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A 100-mA
Institute
of Nuclear
NEGATIVE
G. I. Dimov,
Physics,
Siberian
on Mu~eah
Science,
HYDROGEN-ION
Vo.!..NS-24,
SOURCE
No.3,
June 1977
FOR ACCELERATORS
G. Ye. Dereviankin,
and V. G. Dudnikov
Division,
USSR Academy of Sciences,
Novosibirsk
In the Institute of Nuclear Physics of the USSR
Academy of Science a surface-plasma
source of Hions has been developed,
designed for powerful linear
accelerators
(meson factories)
and also for circular
accelerators
with charge-exchange
injection
of protons,
especially
for boosters of the proton synchrotrons.
Parameters
of the source (nominal values are given in
parentheses)
are: repetition
rate of up to 100 (100) Hz,
current pulse duration of 100-300 (ZOO) msec , output
current of the H- ion beam of up to 150 (100) mA, ion
energy of IO-30 (20) keV.
The source, as is shown in Fig. 1, is mounted
on a metal flange (1) of the forinjector
accelerating
tube (2) inside a cylindrical
shield (3). The magnet
yoke (5) is suspended to the flange on a bar (4). The
magnet is excited by the water-cooled
coils placed in
vacuum-tight
metal housings (6). The magnet is the
most massive part (40 kG). A gas-discharge
chamber
(7) which is fixed on the magnet with high-voltage
A
insulators
(8) is a direct source of the H- ions.
negative-ion
beam is extracted
through the emission
slit of the gas-discharge
chamber with electrodes
(9)
The gas-discharge
chamber
installed
on the magnet.
is under the negative potential
of up to 30 kV. The
beam of H- ions passes between magnet poles (10)
where it is turned at 90’ by the magnetic
field and
injected through holes in the yoke (5) and the shield(3)
into the first accelerating
gap of the tube (2). The
mean radius of the ion trajectory
in the magnetic field
is 7 cm. A gap between the poles on this radius is
field with the
3 cm. The poles form the magnetic
radial droop index n = 1. By virtue of this, together
with the beam separation
in this field an onedimensional
focusing of a beam in the plane parallel
to
for such
the field is carried
out. The necessity
focusing is connected with a large divergence
of the
In the plane
beam in an extraction
gap in this plane.
perpendicular
to the field the beam divergence
is
At the accelerating
tube input the ion
close to zero.
beam has a nearly round cross section of about l-cm
At the sides of the poles (10) the small
diameter.
rectangular
pole pieces are installed
to form the magThe nominal
netic field in the gas-discharge
region.
bending magnetic field on the mean ion trajectory
is
2.9 kG, and about 1 kG in the gas-discharge
chamber
The gas-discharge
chamber
(7) is assembled
region.
together with small pole pieces and extracting
electrodes (9) and installed,
as a unit, on poles (10). The
gas-discharge
chamber feeding with hydrogen,
cooling
air, water, and voltage is performed
through a parThe beam current
tition insulator
in the flange (12).
control at the source output is performed
by the
Rogovsky coil ( 11). It is assumed that the hydrogen
from the source will be pumped out through the holes
in a cylindrical
shield (3) and thrn through an accelThe mean hydrogen
pressure
inside the
erating tube.
shield should be maintained
at the level of 10m4 Torr.
At this pressure
the necessary
hydrogen pumping speed
is up to 2000 l/set.
90, USSR
The source of H- ions has been described
earlier
in Ref. 1. To increase the source lifetime
and make
better ion-optic
characteristics
of a beam, the source
’
design has, to a considerable
‘extent, been improved.
The improved
design of the gas-discharge
chamber
with extracting
electrodes
is presented
in Fig. 2. The
Penning type gas-discharge
cell is mounted in a
chamber body (1) to which the wall with the emission
slit (2) is welded.
An anode insert (3) embracing
molybdenium
cathode (4) is placed into the chamber
body.
There is a slot, in the cathode, parallel
to the
emission
slit, 15 mm long and 5 mm wide.
The
anode insert crosspiece
of 4 mm wide passes through
this slot near its bottom.
A lower free part of’the slot,
adjacent to the emission
chamber wall, 2.7 mm deep, is
is a gas-discharge
region.
Its volume is 2.7X 5 X 15
mm3.
The active surface area of each cathode formed
by the opposite walls of the slot is 2.7x 15 mm2.
The
magnetic field formed by poles ( 10) and lugs (11) is
directed normally
to the active cathode surfaces,
and
in the extracting
gap it is concave toward the emission
The emission
slit in the chamber wall (2) is perslit.
pendicular
to the magnetic field; it has the dimensions
of 0.5mmx
10mm (the thickness
of its edges is 1 mm).
