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22/03/1999A.Blas1 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
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22/03/1999A.Blas2 Hollow bunches (history) F. Pedersen (PSB 1978): h = 5 bucket deposit in a 50 MeV coasting beam and acceleration with h = 5 and 7.10 10 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 f S 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.
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22/03/1999A.Blas3 Pre-requisite BTFM: longitudinal beam transfer function measurement (H. Schonauer, M. Sjöström)
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22/03/1999A.Blas4 Bucket deposition
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22/03/1999A.Blas5 Bucket deposition
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22/03/1999A.Blas6 Bucket deposition
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22/03/1999A.Blas7 Bucket deposition (rf set-up) 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 Time (s) Voltage (kV) 310 315320325330335 Time (s) Frequency (kHz) 310 315320325330 Frequency of the “synchronous” particle C16 frequency Sweep time 40 kHz 4 8 C16 C02 C04 injection
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22/03/1999A.Blas8 1. Single Harmonic 1.2 1D Line density 1D Line density
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22/03/1999A.Blas9 1. Single Harmonic C02=8 kV C16=6 kV Red: 8 ms sweep Black: 1 ms sweep Energy = 50 MeV 3 turns injected (?) 2D density profiles Synchrotron frequency 2D density profiles
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22/03/1999A.Blas10 2. Dual Harmonic 2.4 Line density 2D density profile1D line density
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22/03/1999A.Blas11 2. Dual harmonic 2.1 Amplitude distribution 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 Energy: 50 MeV 3 turns injected 2D Density Profile Synchrotron frequency
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22/03/1999A.Blas12 2. Dual Harmonic 2.2 Emittance 2D density profiles
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22/03/1999A.Blas13 2. Dual harmonic 2.3 Phase space images
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22/03/1999A.Blas14 3. Dual harmonic instability 3.1 Time development C02=8 kV C04=4 kV Energy: 212 MeV at instability (green) 2D density profiles Synchrotron frequency Before instability During instability After instability 129 MeV212 MeV655 MeV
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22/03/1999A.Blas15 3. Dual harmonic instability 3.2 “Phase space” before, during and after
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22/03/1999A.Blas16 3. Dual harmonic instability 3.3 Instability frequency Basic repetitions rate: 4184.3 Hz If octupolar: f = 4184.3 / 4 1046 Hz f synch. =1046 Hz
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22/03/1999A.Blas17 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
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