DOE/ET-51013-209
UC20,a,b,df,g
PFC/RR-87-1
Pumping Effectiveness of a Magnesium Vapor Jet
in the Presence of a High Power Neutral Beam Source
T. J. Farish and R. S. Post
January 1987
Plasma Fusion Center
Massachusetts Institute of Technology
Cambridge, Massachusetts 02139 USA
This work was supported by the U. S. Department of Energy Contract
No. DE-AC02-78ET51013.
-2-
Abstract
An
experimental
study
of
a supersonic
magnesium
vapor
jet
neutralizer has established the effectiveness of the jet in reducing cold
streaming
gas
from
a high current neutral beam source.
The removal of
molecular hydrogen gas by a magnesium jet neutralizer provides a means of
simplifying
jet
beamline
design
by improved pumping characteristics of the
compared with conventional technology while simultaneously improving
performance.
-3-
An
experimental
study
of
a
supersonic
magnesium
vapor
jet
neutralizer has established the effectiveness of the jet in reducing cold
streaming
gas
from
a high current neutral beam source.
The removal of
molecular hydrogen gas by a magnesium jet neutralizer provides a means of
simplifying
jet
beamline
design
by improved pumping characteristics of the
compared with conventional technology while simultaneously improving
performance.
In
the
ion
a
source is used as the charge-exchange target.
prevented
plasma
from
ions
escaping
pumping
up
conventional neutralizer system, hydrogen gas issuing from
flowing
from
from
to
the
undergoing
the machine.
target
mirror
machines,
plasma in order to prevent hot
charge-exchange
reactions in the gas and
Excess gas is limited by baffling and large
speeds of the beamlines.
in
This gas must be
but
Present methods allow plasmas to build
the
plasma
lifetime is still limited by
charge-exchange.
The present experiment tests an improved version of the magnesium
with a full-scale ion beam source capable of 55 amperes extracted at
jet
16.9 kV.
a
cold
The data shows that the magnesium jet is capable of maintaining
gas pressure differential in excess of three orders of magnitude
over the full range of beam source operating conditions.
neutral
the
study
particle
fraction
Measurements of
obtainable with the magnesium jet and a
of possible magnesium plasma formation in the beam-jet interaction
region are underway.
The
valve,
is
vapor conduit and supersonic nozzle.
heated
point
magnesium vapor jet consists of four main components:
oven,
Magnesium metal in the oven
to an operating temperature of 8000 C, well above the melting
of 6540 C.
When the valve is opened,
magnesium vapor flows out of
-4-
the
oven through the conduit to the supersonic nozzle.
The vapor stream
exits the nozzle at supersonic velocities and intercepts the ion beam/gas
stream.
as
The 20 kV beam particles are neutralized by the magnesium vapor
in
a
gas
charge-exchange
cell.
From
the
point of view of the
energetic
beam atoms, the magnesium molecules are almost stationary. For
the
hydrogen
cold
magnesium
stream
molecules
are
the
an
system
mass
flow
and
gas
was
found
formidable
the
barrier.
beam,
however, the
Since the magnesium
by
of
raising the oven temperature and increasing the
the
limited
evaporated
material.
In a real-world system,
by temperature effects on reliability of components
leakage around the pumping region.
that
In
an arbitrarily large amount of gas can be pumped with
simply
rate
is
a
with
jet acts like a diffusion pump to sweep away the gas.
system,
this
pumping
presents
traveling
moving quite fast compared to the hydrogen gas molecules,
magnesium
ideal
molecules
reliability
of
the
In previous experiments,
heating
units
it
deteriorated
for
1 shows the magnesium oven, valve and heating units.
The
operating temperatures above 9000 C.
Figure
oven,
conduit
components.
and
heating
units
are made from commercially available
The electromagnetically operated valve, which consists of a
molybdenum shaft and tungsten plug, was designed for a previous prototype
and
used
with
magnesium
jet
temperature
ceramic
modification
neutralizer
of 8000 C.
fiber
operation,
no
the
system
is
the
current
oven
design.
The
designed to operate with an oven
The oven and conduit are surrounded by layers of
insulation
valve
with
is
and
a
opened
single
for 0.5
steel
to
heat
shield.
