Design Optimization and the Path Towards a 2 MW Spallation Neutron Source Page: 3 of 3
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Table 2: Beam parameter evolution and an end-to-end simulation example across SNS accelerator systems.
LEBT RFQ MEBT DTL CCL SCL 1 SCL 2 HEBT Ring RTBT Unit
Ex (in) 0.065 2.5 2.5 86.8 185.6 387 1000 1000 1000 MeV
Length 0.12 3.8 3.6 36.6 55.1 64.2 94.7 169.5 248.0 150.8 m
C, (rms) 0.09/ 0.19- 0.22- 0.27- 0.29- 0.33 0.34- 0.34- 44+ 44+ pr
0.2 0.21 0.27 0.29 0.33 0.4 0.56 0.59 44 44 pr
eu. (99%) 120+120 120+120 pr
Ax jitter (i) 0.3 0.3 0.3 0.3 mm
AE (ms) 0.005- 0.007- 0.09- 0.13- 0.19- 0.27- MeV
0.01 0.015 0.092 0.15 0.27 0.51 MeV
AE jitter (i) 1.6 0.25 MeV
AE (99%) 4 10 10 MeV
Codes used IGUN PARMTEQ PARMILA PARMILA PARMILA PARMILA PARMILA PARMILA UAL/ORBIT PARMILA
Loss (control) 0.28 - 0.04 - - - - 0-0.001 0.02-0.1 0.04
Loss (uncont.) 0.1 0.2 < 0.01 5.7e-4 1.4e-4 2e-6 1.4e-5 2.8e-5 1.4e-4 le-6
e (rms) growth 5 19 18+ 12+ 14+ 3+ 0-11 5 3 %
e (99%) growth 0-20 10 5 %4.3 Component Implementations
The implementation of ring chromatic sextupoles was
debated. Sextupoles are not used in rings like ISIS. How-
ever, as a higher intensity machine, the SNS requires un-
precedented loss tolerance, and relies upon a large momen-
tum spread for instability damping and a large momentum
aperture for beam-in-gap/momentum cleaning [10]. Chro-
maticity sextupoles, powered in four families, are thus es-
sential in avoiding resonances resulting from a chromatic
tune spread.
In order to reach a large momentum spread without in-
troducing excessive beam halo, longitudinal painting is im-
plemented. To facilitate such a painting scheme, the output
momentum jitter and spread must be strictly controlled by
an energy-correcting RF cavity synchronized to the linac
frequency at an optimized distance from the end of linac to
allow for an adequate beam-phase slippage and a moderate
RF voltage. Thus, these "corrector" and "spreader" cavities
are also essential.
4.4 Design Challenges
The effect of electron-cloud generation [11] imposes a
serious threat to a high-intensity ring like SNS. Efforts to
address this problem focus two fronts: minimization of
electron production, and enhancement of Landau damping.
Implementations to minimize electron production includes
a pair of tapered magnets for electron collection near in-
jection foil, TiN coated vacuum chamber to reduce multi-
pacting, striped (TiN) coating of extraction kicker ferrite,
beam-in-gap kicker to keep a clean beam gap (10 -4), a rel-
atively high vacuum (5x10-9 Torr), ports screening and
step tapering, installation of electron detectors for mon-
itoring, and possible installation of solenoid windings in
unoccupied straight sections. Implementations to enhance
damping includes a high RF voltage (up to 60 kV) along
with the energy spreader to provide momentum acceptance,
lattice sextupole families for chromatic adjustments, and
reserved space for possible wide-band damper systems.5 END-TO-END SIMULATION
Extensive efforts have been made to compare and de-
velop models and codes for linac simulations including
space charge and SRF, and to develop ring simulation codes
(UAL [12] and ORBIT [13]) that handle tracking and map-
ping along with space charge, painting, magnetic errors,
impedances, and collimation. Table 2 is an example of an
end-to-end simulation using various codes. The SNS beam
loss model is based partly on empirical data at existing fa-
cilities and partly on simulations [14].
6 SUMMARY
By adopting superconducting RF technology for the
linac and by fully optimizing the accumulator ring design,
the SNS is following a clear path towards a high-intensity
(2x 101 at 60 Hz), high-power (2 MW) facility.
The authors would like to thank our colleagues of the
SNS teams and our collaborators all over the world for their
contributions and helpful discussions.
7 REFERENCES
[1] R. Kustom, LINAC 2000, p.321.
[2] T. Wangler, RF Linear Accelerators, (Wiley & Sons, 1998);
Workshop on Beam Halo and Scraping, Wisconsin (1999).
[3] J. Wei et al, Phys. Rev. ST-AB 3,080101 (2000).
[4] J. Stovall et al, LINAC 2000, p.605.
[5] I. Hofmann, et al, these proceedings.
[6] J. Wei et al, EPAC 2000, p.981 and p.123.
[7] R. Sundelin; D. Jeon et al, these proceedings.
[8] S. Kim, et al, these proceedings.
[9] J. Wei, et al, these proceedings.
[10] S. Cousineau, et al, these proceedings.
[11] R. Macek, these proceedings.
[12] N. Malitsky, J. Smith, R. Talman, J. Wei, PAC 1999, p. 2713.
[13] J. Galambos et al, ORBIT User's Manual, 1999.
[14] N. Catalan-Lasheras et al, SNS Notes SNS/AP/7 (2001).
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Wei, J.; Blaskiewicz, M.; Catalan-Lasheras, N.; Davino, D.; Fedotov, A.; Lee, Y. Y. et al. Design Optimization and the Path Towards a 2 MW Spallation Neutron Source, article, June 18, 2001; Upton, New York. (https://digital.library.unt.edu/ark:/67531/metadc719059/m1/3/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.