High current density negative ion source for beam line transport study Page: 3 of 3
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transverse ion temperature on the extraction surface is
T;-3 eV. An effective brightness, B=j~/T;= 0.1 A/cm eV,
was relative high, but 10 times smaller then for a pulsed
SPS . For the beam line test the extraction aperture of 1
mm diameter was closed with 0.2 mm stainless steel foil
with 0.3 mm aperture. With this aperture the H- current in
a remote collector after the solenoid lens, collimator and
analyzer was 1 microamp, similar to the proton current
from a Duoplasmatron (3 microamp). After 20 hours of
operation the collector current started to increase and was
8 microamps after 60 hours. This current growth was due
to an aperture increase by sputtering from back
accelerated positive ions. With a molybdenum foil this
sputtering was invisible. In a clean vacuum an increasing
current in the extraction gap was observed, consisting of
electrons and ions, independent of the gas and discharge.
This emission was suppressed by a controlled leak of air.
With a 0.3 mm extraction aperture the H- beam was
transmitted through the 8 m long 1 mm aperture beam line
and final intensity was comparable with the proton beam
from the Duoplasmatron. Repeatability of the H- beam
parameters could to be unstable. In the a configuration a
discharge can start from the top exit of the hollow cathode
with a plasma drift to the right, to insulator (4), with a loss
of ion emission.
Dlscharg cu-rnt Id, A
Figure 3: Collector current versus discharge current for
SPS b with an extraction aperture of 1 mm diameter,
This loss of emission was the reason to change to
configuration b with the hollow cathode opening on only
one side. This version of the SPS was tested in another
test stand with extended operation of up to 4 weeks. This
time was limited by the cesium in the oven. For longer
operation a larger Cs supply is needed, as in the FNAL
magnetron SPS . For long operation it is important to
have at all times a low discharge voltage (Ud<90V) and
good Cs recycling. With an extraction aperture of 1 mm
diameter an extracted H- beam up to 2.5 mA was
repeatedly obtained. Dependence of the beam current at a
test stand collector on the discharge current is shown in
Figure 3. The collector current saturation is determined
not by H- stripping in the plasma, but by an increase of the
cathode temperature and increased Cs desorption.
With a new anode and extraction aperture of 0.4 mm
diameter an H- beam up to 0.9 mA was extracted
(emission current density j=0.7 A/cm2). The discharge
voltage is Ud=80 V, discharge current Id=0.5 A, power
P=40W. The production efficiency at this current density
F=j/P=17.5 A/cm2kW, is much higher than F= 0.25-0.05
A/cm2kW for a good proton source .
_ -. .lo, mkA
500 ;- lex, mA
v400 *ci, mkA
v 200 _
0 5 10 15 20
Extraction voltage Uex, kV
Figure 4: Optimization of the beam current for different
extraction voltage for SPS c with an extraction aperture of
0.4 mm diameter.
The next improvement is shown in Fig.1 c. A hollow
cathode channel was drilled normal to the conical part of
the cathode surface and 45 degrees to the axis. This SPS
minimized the distance for plasma drift to the emitter
surface. The efficiency of negative ion generation has
been improved. Examples of the collector current for
different condition of SPSc operation are shown in Figure
4. A typical discharge voltage is Ud=80V, discharge
current Id=0.4A. Increasing the H2 gas from p=10.6 Torr
to 3x10-6 Torr decreases the collector current from 0.8mA
to 0.3 mA. Traces of sputtering on the anode surface
confirms a good focusing of the negative ions by the
spherical surface of the emitter. Moving the focus point
relative to the extraction aperture could change the
intensity. Conditions for long time operation without any
change in the parameters has been found. Divergence of
this high current density beam was relative large -50
mrad in agreement with a computer simulation by
PBGUN code . With an electrostatic einzel lens after
the extractor, the H- beam can be transformed to a
converging, to parallel or to finely focused on the ruby
ceramic screen. This Semiplanatron SPS could be used for
precision optimization of low aberration beam lines and
for other applications requiring beams with high
 A.Shemyakin, A.Burov, et al., EPAC 2000, Viena,
 V. Dudnikov, Rev. Sci. Instr., 67(3), 915 (1996).
 S. Guharay et al, J.Vac.Sci.Tech.,B14(6),3907(1996).
 A.Bashkeev, V.Dudnikov, AIP Conf. No 210, p.329,
 C.W.Schmidt, V. Dudnikov, J. MacLachlan, PACO1,
 C. W. Schmidt, C. Curtis, IEEE Trans. Nucl. Sci.,
 K. Volk, PhD Thesis, University of Frankfurt, 1993.
 Jack Boers, Thunderbird Simulations, Garfield, TX.
" Ici, mA
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Dudnikov, Vadim & Wendt, Charles W Schmidt and James. High current density negative ion source for beam line transport study, article, July 25, 2001; Batavia, Illinois. (digital.library.unt.edu/ark:/67531/metadc722942/m1/3/: accessed September 19, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.