SOURCE OF INTENSIVE OXYGEN PLASMA FLOWS OF LOW
ENERGY FOR TECHNOLOGICAL A.FLICATICS
Yury V. KYBAREV *,
Vladimir N. CHERNIK **
Abstract
The external magnetic field magnetoplasmadinamic accelerator
cathode using.
Double arc contraction by ferromagnetic intermediate
with
electrode
material
plasma
reduces
pumping
electrode
of
The source produces the oxygen
4*10 .
to
up
contamination
jet
plasma jet with ion current density 5 mA/cm2
the
in
range
energy
free lifetime is some lOOh.
Its maintanance-
20-60eV.
Ar-flow-hollow
by
adopted to produce low energy oxygen plasma jet
is
Introduction
At present the interest has grown
of the intensive
in creation
oxygen plasma or ion flows with the energy lower 100 eV[1].
3aused
the
by
structure
to
requirements
make
thin
films
with
It is
perfect
and also the simulation of action of elemental oxygen on
spacecraft materials.
In the film fabrication technology the film proberties such as
Irystalline
structure,
refractive
index can be controlled by bombarding qrowing film with
film density,
inert or reactive qas ions.
resistivity
electrical
and
The interactions between ions and thin
film surfaces are strongly-dependent on the incident kinetic energy
Considering the energy dependences of damage creation,
of the ions.
the
sticking
probability,
enhanced adatom migration,
deposition
and
epitaxial
the
surface cleaning effect,
the optimum kinetic
energy
growth of the films is
and tne
range
between.a few eV
and 100 eV [2].
*)
-
*) -
Academy of Ingeneering Sciences of Russia. 84/32,
Profsoyusnaya Str., Moscow, 117610, Russia
Moscow
State
Informatics,
I
20,
Akademy
of
Stromynka Str.,
Instrument-making
Moscow,
for
107846,
and
Russia
Spacecraft in
low
earth orbit (200-500Km height) are exposed
to a rarefied atmosphere which is,
atomic
oxygen
(ATOX).
It
can
to a large extent,
composed of
interact with the spacecraft
in a
number of ways and in particular can cause undersirable erosion
degradation
of
spacecraft
materials.
facilities are neened to investigate the
erosion
appropriate
choice
of
Thus
ground-based
principal
materials
test
mechanisms
used
in
or
of
spacecraft
construction[3].
The problem
of material erosion in oxidative plasma is existed
in fusion tecnology too.
impurity
oxygen
wall material
Under plasma-wall
interaction
the
wall
ions are produced intra- cavitarily and sputter of
chemically.
To
predicate
materials
behavior
the
needed
of
simulative tests are requisite [1,4].
All technology
high-intensivity
branches
oxygen
energy lower 100eV.
mentioned
plasma
flow
are
in
source with ion translation
Another common requiments are 1)
density more of 1ma/cm 2 ,
ion
current
2) long durability sufficient to material
tests durations(lOOh). 3) minimal beam contaminants.
Plasma or
ion
sources
usually
suitable for it in little degree.
with
one-grid
hours)
ion
because
of
optics
the
used
in
the
practice
are
Kaufman ion source of low energy
(IO) possesses a small lifetime (a few
rapid
10
destruction
in
the
oxygen.
In addition since most of the conventional high-current
ion sources
utilize
metallic
hight-discharge
hot
current
cathodes
and
and
are
operated
at
considerably low gas pressure,
a
the
liftime of the metallic hot cathodes is
rather short and iL usually
limits
the
lifetime
of the
ion
source.
In opposite a
closed
electron
drift
plasma
accelerator
supplied with Ar-gas-flow hollow cathode.That resultes
in long
operation in oxygen plasma
15),
but the oxygen plasma flow
is
life
energy
is
found
to be too high (100eV and more).
ions can be produced
by
Low translation energy
magnetoplasmadinamic
accelerator
intended for a ground based ATOX simulation [6].
self-induced magnetic field MPD thruster type.
Jet
energy
is
expected
(MPDA)
The accelerator
The
oxygen
to be in the range 3-15eV.
oxygen gas operating the MPDA electrodes effects
is
plasma
Howerer when
limit
pertormance
duration (some minutes) and result in jet contaminations.
Oxygen plasma source design and
performance
This paper concerns the oxygen plasma source advanced. Contrary
to the truster above mentioned the
source
is
based
on
external
magnetic field MPDA configuration [7].
Inside the MPDA the regimes of
acceleration
flows with velocity of 5-20 km/s are known [7].
of this type you can create
along
the
lines
of
conditions
magnetic
forces
when
and
field
and
electron
plasma
air
In the accelerator
the
electrons
move
their directions to a
considerable extent coincide with the lines of
electric
wi.th
plasma
flows.
The
temperature are not great and it
possible to make flows with energy not higher than
ionization
is
one
[8].
When developed the main difficult problems
thermoelectron
arc
cathode
were
to
design
a
with a long lifetime in the oxidative
arc and to purify oxygen plasma jet of contaminants.
