1 of 38 S. N. “ Cavities for Super B-Factory” Sasha Novokhatski SLAC, Stanford University Accelerator Session April 20, 2005 Low R/Q Cavities for Super B- factory
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1 of 38
S.
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Sasha NovokhatskiSLAC, Stanford
University
Accelerator Session April 20, 2005
Low R/Q Cavitiesfor Super B-
factory
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”Why Low R/Q Cavities for
Super B?
Because we need high currents to achieve super high luminosity
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”Low R/Q cavities are:
• To damp multi-bunch instability• To avoid single-bunch instability• To decrease HOM power
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”How we can make low
R/Q?• By decreasing cavity gap
– in this case HOM power goes down – but surface fields go up and bring limit
very soon
• By increasing beam pipe radius– smaller R/Q -
- closer to cut-off frequency
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”Varying cavity gap length
cavity gap cavity gap
35.5 Ohm R/Q 24.3 Ohm
0.39 V/pC HOM loss factor 0.24 V/pC
12.77 MV/m Max surface electric field* 31.12 MV/m
30.16 A/m Max surface magnetic field* 58.78 A/m
*for 1 MeV energy gain, f=952 MHz, bore radius 70 mm
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”Varying beam pipe radius
“Wakefield” calculations
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”Wakefield spectrum
“Wakefield” calculations
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”Electric Field Distribution
Rb=110mm
Rb=90mm
Rb=70mm
“SUPERFISH” calculations
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”Surface fields distribution**1 MeV energy gain. Electric field – Green. Magnetic field - pink
“SUPERFISH” calculations
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”Cavity parameters
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”R/Q and HOM Power
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”Transient time factor and stored
Energy
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”Maximum surface fields
magnetic
electric
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Periodic Structure. Main mode coupling
Distance between cavities 787.5 mm (5)
Zero mode
mode
Coupling: 0.55/952=5.8E-04
TM01
Cut-off
1.04276 GHz
Rb=110mm
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”Trapped TM11 modes
Zero mode
mode
TM11
Cut-off
1.6621 GHz
“MAFIA” calculations
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”Trapped TE11 modes
Zero mode
mode
TE11
Cut-off
798.55 MHz
“MAFIA” calculations
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”Checking single–bunch
stabilityWe need to know:
• Wake potentials
• Number of cavities
• Total voltage
• Momentum compaction
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”Cavity wake Potential
Bunch shortening ?
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”Yes, cavity wake produces shorter
bunches
Bunch Current 3.300 mA Bunch Charge 24.21 nC Zero bunchlength 1.80 mm Moment. compact. 9.400E-04 Ring Energy 3500.0 MeV Energy Spread 2.400 MeV SR Energy loss 0.970 MeV per turn RF Voltage: 52.50 MV Number of cavities 42 Phase Angle 1.059 degree (0.926 mm) Harmonic Number 6984 Rev. frequency 136.2707 kHz Synchrotron freq. 17.045 kHz (7.995 Turns) Damping turns 4100.000
1.8 mm 1.75 mm
1.25MV/cav
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”Resistive-Wall Wake (bunch lengthening)
PowerSS: 39.44 MW Al: 7.88 MWCu: 6.06 MW
SR: 22.3 MW
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”IP wake (large additional
part)PowerIP HOMs: 4.0 MW
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”All wakes included
Bunch Current 3.300 mA Bunch Charge 24.21 nC Zero bunchlength 1.80 mm Moment. compact. 9.400E-04 Ring Energy 3500.0 MeV Energy Spread 2.400 MeV SR Energy loss 0.970 MeV per turn RF Voltage: 52.50 MV Number of cavities 42 Phase Angle 1.059 degree (0.926 mm) Harmonic Number 6984 Rev. frequency 136.2707 kHz Synchrotron freq. 17.045 kHz (7.995 Turns) Damping turns 4100.000
1.83 mm
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”Bunch length in the ring
IP
cavities
1
3 2
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IP
1.83 mm
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2.00 mm
Before cavities
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2.00 mm
after cavities
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IP
1.83 mm
IP
Cavities
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”More Voltage
• 1.5 MV/cavity * 42 = 63 MV
• Momentum compaction goes to 1.128E-03 to have the same bunchlength
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IP
1.77 mm
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2.02 mm
Before cavities
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2.02 mm
after cavities
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IP
1.77 mm
IP
Cavities
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”More focusing
• We can increase momentum compaction more to bring bunch length to 1.8mm at IP
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IP
1.816 mm
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2.085 mm
Before cavities
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2.085 mm
after cavities
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IP
1.816 mm
IP
Cavities
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”Conclusions
• Low R/Q cavities are needed for super high luminosity factories. These cavities are superconducting cavities.
• Low R/Q is achieved by using large beam pipe. Cut-off frequency is very closer to the working frequency.
• Trapped transverse modes must be damped using external loads.
• High voltage and correspondent momentum compaction give additional bunch shortening at interaction point.
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