30 V led to rise of T e from 0.5 to 8 eV (Table
1). Electron temperature in Pb discharge measured at the
same area decreased from 0.7 to 0.4 eV with arc current
increase from 18 to 37 A (Table 1). Arc voltage at these
regimes was about 15 V.
In Gd discharge with voltage increase from 3.5 to 8 V
average charge of heavy particles behind the anode Z a
increased from 0.5 to 1.5 e (e is an electron charge). In Pb
discharge average charge Z a decreased from 0.25 to 0.17
e with the current growth from 18 to 37 A (Table 1). In
general, the values of average charges Z a and electron
temperatures T e are in qualitative agreement with each
other. In these estimations we assumed that condensation
coefficient of particles is equal unit.
In Table 1 there are presented values of specific cathode
erosion χ in units of atom/electron. Value χ is a ratio of
evaporated atoms during experiment to total charge
transferred through discharge gap.
Measured average ion charge is in agreement with
results of spectral diagnostics. Electron temperature in Pb
and Cr discharges was relatively small and plasma
spectrum consisted only of atom and singly ionized atom
lines.
In Gd discharge spectrum lines of double ionized atoms
were observed. Behaviour of ion lines was non monotonic
with arc voltage increase. At I = 52 A intensity of singly
ionized atoms had a maximum at arc voltage about 5 V.
At voltages 5 V and 7 V lines intensity was about 30 % of
maximum value. Lines of double ionised Gd had
maximum at voltage of 10 V and at 7 V and 13 V
intensity of these lines was about 30 % of maximum
value. Line intensity of atoms had a monotonic fall with
increase of arc voltage (and decreasing crucible
temperature respectively). Plasma radiation spectra
recorded at 3 mm distance from crucible surface.
In experiment with Pb cathode it was managed to
estimate the plasma pressure in the crucible. In this test
the plasma pressure dropped the crucible cap. By
estimations it would possible if the plasma pressure was
equal 0.87 kPa. It happened at cathode temperature
1.38 kK, when saturated vapour pressure of Pb is 0.68
kPa [6]. Thus the plasma pressure corresponds to
saturated pressure of Pb.
It was established in work [2-5] the discharge with CDS
on Gd and Cr cathodes is characterized by the absence of
micro particles in the cathodes jets. The discharge on the
Pb cathode also possessed this peculiarity, the micro
particles were observed in transient regimes. In our
experiments the current contraction on the anode was not
observed.
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4. Discussion
Presented results show the qualitative difference
between discharge on Gd cathode and Pb or Cr cathodes.
This distinction may be caused by the difference in the
charge transfer mechanisms on these cathodes. On
thermionic cathode from Gd proportion of thermionic
current S e is about 90% of arc current [2]. For discharges
on Cr and Pb cathodes situation is radically different. As
shown in paper [3] no one electron emission mechanism
can provide the charge transfer on Cr cathode and S e ≈ 0.
Thermionic current density from Pb cathode at the typical
temperature 1.35 kK as follows from calculations is
J e ≈ 0.2 mkA/cm2. It is almost eight orders of magnitude
less than average arc current density J c . At the same time
density of heavy particles at cathode surface is enough to
provide the charge transfer.
Maximum ion current density J im on cathode can be
calculated in assumption that all evaporated atoms return
to cathode as ions [2]:
𝐽𝑖𝑖 = 𝑒
𝑝𝑠
�2𝜋𝑚𝑎 𝑘𝐵 𝑇𝑐
(1)
Where m a – mass of the atom, k B – Boltzmann
constant. Saturated vapour pressure of Pb at typical
temperature 1.35 kK is p s ≈ 0.48 kPa and equilibrium
density is n s ≈ 2.5*1016 cm-3[6]. From (1) it follows that
J im = 40 A/cm2. Thus if 25% of evaporated atoms return
to cathode as ions it will be enough to provide charge
transfer on its surface.