The development
working
of
directly
durability
to
traditional
in
the
MPDA
oxidative
scheme
are
with
permited
the filament
the
source
be no more than a few hours due to cathode erosion.
Zirconium or hafnium cathodes known as thermo chemical have a
durability
in
atmospheric
oxygen
arc
long
owing to protective oxide
layer creation and maintenance in oxidative plasma.
The application
of the thermochemical Hf emitter oxygen failed
to increase the source lifetime because of the destruction
1
of
the
protective emissive layer by ion bombardment in low pressure arc of
MPDA
.e
t'e
[9].
long
hollow
emItter
lifetime
inert gas
cathode
prototype [10].
of
5-.
th
cathode was
developed
,mp
to
lOOh)
employed. The
for
the
"FOBOS
was
ensured w en
diaphragmed
program
LaS
served
as
The finished cathode assembly of the source is mace
in the form of a diaphragmed Mo pipe
which is half filled with LaB6
grains. The pipe is surrounded with the spiral start-up
electric
heater. The cathode assembly is arranged inside
the cylindrical
thermoscreen. The inert gas fills the screen hollow
flowing through
the pipe, and in such a way it protects not only
LaBg emitter but
also the exterior pipe and heater surfaces inside
the screen.
In
this way the source life time in between
LaB 6 grains substitutions
is found to be near 100 hours in dependence on cathode
flow-Ar-gas
Purity.
The plasma jet contamination problem being
minor
is important
for
technology source.
for
thruster
The cathode erosion products
and inert gas are the impurities of the oxygen
plasma
jet of tne
MPDA. The electrode material atoms are transported
by plasma jet to
an irradiated surface, are deposited and
disturbed its behaviors.
The inert gase is pumped poorly by some type
of vacuum pumps as
sputter-ion ones.
To reduce
the
impurities
yield
employed magnetic and geometric arc
intermediate electrode
(IE)
as
in
to
the
contraction
the
plasma
by
stream
we
ferromagnetic
duoplasmatron [11].
The
contraction creates potential barriers
limiting cathode plasma ions
(cum contaminants) moving towards the anode
plasma in which the jet
is formed. For the more complete decrease
of the neutral impurities
yield
to the anode plasma, the IE hollow is
pumped complementary.
The pumping reduces pressure in the IE and maintains in the
E'
channel gas flow which is contrary the
eiectrons[12].
ship between intracavity IE pressure
and plasma jet
The relationion
currents
I
i27
1ratio
of Xe-impurity to oxygen is shown in fig.
1.
The schematic diagram of the source is shown in figure
anode 1,
IE
2,
cathode
inside the solenoid 4.
XEOHW
OXYGEN
of
*
4.2
S-
magnetic
distribution in
hum
distribution
in the anode and
to
corresponds
downstream
the MPDA,
its
the IE and at
The
3 are arranged
The
induction
2.
and in
front of
is
it
channel
its
ana-
40
0
4
2
6
IE PRESSURF- Pa
Fig.
logous
to
the
distribution in the duo-
plasmatron.
The plasmaforming gas oxygen
is
the
in
fed
anode and the inert gas
argon or xenon runs through
by pipelines
pumped
through
The source
vacuume
line
performances
5
and 6.
by
7.
were
investigated
in
two-probe
technique,
a
the
1
vacuum
m" /s
and
three-grid energy
analyser and a monopolar mass-spectrometer [13]. The oxygen
flow measured at a distance of 0.2 m from the anode had
plasma
ion current
density up to 5 mA/cm with 3 cm diameter on level 0.7 max.
Varying
the
you can
magnetic field,
gas and electric supply consumption,
control the ion current density.
Figure 3 shows
density as
current.
2
1
Ithe
6
O
o'
5
Fig.2
a
The
the
function
ion
of
current
discharge
The average ion energy
range
+
ratio 0 /Ois
of 20-60 eV,
in
and the fluxes
at the interval of
indicated
is
0.3-3.
perfomances are obtained
with oxygen gas flow equal to 0.5 cm /s,
argon
I
cathode
The IE cavity is
facilities with differential diffusion pumping 2 and
plasma diagnostics
the
or
xenon flow equal to 0.1 cm/s,
I
6-12
A
are
current.
O-100
V
arc
voltage.
16 -
The
s '
0
o
8
impurity
materials
12
DISCHARGE CURRENT-A
not
construction
cathode
observed
massspectrometry
Fig.3
of
inert
and
were
16
ions
of
5*104 sensitivity
gas
(Xe)
during
the
plasma flow
with
limit.
Construction
materials contaminates were measured by deposition probe
with Rutherford Back Scattering method. The Fe atoms
technique
were found to
be some 4 per million oxygen ions in plasma jet [14].
The described
source
was
used
in
the
degradation of spacecraft and thermonuclear
investigations
reactor
of
materials
in
oxygen plasma flows [15,16].
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