Peculiarities of self-sustained discharge in closed electron drift accelerator based on permanent magnets
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| Опубліковано в: : | Вопросы атомной науки и техники |
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| Дата: | 2002 |
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України
2002
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| Цитувати: | Peculiarities of self-sustained discharge in closed electron drift accelerator based on permanent magnets / S.N. Pavlov, A.A. Goncharov, A.N. Dobrovolsky, I.M. Protsenko // Вопросы атомной науки и техники. — 2002. — № 5. — С. 133-135. — Бібліогр.: 5 назв. — англ. |
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Pavlov, S.N. Goncharov, A.A. Dobrovolsky, A.N. Protsenko, I.M. 2015-03-30T09:20:52Z 2015-03-30T09:20:52Z 2002 Peculiarities of self-sustained discharge in closed electron drift accelerator based on permanent magnets / S.N. Pavlov, A.A. Goncharov, A.N. Dobrovolsky, I.M. Protsenko // Вопросы атомной науки и техники. — 2002. — № 5. — С. 133-135. — Бібліогр.: 5 назв. — англ. 1562-6016 PACS: 52.75.-d https://nasplib.isofts.kiev.ua/handle/123456789/79279 This work is supported by STCU grant #1596. en Національний науковий центр «Харківський фізико-технічний інститут» НАН України Вопросы атомной науки и техники Low temperature plasma and plasma technologies Peculiarities of self-sustained discharge in closed electron drift accelerator based on permanent magnets Article published earlier |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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| title |
Peculiarities of self-sustained discharge in closed electron drift accelerator based on permanent magnets |
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Peculiarities of self-sustained discharge in closed electron drift accelerator based on permanent magnets Pavlov, S.N. Goncharov, A.A. Dobrovolsky, A.N. Protsenko, I.M. Low temperature plasma and plasma technologies |
| title_short |
Peculiarities of self-sustained discharge in closed electron drift accelerator based on permanent magnets |
| title_full |
Peculiarities of self-sustained discharge in closed electron drift accelerator based on permanent magnets |
| title_fullStr |
Peculiarities of self-sustained discharge in closed electron drift accelerator based on permanent magnets |
| title_full_unstemmed |
Peculiarities of self-sustained discharge in closed electron drift accelerator based on permanent magnets |
| title_sort |
peculiarities of self-sustained discharge in closed electron drift accelerator based on permanent magnets |
| author |
Pavlov, S.N. Goncharov, A.A. Dobrovolsky, A.N. Protsenko, I.M. |
| author_facet |
Pavlov, S.N. Goncharov, A.A. Dobrovolsky, A.N. Protsenko, I.M. |
| topic |
Low temperature plasma and plasma technologies |
| topic_facet |
Low temperature plasma and plasma technologies |
| publishDate |
2002 |
| language |
English |
| container_title |
Вопросы атомной науки и техники |
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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Article |
| issn |
1562-6016 |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/79279 |
| citation_txt |
Peculiarities of self-sustained discharge in closed electron drift accelerator based on permanent magnets / S.N. Pavlov, A.A. Goncharov, A.N. Dobrovolsky, I.M. Protsenko // Вопросы атомной науки и техники. — 2002. — № 5. — С. 133-135. — Бібліогр.: 5 назв. — англ. |
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2025-11-26T16:15:58Z |
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2025-11-26T16:15:58Z |
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| fulltext |
PECULIARITIES OF SELF-SUSTAINED DISCHARGE IN CLOSED
ELECTRON DRIFT ACCELERATOR BASED ON PERMANENT MAGNETS
S.N. Pavlov *, A.A. Goncharov, A.N. Dobrovolsky, I.M. Protsenko
Institute of Physics NAS Ukraine, Kiev;
*Institute of Nuclear Researches NAS Ukraine, Kiev
PACS: 52.75.-d
INTRODUCTION
Investigation of accelerators with closed electron
drift is a subject of extensive research [1]. However, for
their half-century history these accelerators were mostly
used for propulsion. That’s why majority of investigations
was directed to achieving maximum possible velocities of
plasma flux and attaining gas economy of accelerator.
In the last years the interest to treatment of
surfaces of hard materials by plasma fluxes with moderate
energies has grown up very much [2]. Those are used in a
number of electronic and optical devices. Plasma flux
modifies the surface prior to deposition of functional
coatings onto it, and improves their adhesion, strength,
electrical and chemical properties. Such tendency has led
to changes in a number of requirements to the
accelerators. The first place was taken by homogeneity of
surface treatment, absence of incandescent elements in
the system, servicing simplicity. It, in turn, led to the
necessity of reconsidering a set of statements used in
design of accelerators. Besides, switching to new
operation regimes surely influences dynamics of
discharge glowing in the systems of such kind.
