Progress with the SNS front-end systems Page: 2 of 3
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Table 1. FES Key Performance Parameters
Ion species H-
Output energy (MeV) 2.5
H- peak current:
MEBT output (mA) 38
Nominal ion-source output (mA) 50
Output normalized transverse rms emittance (t mm mrad) 0.27
Output normalized longitudinal rms emittance (n MeV deg) 0.126
Macro pulse length (ms) 1
Duty factor (%) 6
Repetition rate (Hertz) 60
Chopper system:
Rise, fall time (ns) 10
Off/on beam-current ratio 10-4
2 ION SOURCE AND LEBT
The H- ion source and LEBT are shown in Fig. 2. The
source plasma is sustained by pulsed 2-MHz-rf power and
confined by a multi-cusp magnet configuration. A mag-
netic dipole filter separates the main plasma from a
smaller H- production region (darker area in Fig. 2) where
low-energy electrons help generating copious amounts of
negative ions. A heated collar, equipped with eight ce-
sium dispensers, surrounds this H- production volume.
The outlet plate of the ion source contains another di-
pole-magnet configuration that creates a deflecting field
across the extraction gap in order to separate extracted
electrons from the ion beam and steer them towards a
'dumping' electrode biased at 5 kV with respect to the
outlet plate. Because this dumping field steers the ion
beam as well, the entire plasma generator is tilted at an
adjustable angle of about 30 against the LEBT axis to
compensate for this effect.
Plasma DumpIn magnets Second lens/steerer/chopper
Permanent magnets Cesiumcollar Dumpingelectrode Frsliens Chopper target/RFQ
entrance flange(qron.d)
Window\
RF antenna Flter rngnets Outlet eectrode Extracto electrode. Gru eletr d.
Figure 2: Schematic of the ion source and LEBT. Note
that the actual filter and electron-dumping magnetic fields
are oriented orthogonally to the illustration plane. The
width of the ion beam is exaggerated in this schematic to
emphasize the focusing action of the double-lens system.
The LEBT structure has to serve five main purposes,
i.e., beam formation, 2-parameter matching into the RFQ,
steering in angle and transverse offset, pre-chopping, and
gas pumping. A fully electrostatic system with two einzel
lenses as focusing elements was chosen for the SNS
LEBT. The second one of these lenses is split into four
quadrants which can be biased with d.c. and pulsed volt-
ages to provide angular steering as well as pre-chopping.
The LEBT can also be offset against the RFQ axis.The last LEBT electrode is part of the RFQ entrance
wall, and on its upstream side it carries a diagnostic ele-
ment made again from four insulated quadrants. During
the pauses in between mini pulses, chopping voltages of
2.5-kV amplitude and 300-ns duration are applied to op-
posing pairs of lens quadrants in a rotating pattern, di-
recting the chopped beam alternatingly towards each of
the four separation zones between the diagnostic-electrode
quadrants. In this way, any parts of the beam that are not
intercepted by the diagnostic electrode are prevented from
hitting the RFQ vane tips whose accurate shapes could
otherwise gradually be eroded by sputtering.
The LEBT-electrode shapes were optimized by simu-
lating proton beams, using the 2-d positive-ion code
IGUN [3] in a novel way that allows introduction of finite
ion temperatures into the calculation without experiencing
unrealistic deformations of the plasma meniscus [4]. De-
tails of the beam-formation and electron-dumping proc-
esses were modeled [5] using the 2-d code PBGUNS [6]
with actual H- ion species input and handing the trajectory
data over to the 3-d code SIMION [7]. These simulations
were helpful to determine the source tilt-angle and prove
that no lateral axis offset was needed to obtain low emit-
tances.
The ion source and LEBT have been commissioned,
and average beam pulse-currents of 50 mA have been
obtained at 6% duty factor and transported through the
LEBT. Our simulations predict transmission values >85%
through the RFQ with the actual emittances measured by
an Allison scanner and shown in Figure 3. Peak beam-
current values up to 68 mA were measured at the begin-
ning of the pulses.Vytim ,,
63713 rt im mradHorizontal
0.33 n mm mradFigure 3: Transverse emittances of a 50-mA beam, taken
at the LEBT exit.
The remaining issues for the ion source are the layout
of the rf impedance matcher and facilitation of the dis-
charge ignition. The most promising approach consists in
using low-amplitude cw rf-power of 13.56-MHz frequen-
cy to create a steady-state, low density plasma with neg-
ligible H- abundance, in addition to the main 2-MHz
pulsed power. Details of a newly developed matcher de-
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Keller, R.; Abraham, W.; Ayers, J. J.; Cheng, D. W.; Cull, P.; DiGennaro, R. et al. Progress with the SNS front-end systems, article, May 1, 2001; California. (https://digital.library.unt.edu/ark:/67531/metadc717726/m1/2/: accessed April 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.