In the wall (2) behind the emission
slit, the anode
groove is made of 1 mm deep.
cell has been
The geometry
of the gas-discharge
chosen according
to the concepts of the surfaceplasma source operation
principles2
taking into account
conditions
for the discharge
ignition and burning as
well as the work duration.
The H- ions producing
on
electrodes
due to the secondary
emission
and the
reflection
of hydrogen particles
come to the anode
groove at a relatively
high energy.
As a result of
charge exchange of these ions with atomic hydrogen,
slow negative
ions are produced in the anode groove.
Then slow negative ions come through the emission
slit to the extraction
gap.
Both direction
and divergence
of the beam being
extracted
in the plane parallel
to the magnetic field,
are sensitive
to the extraction-gap
geometry.
To
maintain
the beam geometry
invariable
a very fast
fixation
of extracting
electrodes
(13) and the emission
wall of the gas-discharge
chamber (2) are necessary
as well as the limitations
for their distortions
due to
gap is
The geometry
of the extraction
heating.
accepted to be similar
to the Pirs geometry;
the
extraction
gap length is 1.5 mm, the gap between
extracting
electrodes
is 0.8 mm.
In the nominal regime the mean power of the gasdischarge
is 250-300 W. According
to the measurements3 70-75s of this power is released on the cathode.
The cathode is cooled with the cooler (5) which has a
thermal
contact with the cathode and through which the
The cathode temperature
is mainwater is passed.
tained at the level of 600’ C. The anode is cooled with
wall
air passed through channels (14) in the emission
1545
With the hydrogen density increase
in the discharge
and the magnetic
field decrease,
the random fluctuations are first converted
into harmonic
oscillations
with frequences
of 17-18 mHz and decreased
then
below the recorded
level.
The hydrogen density in the
discharge
chamber necessary
for the noiseless
regime is about 1.5 times higher than that required
for
the discharge
ignition.
of the chamber (2). The temperature
of this wall is
maintained
at the level of $00’ C, and together
with
the limitation
for its thermal
expansion,
provides
optimum
covering
of cathode surfaces by cesium and
also limits the Cs-vapor
yield through the emission
slit within time intervals
between the gas-discharge
pulses.
Cesium is led in the gas-discharge
region along
the channel in the anode, through the metal wick from
the preheated
container
with cesium (17). The cesium
consumption
is 0.1 g per 100 hours of the source
operation.
The hydrogen is led through channels of
the anode insert crosspiece
in the gap between the
The gas efficiency
of the
crosspiece
and cathode.
source is rather high due to separation
of the hydrogen inlet from the emission
slit by the gas-discharge
region in which the hydrogen molecules
are efficiently
ionized by electrons.
The hydrogen filling is performed
in 200 msec
pulses with an electromagnetic
valve4 at a large work
resource
(more than 109 switchings).
The hydrogen
consumption
is mostly determined
by a volume of the
gas-discharge
cell and by a hydrogen density necessary
for the discharge
ignition and pulse frequency.
The
hydrogen consumption
is about 1 cm3 Torr per
impulse.
The source and extraction
voltage is provided
by
the pulsed electric
power supply.
Voltages on the
discharge
and extraction
gap as well as corresponding
currents
are presented
by oscillograms
in Fig. 3. The
discharge
voltage is close to 100 V, the discharge
current in the nominal regime is 100-120 A. The
extraction
circuit
current is 200-200 mA.
The reduced microperviance
of the nagative-ion
beam with nominal parameters
is quite high, about
1.5. This means that for the beam transportation
either a very strong focusing or compensation
of the
negative space charge in a beam with positive
ions is
Accumulation
of positive
ions in the beam
required.
due to the residual
gas ionization
is accompanied
with
elimination
of some fraction
of negative ions in the
Therefore,
with the compensation
time
beam.
decrease due to the residual
gas-density
increase,
the
beam attenuation
becomes higher.
At the residual
gas
pressure
of 5x 10-5 Torr,
the observed compensation
time of the beam is several tens of microseconds:
the
output current
of negative ions in the beam (25 cm
from the emission
slit) is 90-950/o of the total current
With the pressure
increase
of the extracted
H- ions.
up to 10m4 Torr,
the compensation
time decreases
respectively,
and the output current is reduced down
to 80% of the total current of the extracted
H- ions.
If the low-frequency
fluctuations
are available
in the
discharge
and the residual
gas pressure
around the
beam is several times lower than 10-4 Torr,
the
negative-ion
beam diameter
at the output increases
up
to 3.5 cm, and its normalized
emittance
reaches the
value of 4X 10-5 cm rad X 2 X 10-4 cm rad (the first
number concerns the direction
parallel
to the magnetic
field, the second the perpendicular
direction).
