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Hollow bunches A. Blas, S. Hancock, S. Koscielniak, M. Lindroos, F. Pedersen, H. Schonauer

Hollow bunches A. Blas, S. Hancock, S. Koscielniak, M. Lindroos, F. Pedersen, H. Schonauer. Why: to improve space charge related problems. How: Increase the I mean / I peak = BF value by creating a hollow distribution in the longitudinal phase space. Hollow bunches (history).

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Hollow bunches A. Blas, S. Hancock, S. Koscielniak, M. Lindroos, F. Pedersen, H. Schonauer

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  1. Hollow bunchesA. Blas, S. Hancock, S. Koscielniak, M. Lindroos, F. Pedersen, H. Schonauer • Why: to improve space charge related problems. • How: Increase the Imean / Ipeak= BF value by creating a hollow distribution in the longitudinal phase space A.Blas

  2. Hollow bunches (history) • F. Pedersen (PSB 1978): h = 5 bucket deposit in a 50 MeV coasting beam and acceleration with h = 5 and 7.1010 p (no fast FB!) . • K. Schindl (PSB 1978): Use of dual harmonic debuncher in the Linac transfer line • Results: loss of hollowness when closing any loop. Abandoned because of the success of dual harmonic (h5 + h10) operation. • R. Garoby, S. Hancock (PS 1992): Phase shaking at ~ 0.94 fS and homogenization with 200 MHz cavity. Very successful from 1GeV to 26 GeV trough transition. • S. Hancock (PSB 1997): same method as above, but h=16 frequency control too poor at that time to get proper results. A.Blas

  3. Pre-requisite • BTFM: longitudinal beam transfer function measurement (H. Schonauer, M. Sjöström) A.Blas

  4. Bucket deposition A.Blas

  5. Bucket deposition A.Blas

  6. Bucket deposition A.Blas

  7. Bucket deposition (rf set-up) Voltage (kV) 8 • An “empty” bucket is brought into the injected beam before capture • C16 cavity for “hole-generation” • C02 cavity for acceleration (h=1) • C04 cavity for bunch shaping C02 4 C16 C04 Time (s) 310 315 320 325 330 335 Frequency (kHz) Sweep time C16 frequency 40 kHz Frequency of the “synchronous” particle injection Time (s) A.Blas 310 315 320 325 330

  8. 1. Single Harmonic1.2 1D Line density 1D Line density A.Blas

  9. 1. Single Harmonic • C02=8 kV • C16=6 kV • Red: 8 ms sweep • Black: 1 ms sweep • Energy = 50 MeV • 3 turns injected (?) Synchrotron frequency 2D density profiles 2D density profiles A.Blas

  10. 2. Dual Harmonic2.4 Line density 2D density profile 1D line density A.Blas

  11. 2. Dual harmonic2.1 Amplitude distribution • Energy: 50 MeV • 3 turns injected • C02=8 kV • C04=4 kV • C16 • red: 0 kV and no sweep • green: 3 kV and 0.5 ms sweep • blue: 3 kV and 8 ms sweep • lilac: 6 kV and 8 ms sweep 2D Density Profile Synchrotron frequency A.Blas

  12. 2. Dual Harmonic2.2 Emittance 2D density profiles A.Blas

  13. 2. Dual harmonic2.3 Phase space images A.Blas

  14. 3. Dual harmonic instability3.1 Time development • C02=8 kV • C04=4 kV • Energy: 212 MeV at instability (green) Before instability During instability After instability Synchrotron frequency 2D density profiles 129 MeV A.Blas 212 MeV 655 MeV

  15. 3. Dual harmonic instability3.2 “Phase space” before, during and after A.Blas

  16. 3. Dual harmonic instability3.3 Instability frequency Basic repetitions rate: 4184.3 Hz If octupolar: f = 4184.3 / 4  1046 Hz fsynch.=1046 Hz A.Blas

  17. Conclusion • Bunching factor is increased 0.28 to 0.38 in the single harmonic case 0.45 to 0.55 in the dual harmonic case (values at 100 MeV) • Single harmonic acceleration was successful ...but loss of hollowness after 390 ms • Dual harmonic operation unstable ...from 50 ms to 250 ms A.Blas

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