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BEAM STUDIES AT THE SNS LINAC

BEAM STUDIES AT THE SNS LINAC. Yan Zhang On behalf of the SNS team. 42nd ICFA, HB2008, Nashville, USA, August 25-29, 2008. Outline. Introduction to the SNS Linac Longitudinal Beam Dynamics Studies Transverse Beam Dynamics Studies Unsolved Puzzles Summary. The SNS Linac. To Ring.

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BEAM STUDIES AT THE SNS LINAC

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  1. BEAM STUDIES AT THE SNS LINAC Yan Zhang On behalf of the SNS team 42nd ICFA, HB2008, Nashville, USA, August 25-29, 2008

  2. Outline • Introduction to the SNS Linac • Longitudinal Beam Dynamics Studies • Transverse Beam Dynamics Studies • Unsolved Puzzles • Summary Presentation_name

  3. The SNS Linac To Ring 402.5 MHz 805 MHz HEBT MEBT DTL CCL RFQ SCL, ß=0.61 SCL, ß=0.81 Linac dump Injector 2.5 MeV 86.8 MeV 186 MeV b=0.55 391 MeV b=0.71 1 GeV b=0.87 • Length ~260 m, 96 independently phased RF cavity/tanks • Normal conducting linac from the H- ion source to 186 MeV • Superconducting linac from 186 MeV to 1 GeV • Beam commissioning of the SCL began in August 2005 • Achieved the design repetition rate 60 Hz,maximum beam energy 1.01 GeV, peak beam current 40 mA, pulse length 1 ms, beam power on the mercury target 520 kW. Presentation_name

  4. Longitudinal Lattice RMS phase in the warm linac, linear map and zero current Presentation_name

  5. Y. Zhang, S. Henderson, this proceedings. Phase oscillation in one of the SCLcommissioning lattice Presentation_name

  6. Longitudinal beam emittance increase in the SCL commissioning lattice IMPACT, beam current 20 mA Presentation_name

  7. Y. Zhang, et. al., NIM B 261 (2007) Beam phase oscillation and damping are sensitive to RF errors Presentation_name

  8. SCL phase oscillation measured in February 2006 First cavity gradient 10% & 15% reduction, model & measurement Presentation_name

  9. A. V. Aleksandrov, et. al., EPAC2008 RF Shaker, warm linac model prediction and BPM measurement Presentation_name

  10. SCL phase oscillation, dominated by random errors (~2 phase, ~2% amplitude) and likely caused by RF driftings Presentation_name

  11. Longitudinal Beam Emittance Measurement A. V. Feschenko, et. al., PAC2007 Beam phase profile measured with a BSM in CCL1 Presentation_name

  12. Y. Zhang, et. al., submitted to PRST-AB Model predicted the SCL longitudinal acceptance BCM measured acceptance SCL longitudinal acceptance measurement with BCM and BLMs Presentation_name

  13. Bunch shape measurement Energy profile measurement Bunch shape and beam energy profile measured with BCM Presentation_name

  14. Longitudinal beam contours at the SCL entrance (Cav_01b) RMS emittance: 2.9 MeV*deg ~ 2.2 times the nominal design Cav_01a reference phase set at -85, -90 and -95 degree Presentation_name

  15. 12/23/2007 Phase tails > 40 deg 06/15/2008 Presentation_name

  16. 12/23/2007 Energy tails > 3 MeV 06/15/2008 Presentation_name

  17. Transverse Beam Dynamics Beam emittance and twiss parameter measurement at MEBT • Multiple wire scans measure beam profiles, and fit the measured beam RMS size with the accelerator models • On-line model in the XAL is based on TRACE3D. • Transverse – longitudinal phase space coupling. • Higher order effects: emittance growth in RF gaps, chromatic aberrations in “short” quadrupoles. Presentation_name

  18. Y. Zhang, J. Qiang, this proceedings MEBT beam parameter measurement using the IMPACT model Presentation_name

  19. Beam Trajectory Correction T. Pelaia, et. al., ICALEPCS 2007 Warm Linac SCL Presentation_name

  20. A. Shishlo, A. V. Aleksandrov, EPAC2008 x Before quads shaker Y x After quads shaker - model based alignment Y Presentation_name

  21. Unsolved Puzzles Linac residual activations after neutron productions Presentation_name

  22. SCL Longitudinal Acceptance and … SCL Acceptance, MEBT Particles and CCL Particles Presentation_name

  23. Longitudinal emittance in the warm linac The nc. linac could be a halo filter (scrapper) if no error existed Presentation_name

  24. RFQ Beam Tails plus Linac RF Errors ? Beam loss in the linac, No Error, RF Error, MEBT RBs gradients Presentation_name

  25. Beam Loss In the SNS Linac (500 kW) Presentation_name

  26. Partially chopped beams ? CCL1 BPM amplitude and phase measurement for 4 mini-pulse Presentation_name

  27. Increase the SCL acceptance ? s = -35 (design: -20), sacrifices ~100 MeV output energy Presentation_name

  28. Summary • Beam dynamics studies at the SNS linac are performed with several conventional and newly developed techniques. • Beam longitudinal tails/halo shown in the measurements, but the exact cause of the longitudinal halo is not fully understood. • Simulation study of halo in the linac does not have a complete picture, might need 3D particle tracking from the ion source. • Characterize beam halo in the order of 10-5 to 10-4 and reduce the fractional beam loss to below 10-4 in the SCL is a challenge, existing problems should be fixed first. E.g., MEBT rebuncher, LEBT and MEBT beam chopper, performance of the RFQ, and transverse beam matching through the entire linac correctly… • Beam loss in the linac especially in the SC linac is one of the major concerns to further ramp up the beam power, a factor of two to three beam loss reduction is needed. Suggestions and ideas are welcome. Presentation_name

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