If one assumes that all charge on cathode is transferred
by ions, then their energy on the cathode may be
estimated from energy balance. Typical value of VE for
discharge in Pb vapour is V c = 6 V (Table 1), i.e. every
ion transmits the cathode energy about 6 eV. This energy
consists of energy released due to ion neutralization
(ionization potential minus work function, for Pb – 3.4
eV) and kinetic energy. Thus ion kinetic energy in this
regime is 2.6 eV and ion velocity equals 1.1*105 cm/s.
Then at ion current density J c ≈ 10 A/cm2 ion density at
cathode surface is 6*1014 cm-3.
Obtained results are in accord with qualitative model of
cathode layer of arc discharge with ion charge transfer
described in [7]. In accordance with this model the
electron diffusion plays significant role in the processes in
cathode layer. Near the cathode the diffusion and drift
electron flows are approximately equal in value and are
opposite directed, and the resulting density of electron
current on cathode is much less than arc current density.
For ions diffusion and drift flows are co-directed. As a
result of electron diffusion there is an alignment of
charged particles density; and the space charge near
cathode is practically absent. Estimated from energy
balance relatively low ion energies (about 1 eV) support
this assumption. Distributions of electrical potential and
electron temperature are non-monotonic. Maximums of
potential (≈ 10 V) and electron temperature (≈ 2 eV) are
situated from the cathode at the distance of ionization
3
length. The value of potential maximum was estimated
from the energy balance in the ionization layer. The
electron thermal conductivity provides the source of
energy in the ionization layer. High rates of density and
temperature of electrons in cathode layer provides great
efficiency of step ionization.
Apparently there is potential maximum near the surface
of Gd cathode. Inhomogeneity in potential distribution is
about T e /e. As electron temperature in this discharge is
high enough then potential maximum is noticeably greater
than in Pb and Cr discharges. Existence of potential
maximum clears a mechanism of cathode heating in Gd
discharge at relatively low part of ion current. In paper [2]
there was a hypothesis that cathode heating provided by
multicharged ions herewith cathode potential drop was
assumed to be close to total voltage drop in arc. Existence
of potential maximum increases the contribution of ion
kinetic energy in cathode heating process.
5. Conclusion
Presented results demonstrate that in vacuum arc on
plumbum and chromium cathodes the charge could
almost completely be transferred by ions. This charge
transfer mechanism can be expected on other cathodes
with atom-electron ratio S ae >> 1: mercury, copper,
bismuth, alkali metals and others. The peculiarities of the
cathode processes on different metals are determined by
the atom-electron ratio.
We should note that this statement is correct only for
steady state of arc. For example, autoelectronic emission
can provide notable charge transfer on these cathodes at
room temperature at the moment of arc ignition due to
vacuum breakdown. However the discharge on this stage
can’t be regarded as arc due to values of its current and
applied voltage.
6. Acknowledgements
The authors are also grateful to V.I. Kiselev for his
great help in preparing and carrying experiments.
The study was supported by grant from the Russian
Scientific Fund (project № 14-29-00231).
7. References
[1] Y.P. Raizer. Gas Discharge physics. Berlin: Springer
(1991)
[2] S.N. Paranin, V.P. Polistchook, P.E. Sychev and other.
High temperature (in Russian), 24, 422 (1986)
[3] V.M. Batenin, I.I. Klimovskii, V.P. Polistchook and
V.A. Sinelshikov High Temperature, 41, 586 (2003)
[4] S.Y. Bronin, V.P. Polistchook, P.E. Sychev and other.
High Temperature (in Russian), 31, 29 (1993)
[5] A.I. Vasin, A.M. Dorodnov and V.A. Petrosov.Soviet
Technical Physics letters (in Russian),5, 1499 (1979)
[6] I. Grigoriev and E. Meylikhov (eds). Physical
Quantities.Handbook. Moscow: Energoatomizdat (1991)
[7] V.P. Polistchook. TRANSACTIONS of Institute for
High Energy Densities of AIHT of RAS (in Russian),
5, 185 (2003)
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