Progress with the SNS front-end systems Page: 3 of 3
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sign are given elsewhere [Staples**]; efforts are under-
way to finalize the implementation of this scheme.
3 RFQ
The SNS RFQ is 3.72-m long overall and consists of
four modules built as composite structures with an outer
GlidCop shell and four oxygen-free copper vanes. Peak
surface fields reach 1.85 Kilpatrick, and the total rf power
consumption is 800 kW during pulses. Water-cooled Tr-
mode stabilizers separate unwanted dipole modes from
the main quadrupole mode. Static frequency tuning is
achieved by 20 slug tuners per module, and dynamic tun-
ing by adjusting the temperature difference between vane
tips and the outer walls of the modules.
Figure 4 shows the assembled first module before the
final brazing operation. This module has now been com-
missioned, reaching the full rf gradient. The resonance
frequency with slug tuners at nominal positions is very
close to the design frequency of 402.5 MHz, and the field
flatness is better than 1% peak-to-peak. The dynamic
tuning procedure involving regulation of the vane-to-wall
temperature difference was successfully tested with this
module. The other three modules are all in advanced
stages of fabrication; Module #2 has been conditioned to
full rf amplitude as well.
a t r_
- Mk W !
Figure 4: End-on view of the assembled RFQ Module #1
prior to the final brazing operation. The upstream ends of
the four vanes are seen at the center, with Tr-mode stabi-
lizers penetrating the vanes horizontally and vertically.
The first rf-accelerated SNS beam was achieved on the
first day after connecting RFQ Module #1 to the LEBT
tank. These experiments resulted in validating the struc-
ture modeling efforts and the LEBT-chopper design. As
an example, Figure 5 shows the simulated and measured
beam transmission values as a function of cavity excita-
tion. The mini-pulse rise and fall times generated by the
LEBT chopper amounted to 25 ns, twice as fast as had
been assumed for the design of the MEBT chopper target.0
FTransmission vs. Gradient
100
80
60 ______
40 ______
20 ______
0
0 20 40 60 80 10
Percentage of Operating Gradient0
Figure 5: Simulated (red squares) and measured (green
crosses) transmission values of RFQ Module #1, normal-
ized to 100% maximum, for an input beam of 35-mA.
4 MEBT
The 3.67-m long Medium-Energy Beam Transport
(MEBT) structure shown in Figure 1 has three main func-
tions, i.e., matching the beam from the RFQ exit plane
into the MEBT chopper and its target, cleanup chopping,
and guiding the remaining particles into the Drift-Tube
Linac currently being built by LANL. Matching in both
transverse and in the longitudinal direction is provided by
14 quadrupole magnets, arranged in three families, and
four rebuncher cavities. An anti-chopper will direct all
particles back on axis that were deflected by the chopper
during the rising and falling pulse flanks and not inter-
cepted by the target.
The MEBT will also contain diagnostic elements such
as beam-position monitors that will also gather phase in-
formation, profile monitors, and two fast current trans-
formers.
All MEBT elements are grouped on three rafts that can
be individually aligned. At present, most major beamline
components, including power supplies, have been fabri-
cated and received at LBNL; power tests of the first re-
buncher cavity are scheduled for June 2001.
REFERENCES
[1] R. Keller for the FES Team, "Status of the SNS Front-End Sys-
tems, Paper MOP5B04, EPAC 2000, Wien (2000).
[2] T. Mason, "The Spallation Neutron Source: A Powerful Tool for
Materials Research," Paper MOAL04, these conference proceed-
ings, PAC '01, Chicago, IL (2001)
[3] R. Becker, "New Features in the Simulation of Ion Extraction with
IGUN," EPAC '98, Stockholm (1998).
[4] J. Reijonen, R. Thomae, and R. Keller, "Evolution of the LEBT
Layout for SNS," Paper MOD19, Linac 2000, Monterey (2000).
[5] R. Welton at al., "Simulation of the ion source extraction and
low-energy beam transport systems for the Spallation Neu-
tron Source," submitted to ICIS '01, Oakland (2001).
[6] J. Boers, "PBGUNS," Thunderbird Simulations, 626 Bradfield Dr.,
Garland, TX 75042
[7] D. Dahl, "SIMION 3D v.7.0," Idaho National Engineering and
Environmental Laboratory Idaho Falls, ID 83415 (2000)
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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/m1/3/: accessed March 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.