During
2 seconds per shot, with
-5-
approximately
shows
the
ion
grams
experimental
ragnesium
conduit
0.1
oven
and
beam
nozzle
is
transversely
actual
experiment
Tara
evaporated per second.
Figure 2
The
in its own vacuum chamber) is mated to a long
inside the test stand vacuum chamber.
also
A full-scale
mounted on the test stand, with the ion beam
through
delineate the chambers P1,
the
magnesium
setup used in the gas pumping experiments.
(residing
source
directed
of
the
magnesium
jet.
The baffles which
P2 and P3 have been designed so as to simulate
relative
volumes and conductances in the Tara Tandem Mirror
beamline.
In this experiment, chamber P1 corresponds to the
source tank, chamber P2 corresponds to the plug region (which is to
b-e screened by the jet) and chamber P3 represents a jet blow-down region.
The magnesium vapor is collected on a liquid nitrogen-cooled surface used
as
part
of
the
milliseconds,
module.
vacuum
and
the
jet
is
nilliseconds.
The
ion
beam source is fired for 50
the pressures in the chambers are recorded by a CAMAC
After
magnesium
system.
system
turned
has
on
returned
and
the
to
its
source
base pressure, the
fired
again
for
50
The results are shown in Figures 3, 4 and 5 which compare
the pressure in the simulated plug region for low and high beam power and
low
and
high
temperature
jet
can
brought
case
be
up
magnesium
temperature.
Figure
3
shows
a
low
(oven at 7500 C) in which some gas leakage through the
seen.
to
oven
As shown in Figure 4, when the oven temperature is
its
operating
value
of
8000 C,
the pressure pulse is
virtually
eliminated
from
ion
source
at full operating power of 55 amperes extracted at
beam
is
the simulated plug region.
16.9 kV, and the oven is at 8000 C.
In Figure 5,
the
-6-
This
reducing
the
data
cold
presence
demonstrates
of
also
in
a
magnesium
jet is capable of
full scale ion beam developed for use with the Tara
Such a gas reduction would allow a substantial
plasma lifetime.
to
work
the
hydrogen gas flow by three to four orders of magnitude in
Tandem Mirror experiment.
increase
that
reduce
cold
In addition, a magnesium jet system would
gas
flow
from a beam dump into the plasma
region.
The
oven
heaters
and valve operated very reliably with an oven
temperature of 8000 C. When the vacuum chamber was opened, all magnesium
deposits
magnesium
the
were
found
to
scavenger.
beamline
was
reside
on the liquid nitrogen panel used as a
Although no evidence of magnesium migration down
seen,
magnesium flow is underway.
a
more
sensitive
experiment to detect stray
-7-
Acknowledgment
Grateful
F. Yarworth,
with this work.
acknowledgment
is extended to Richard P. Torti, George
Vitaly J. Berkman and Marcel P. J. Gaudreau for their help
-8-
EL
VALVE
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O O O
0 0 O0
CONDUIT
0
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00
00
0
0
0
HEATERS
7
0
MG
OVEN
o
0
0
MAGNESIUM OVEN
Figure 1.
0
0
0
0
lo
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Figure 2.
-10-
PRESSURE RISE DURING SHOT - LOW BEAM POWER
10
I
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Mg jet ON, T-a750
cc
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C
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pvf~.~w
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2.2
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3.8
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TIME (SEC)
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(n'
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cc
a-
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cv,
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2.2
3.8
ThiuE (SEC)
Figure 3.
5. 4
(.U
-11-
PRESSURE RISE DURING SHOT - LOW BEAM POWER
10
g
cc
jet
ON,
T-800
C
ff
a
U'
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fir
nA
-
10
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10
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2.2
3.8
TIME (SEC)
7.0
5.4
7.0
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1g
cc
C
5.4
jet
OFF
lo's
cc
n~
cc
10-1
10-1
-1 .0
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2.2
TIME (SEC)
Figure 4.
3.8
-12-
PRESSURE RISE DURING SHOT - HIGH BEAM POWER
10
Mg jet ON, T=800
r
C
lo-s
cc
4D
a-
r--
-1.0
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2.2
TIME (SEC)
3.8
5.4
7.0
5.4
7.0
10'
jet OFF
rMm
t7i
a:
cc
C-
1
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2.2
3.8
TIME (SEC)
Figure 5.