The SNS front-end, an injector for a high-power hydrogen-ion accelerator Page: 2 of 3
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Ion 'Sour-e - .
Repetition rate (Hertz) 60
Rise, fall time (ns) 10
Off/on beam-current ratio to,
The LEBT serves the main purposes 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 fo-
cusing elements was chosen for the SNS LEBT. The sec-
ond one of these lenses is split into four quadrants that
can be biased with d.c. and pulsed voltages to provide
angular steering as well as pre-chopping. The LEBT can
also be mechanically offset against the RFQ axis.
Average beam pulse-currents up to 50 mA have been
transported through the LEBT at 6% duty factor during
the dedicated commissioning phase for ion source and
LEBT. Peak beam-current values up to 68 mA have been
measured at the beginning of the pulses. The emittances
show pronounced effects of aberration, but there is rather
little beam current contained in the distorted wings.
- il Lilil
Figure 2. Ion source and LEBT.
The 3.72-m long RFQ consists of four modules, built
as composite structures with an outer GlidCop shell en-
closing four oxygen-free copper vanes. Peak surface
fields reach 36 MV/m, and the total rf power is 640 kW
during pulses. Water-cooled it-mode stabilizers  sepa-
rate unwanted dipole modes from the main quadrupole
mode. Static frequency tuning is achieved by 20 slug
tuners per module, and dynamic tuning by adjusting the
temperature difference between vane tips and the outer
walls of the modules.
All modules were conditioned together to full nominal
rf gradient at 6% duty factor, and the measured field flat-
ness is better than 1% peak-to-peak.
The RFQ was commissioned with beam at duty factors
around 0.1%. The main topics for this effort were trans-
mission vs. rf power as shown in Fig. 3, transmission vs.
injection energy, influence of LEBT steering and mat-
ching, and emittances vs. rf power and beam current.
Emittances were measured in one direction at a time,
using a rotatable slit/wire-harp device. The effects of slit
scattering as measured in dedicated tests have been sub-
tracted from all RFQ (and later MEBT) emittance data,
but no thresholding was applied on any of them. A meas-
ured vertical RFQ emittance is shown in Fig. 4.
A maximum beam current of 32 mA was recorded be-
hind the RFQ with the ion-source extraction gap in-
creased by 4 mm to obtain better LEBT matching at
moderate currents. This result indicates an actual trans-
mission through the RFQ above 90% and implicitly re-
duces the actual beam-current goal for the ion source to
about 43 mA, in order to reach 1.4-MW performance for
the SNS accelerators.
The mini-pulse rise and fall times generated by the
LEBT chopper were 25 ns, twice as fast as had been as-
sumed for the design of the MEBT chopper target.
Figure 3. Simulated and measured transmission values
III . . ..
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for the RFQ, with an input beam of 35-mA, show excel-
Figure 4. Vertical emittance of a 21.8-mA RFQ beam.
The measured rms size of 0.325 it mm mrad includes a
65-keV beam fraction that carries very little current.
The MEBT, shown in Figure 1, matches the beam from
the RFQ through the MEBT chopper system and into the
drift-tube linac currently being built by LANL. Fourteen
quadrupole magnets and four rebuncher cavities provide
transverse and longitudinal matching. An anti-chopper
directs all particles back on axis that were deflected by
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Keller, R. The SNS front-end, an injector for a high-power hydrogen-ion accelerator, article, February 1, 2002; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc740419/m1/2/: accessed April 23, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.