In experiments described in [3,4] it is shown that
it is reasonable to inlet working gas not through the
accelerator cathode or anode (classic option), but
immediately into vacuum chamber. In that case cleaning
rate increases approximately twice, and working pressure
shifts towards the range of higher values. For this reason,
in modernized configuration of accelerator exactly such
system of gas inlet was used.
Described accelerator provides high surface
cleaning rates at a level of 1 nm/s in narrow pressure
range near 9⋅10-4 Torr. At pressures higher than 1,2⋅
10-3 Torr cleaning is replaced by deposition.
Obviously, clarifying physical nature of such
behavior of accelerator is a base for its further
modernization.
In the present work peculiarities of self-sustained
discharge in crossed ExH field on permanent magnets
with closed electron drift are studied. The cases of
discharge glowing onto closely placed conductive and
dielectric collectors are researched, as well as that onto
remote metal collector. Dependencies of discharge current
on pressure in the system for various conditions of
discharge glowing are presented.
RESULTS OF EXPERIMENTS
The experiments were conducted at setup
described in [3] in configuration described in [4]. Width
of accelerator channel is 1 cm. Since the system was
intended for technological purposes, for making it simpler
and cheaper its single-stage version with the use of
permanent magnets was developed. The same reason
served as a base for rejection of forced space charge
compensation of ion beam. The accelerator cathode was
grounded, and constant voltage of 900 V value was
supplied to the anode. Argon is used as plasma forming
gas.
As it was shown by previous investigations, two
factors may lead to described behavior of accelerator. The
first of those is, that floating potential of target placed at
6-50 cm distance from the accelerator increases very
much in pressure range above 10-3 Torr. The potential
increases from 300 up to 800 V. It surely results in
retarding ion beam and lowering sputtering coefficient. At
second, in that pressure range arch-shaped luminescence
occurs at front side of accelerator, and at further pressure
growth this luminescence fills the whole vacuum chamber
volume. It gives evidence to ignition of stray discharge,
which results in intensive sputtering of accelerator jaws
and structural elements of vacuum chamber.
With an aim mentioned above it should be
determined, what is the origin – stray discharge ignition
leads to the beam decompensation, or vice versa, the
beam decompensation results in stray discharge ignition.
Let us consider at first stray discharge. As it was
shown by more detailed researches, actually three types of
stray discharge exist. This subdivision is due to magnetic
field structure near metal pole of magnetic core. Fig. 1
exhibits schematically the cross section of accelerator
channel with polepieces. Magnetic field lines may be
closed between them on two directions – A and B. There
also exists separatrix C – magnetic field line going to the
infinity. Main discharge is glowing in «O» zone.
The ways of suppression of stray discharges in
the devices with crossed fields are well known. For that
purpose one should install metal shield under floating
potential near the surface, which the discharge is glowing
onto, or to cover the surface by dielectric. In our case the
first stray discharge was suppressed by metal shield, and
the second and the third ones were suppressed by
dielectrics. By subsequent adding of those elements, it is
easy to determine thresholds of discharge ignitions and
their relative contributions to the discharge current.
The lowest ignition threshold (in pressure) is
possessed by the first stray discharge (about 7⋅10-4 Torr).
It glows in zone I (see Fig.1). Current value for this
discharge is of same order of magnitude, as that for main
discharge (see Fig.2). Magnetic field value near the
surface of polepieces at C line is about 150 Oe. It
complicates coming of electrons from zone I to zone II.
For that reason threshold of second stray discharge
ignition appears to be somewhat higher (1-1,2⋅10-3 Torr).
Problems of Atomic Science and Technology. 2002. № 5. Series: Plasma Physics (8). P. 133-135 133
Its cathode is represented by zone of polepieces placed
behind separatrix C. Contribution of this discharge to the
whole discharge current is small – about 10%. Finally, at
pressure 1,4-1,6⋅10-3 Torr the third stray discharge is
ignited with cathode represented by the discharge
chamber walls. Current of this discharge depends
essentially on distance between the target and the
accelerator, which in turn defines the cathode square for
this discharge. Its contribution to overall discharge
current may vary from 10 up to 80%. It is easy to see that
ignition thresholds for all these discharges are close
enough, or even overlapped. It is due to fact that in
mentioned pressure range floating potential grows up
rapidly.