With
the residual
gas pressure
increase
up to the value of
iOv4 Torr,
the beam emittance
decreases
1.5-Z times
for each of the directions,
At the noiseless
region of
the discharge
the measured
normalized
beam emittance
at the output is equal to 3 X 10-6 cm rad X 2 X 10w5 cm
rad5 at nominal parameters.
The current distribution
in the output beam cross section is given in Fig. 4
where it is seen that the beam diameter
is close to 1
cm. The total angular spread of H- ions at the output
is iOv3 rad along the magnetic
field and 10m2 rad
across the field.
The transverse
energies on the
emission
surface are - 8 and -2 eV, respectively.
The
observed dependence of the phase volume of a negative
ion beam on the noises in the discharge
and the gas
density around the beam is connected with emission
surface fluctuations
and spreading the ions in the
angles on undercompensated
and overcompensated
domains of the beam which are formed due to the
fluctuations
of its intensity.
The arise of instability
of a compensated
beam is also possible.
Resource tests of the source have shown that the
erosion of the gas-discharge
chamber components
does not lead to significant
change in its parameters
during 300 hours of continuous work.
References
A phase volume of the output beam considerably
depends on the level and kind of noises in the gasdischarge
as well as on the gas density in the beam
region.
This discharge
in hydrogen with Csadmixture
at the lower hydrogen density and higher
strength of the magnetic field is characterized
by the
random fluctuations
of volta e and discharge
current
The fluctuation
level
with frequencies
of i05- 10BHz.
ln this case, the
may be of several tens per cent.
beam intensity
of the extracted
H- ions fluctuates
too.
1546
i.
V. G. Dudnikov,
Trudy 4-go Vsesoyuznogo
soveshchania
po uskoriteliam
zariazhennykh
chastits,
Moscow,
1975, tom I, p. 323.
2.
Y. I. Bel’chenko,
V. G. Dudnikov,
68 (1975).
G. I. Dimov,
and
Zhurnal Tekhnicheskoi
Fiziki
45,
3.
Y. I. Bel’chenko
and V. G. Dudnikov,
IV
Vsesoyusnaia
konferentsia
po vzaimodeystviyu
atomnykh chastits s tverdym
telom,
Khar’kov,
1976, chast’ 3, 180.
4.
G. Ye. Dereviankin,
V. G. Dudnikov,
and
P. A. Zhuravlev,
Pribory
i tekhnika
eksperimenta,
1975, N 5, 168.
5.
G. Ye. Dereviankin,
V. G. Dudnikov,
and
V. S. Klenov,
Preprint
IJaF, 77-8, Novosibirsk,
1977.
I
0
I
L I I
5
to
I
d
-t-\. :*I,
ii_.-.-..
t.ti-T_.
i-m-
L
20 GM
Fig. 1. Source Design.
1 - high-voltage
flange of
accelerating
tube; 2 - accelerating
tube; 3 - source
shield; 4 - supported bar; 5 - magnet yoke; 6 - magnet coils; 7 - gas-discharge
chamber;
8 - highvoltage insulators;
9 - extracting
electrodes;
10 bending magnet poles ; 11 - Rogovsky coil; 12 partition
insulator.
‘I-
Fig.
Oscillograms:
Up - voltage on gas-discharge
current;
U, - extracting
voltage;
- discharge
I - &rent
in the extraction
circuit;
I- - current
in
I?- beam at the source output.
gap;
I_-_LL--.
r I 2
J
3.
I
1c.
Source.
1 - gasFig. 2. Design of Negative-Ion
discharge
chamber body; 2 - emission
wall of chamber; 3 - anode insert; 4 - cathode; 5 - Cooler of
cathode; 6 - cathode insulator;
7 - high-voltage
insulators; 8 - corbel-pieces
embracing
insulators;
9 shields of insulators;
10 - magnetic
pole pieces; 11 lugs of poles; 12 - base piece; 13 - extracting
electrodes; 14 - channels for cooling anode; 15 - tubes
for cooling anode; 16 - tubes for cooling cathode;
17 - cesium feeder; 18 - hydrogen valve; 19 - hydrogen inlet pipe; 20 - tubes for cooling valve.
-45
0
UC5
XCM
-0.5
10.5
f”
1
in the
Fig. 4. The negative ion current distribution
output beam cross section.
On the left - distribution
along the direction
parallel
to the magnetic field.
On
along the direction
perpendicthe right - distribution
ular to the magnetic
field.
1547