22/03/1999a.blas1 hollow bunches a. blas, s. hancock, s. koscielniak, m. lindroos, f. pedersen, h....
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22/03/1999 A.Blas 1
Hollow bunchesHollow bunchesA. Blas, S. Hancock, S. Koscielniak, M. Lindroos, F. Pedersen, A. Blas, S. Hancock, S. Koscielniak, M. Lindroos, F. Pedersen,
H. SchonauerH. Schonauer
Why: Why: to improve space charge related to improve space charge related problems.problems.
How: How: Increase the IIncrease the Imean mean / I/ Ipeakpeak= BF value by = BF value by creating a hollow distribution in the creating a hollow distribution in the longitudinal phase spacelongitudinal phase space
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Hollow bunches (history)Hollow bunches (history)
F. Pedersen (PSB 1978): F. Pedersen (PSB 1978): h = 5 bucket deposit in a 50 MeV coasting h = 5 bucket deposit in a 50 MeV coasting beam and acceleration with h = 5 and 7.10beam and acceleration with h = 5 and 7.101010 p (no fast FB!) . p (no fast FB!) .
K. Schindl (PSB 1978): K. Schindl (PSB 1978): Use of dual harmonic debuncher in the Use of dual harmonic debuncher in the Linac transfer lineLinac transfer line
ResultsResults: : loss of hollowness when closing any loop. Abandoned because loss of hollowness when closing any loop. Abandoned because of the success of dual harmonic (h5 + h10) operation.of the success of dual harmonic (h5 + h10) operation.
R. Garoby, S. Hancock (PS 1992): R. Garoby, S. Hancock (PS 1992): Phase shaking at ~ 0.94 fPhase shaking at ~ 0.94 fSS and and homogenization with 200 MHz cavity. Very successful from 1GeV homogenization with 200 MHz cavity. Very successful from 1GeV to 26 GeV trough transition.to 26 GeV trough transition.
S. Hancock (PSB 1997): S. Hancock (PSB 1997): same method as above, but h=16 same method as above, but h=16 frequency control too poor at that time to get proper results.frequency control too poor at that time to get proper results.
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Pre-requisitePre-requisite
BTFM: BTFM: longitudinal beam transfer function longitudinal beam transfer function measurement (H. Schonauer, M. Sjöström)measurement (H. Schonauer, M. Sjöström)
Energy width measurement for 1.1 turns injected (1E12 pp).Left fig: BTFM (550 keV FW) right fig: Linac spectrometer (405 keV FW).
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Bucket deposition Bucket deposition T [eV]
t [s]
C16 Buckets
Coasting beam
T [eV]
t [s]
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Bucket depositionBucket deposition
Computer simulation of longitudinal phase space after deposition of V16=1kV empty buckets; T=5ms
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Bucket depositionBucket deposition
Energy spectrum (from BTFM) after deposition of empty buckets with V15
=4kV, 8ms frequencysweep
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Bucket deposition (rf set-up)Bucket deposition (rf set-up)
An “empty” bucket is An “empty” bucket is brought into the brought into the injected beam before injected beam before capturecapture C16 cavity for “hole-
generation” C02 cavity for
acceleration (h=1) C04 cavity for bunch
shaping
Time (s)
Voltage (kV)
310 315 320 325 330 335
Time (s)
Frequency (kHz)
310 315 320 325 330
Frequency of the “synchronous” particle
C16 frequency
Sweep time
40 kHz
4
8
C16
C02
C04
injection
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1. Single Harmonic1. Single Harmonic1.2 1D Line density1.2 1D Line density
1D Line density
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1. Single Harmonic 1. Single Harmonic
C02=8 kVC02=8 kV C16=6 kVC16=6 kV
Red: 8 ms sweep Black: 1 ms sweep
Energy = 50 MeVEnergy = 50 MeV 3 turns injected (?)3 turns injected (?)
Red Black
Emittance(90%) eVs
1.09 1.15
Emittance(rms) eVs
1.80 1.39
2D density profiles
Synchrotron frequency
2D density profiles
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2. Dual Harmonic2. Dual Harmonic2.4 Line density2.4 Line density
2D density profile 1D line density
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2. Dual harmonic2. Dual harmonic2.1 Amplitude distribution2.1 Amplitude distribution
C02=8 kVC02=8 kV C04=4 kVC04=4 kV C16C16
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 MeVEnergy: 50 MeV 3 turns injected3 turns injected
2D Density Profile
Synchrotron frequency
fS
p/eV.s
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2. Dual Harmonic2. Dual Harmonic2.2 Emittance2.2 Emittance
Red Green blue lilac
Emittance(90%) eVs
0.79 0.92 0.94 0.87
Emittance(rms) eVs
0.82 1.00 1.34 1.56
2D density profiles
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2. Dual harmonic2. Dual harmonic2.3 Phase space images2.3 Phase space images
p/eV.s
fS
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3. Dual harmonic instability3. Dual harmonic instability3.1 Time development3.1 Time development
C02=8 kVC02=8 kV C04=4 kVC04=4 kV Energy: 212 MeV at Energy: 212 MeV at
instability (green)instability (green)
2D density profiles
Synchrotron frequency
Black Green Red
Emittance(90%) eVs
1.15 1.19 1.18
Emittance(rms) eVs
1.17 1.20 1.11
Before instability
During instability
After instability
129 MeV 212 MeV 655 MeV
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3. Dual harmonic instability3. Dual harmonic instability3.2 “Phase space” before, during and after3.2 “Phase space” before, during and after
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3. Dual harmonic instability3. Dual harmonic instability3.3 Instability frequency3.3 Instability frequency
Basic repetitions rate: 4184.3 HzBasic repetitions rate: 4184.3 Hz
If octupolar: f = 4184.3 / 4 If octupolar: f = 4184.3 / 4 1046 Hz 1046 Hz
fsynch.=1046 Hz
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ConclusionConclusion
Bunching factor is increased Bunching factor is increased 0.28 to 0.38 in the single harmonic case0.28 to 0.38 in the single harmonic case
0.45 to 0.55 in the dual harmonic case0.45 to 0.55 in the dual harmonic case
(values at 100 MeV)(values at 100 MeV)
Single harmonic acceleration was Single harmonic acceleration was successfulsuccessful...but loss of hollowness after 390 ms...but loss of hollowness after 390 ms
Dual harmonic operation unstableDual harmonic operation unstable
...from 50 ms to 250 ms...from 50 ms to 250 ms