Fig.3 shows the dependencies of metal target
floating potential. Curve 1 corresponds to conditions,
when no steps are taken for suppression of stray
discharges. Curve 2 corresponds to conditions, when all
stray discharges are suppressed. In both cases in pressure
range above 8-10⋅10-4 Torr rapid growth of floating
potential is observed. It gives evidence to fact that the
origin of processes occurring in this pressure range
consists in the growth of space potential. On one hand, it
results in ion beam deceleration, and on the other hand–
to ignition of stray discharges. Draw an attention that
floating potential for pressure range 1-9⋅10-4 Torr is
somewhat less, if arch-shaped discharge exists. It gives
evidence to fact that this discharge partially supplies
electrons to ion beam zone. Basically, there is nothing
strange, if ion beam leaves the accelerator in
decompensated state. It is put in the nature of accelerator
with anode layer [4]. More surprising is fact that in
pressure range below 10-3 Torr partial compensation takes
place. Arch-shaped discharge supplies just a portion of
electrons to the beam. It is currently unknown, where the
rest portion of electrons is taken from. Studying this issue
requires additional investigation.
Let us consider the nature of stray discharge
ignition. Purely hypothetically, one can assume that glow
discharge is ignited in the volume. However, simple
calculations show that for existing cathode material and
working gas pressure range the distance between anode
and cathode should be a few meters, what is unreal.
Another possibility is self-sustained arch-shaped
discharge in crossed magnetic and electric fields. Indeed,
magnetic field strength at front side of accelerator is high
enough and, as shown by estimations, is able to provide
about ten turns of electrons before its coming to cathode.
For verification of this assumption main discharge was
turned off. Auxiliary electrode was installed near
accelerator front side in arch-shaped discharge zone, and
potential of about 1000 V was applied to that electrode.
This verification has shown that arch-shaped discharge is
not self-sustained one.
134
1 2
3
<<O>>
III
III
AB
C
Fig.1. The Scheme of accelerator cross section with
magnetic field structure and zones of stray discharges:
1- permanent magnets, 2- magnetic circuit, 3- anode; A,
B, C – magnetic field lines;
I, II, III- zones of stray discharge glowing
Fig.2. Dependencies of discharge current on pressure:
1- stray discharges are not suppressed; 2- the first stray
discharge is suppressed; 3- the first and the second stray
discharges are suppressed; 4- all stray discharges are
suppressed
Fig. 3. Dependencies of target floating potential on
pressure in the system. Curve 1 corresponds to
conditions, when no steps are taken for stray discharge
suppression. Curve 2 corresponds to conditions, when all
stray discharges are suppressed
Finally, the possibility of non-self-sustained
discharge ignition exists in this region. It is readily to
understand its nature, if we recall classic equation for
breakdown in the gas γ(eαd-1)≥1, where γ is coefficient of
secondary ion-electron emission from the cathode, α is
electron multiplication coefficient in a gas, d is effective
distance between electrodes. Usually γ value is 10-2 – 10-3.
It requires large values for eαd about 100-1000. Such
necessity disappears when we have non-self-sustained
discharge. Let us imagine model task, in which quasi-
neutral plasma flux flows into the gap between two plates
with potential difference applied to them. If electric field
strength is high enough, and electron free path is small
enough, it is not difficult to reach eαd values at a level of
2-5 instead of hundred or thousand. In this case we get the
discharge current growth by factor of 3-6, which exactly
is observed in the experiment.
CONCLUSIONS
Thus, in the present proceeding it is shown that:
1. Non-monotonous character of cleaning rate
dependence on pressure for single-stage AAL is
determined, first of all, by the growth of space charge
potential in high pressure range. Although sputtering of
accelerator channel walls is present, this effect is second-
order one.
2. Besides main discharge, which glows in zone of
maximum magnetic field, three more non-self-sustained
discharges are present in the system, that distort the
discharge characteristics. Presence of those discharges
results in undesirable sputtering of technological surfaces
of device and unreasonable increase of consumed power.
ACKNOWLEDGEMENTS
This work is supported by STCU grant #1596.
REFERENCES
1. Ion injectors and plasma accelerators. Edited by
A.I.Morozov and N.N.Semashko, Moscow,
Energoizdat, 1990 (in Russian).
2 Ide-Ektessabi, Yasiei, Kkuyami, Rev. Sci. Instr.,
2002, v. 73, N 2, p. 873-876.
3 A.A. Goncharov, A.M. Dobrovolsky, O.A.
Panchenko, S.N. Pavlov, I.M. Protsenko. Problems of
Atomic Science and Technology, #6 (2000), Series:
Plasma Physics (6), pp. 160-162.
4 A.A. Goncharov, A.M. Dobrovolsky, O.A.
Panchenko, S.N. Pavlov, I.M. Protsenko. Problems of
Atomic Science and Technology, #4 (2002), Series:
Plasma Physics (7), pp. 176-178.
5 S.D.Grishin, L.V.Leskov, N.P.Kozlov, Plasma
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Institute of Physics NAS Ukraine, Kiev;
CONCLUSIONS
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