S. N. “ Cavities for Super B-Factory” 1 of 38 Sasha Novokhatski SLAC, Stanford University Accelerator Session April 20, 2005 Low R/Q Cavities for Super B-factory

S. N. “ Cavities for Super B-Factory” 2 of 38 Why Low R/Q Cavities for Super B? Because we need high currents to achieve super high luminosity

S. N. “ Cavities for Super B-Factory” 3 of 38 Low R/Q cavities are: To damp multi-bunch instability To avoid single-bunch instability To decrease HOM power

S. N. “ Cavities for Super B-Factory” 4 of 38 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

S. N. “ Cavities for Super B-Factory” 5 of 38 Varying cavity gap length cavity gap  cavity gap  35.5 OhmR/Q 24.3 Ohm 0.39 V/pC HOM loss factor 0.24 V/pC MV/m Max surface electric field * MV/m A/mMax surface magnetic field * A/m * for 1 MeV energy gain, f=952 MHz, bore radius 70 mm

S. N. “ Cavities for Super B-Factory” 6 of 38 Varying beam pipe radius “Wakefield” calculations

S. N. “ Cavities for Super B-Factory” 7 of 38 Wakefield spectrum “Wakefield” calculations

S. N. “ Cavities for Super B-Factory” 8 of 38 Electric Field Distribution Rb=110mm Rb=90mm Rb=70mm “SUPERFISH” calculations

S. N. “ Cavities for Super B-Factory” 9 of 38 Surface fields distribution* *1 MeV energy gain. Electric field – Green. Magnetic field - pink “SUPERFISH” calculations

S. N. “ Cavities for Super B-Factory” 10 of 38 Cavity parameters

S. N. “ Cavities for Super B-Factory” 11 of 38 R/Q and HOM Power

S. N. “ Cavities for Super B-Factory” 12 of 38 Transient time factor and stored Energy

S. N. “ Cavities for Super B-Factory” 13 of 38 Maximum surface fields magnetic electric

S. N. “ Cavities for Super B-Factory” 14 of 38 Periodic Structure. Main mode coupling Distance between cavities mm (5  ) Zero mode  mode Coupling: 0.55/952=5.8E-04 TM01 Cut-off GHz Rb=110mm

S. N. “ Cavities for Super B-Factory” 15 of 38 Trapped TM 11 modes Zero mode  mode TM11 Cut-off GHz “MAFIA” calculations

S. N. “ Cavities for Super B-Factory” 16 of 38 Trapped TE 11 modes Zero mode  mode TE11 Cut-off MHz “MAFIA” calculations

S. N. “ Cavities for Super B-Factory” 17 of 38 Checking single–bunch stability We need to know: Wake potentials Number of cavities Total voltage Momentum compaction

S. N. “ Cavities for Super B-Factory” 18 of 38 Cavity wake Potential Bunch shortening ?

S. N. “ Cavities for Super B-Factory” 19 of 38 Yes, cavity wake produces shorter bunches Bunch Current mA Bunch Charge nC Zero bunchlength 1.80 mm Moment. compact E-04 Ring Energy MeV Energy Spread MeV SR Energy loss MeV per turn RF Voltage: MV Number of cavities 42 Phase Angle degree (0.926 mm) Harmonic Number 6984 Rev. frequency kHz Synchrotron freq kHz (7.995 Turns) Damping turns mm  1.75 mm 1.25MV/cav

S. N. “ Cavities for Super B-Factory” 20 of 38 Resistive-Wall Wake (bunch lengthening) Power SS: MW Al: 7.88 MW Cu: 6.06 MW SR: 22.3 MW

S. N. “ Cavities for Super B-Factory” 21 of 38 IP wake ( large additional part) Power IP HOMs: 4.0 MW

S. N. “ Cavities for Super B-Factory” 22 of 38 All wakes included Bunch Current mA Bunch Charge nC Zero bunchlength 1.80 mm Moment. compact E-04 Ring Energy MeV Energy Spread MeV SR Energy loss MeV per turn RF Voltage: MV Number of cavities 42 Phase Angle degree (0.926 mm) Harmonic Number 6984 Rev. frequency kHz Synchrotron freq kHz (7.995 Turns) Damping turns mm

S. N. “ Cavities for Super B-Factory” 23 of 38 Bunch length in the ring IP cavities 1 3 2

S. N. “ Cavities for Super B-Factory” 24 of 38 IP  1.83 mm

S. N. “ Cavities for Super B-Factory” 25 of 38  2.00 mm Before cavities

S. N. “ Cavities for Super B-Factory” 26 of 38  2.00 mm after cavities

S. N. “ Cavities for Super B-Factory” 27 of 38 IP  1.83 mm IP Cavities

S. N. “ Cavities for Super B-Factory” 28 of 38 More Voltage 1.5 MV/cavity * 42 = 63 MV Momentum compaction goes to 1.128E-03 to have the same bunchlength

S. N. “ Cavities for Super B-Factory” 29 of 38 IP  1.77 mm

S. N. “ Cavities for Super B-Factory” 30 of 38  2.02 mm Before cavities

S. N. “ Cavities for Super B-Factory” 31 of 38  2.02 mm after cavities

S. N. “ Cavities for Super B-Factory” 32 of 38 IP  1.77 mm IP Cavities

S. N. “ Cavities for Super B-Factory” 33 of 38 More focusing We can increase momentum compaction more to bring bunch length to 1.8mm at IP

S. N. “ Cavities for Super B-Factory” 34 of 38 IP  mm

S. N. “ Cavities for Super B-Factory” 35 of 38  mm Before cavities

S. N. “ Cavities for Super B-Factory” 36 of 38  mm after cavities

S. N. “ Cavities for Super B-Factory” 37 of 38 IP  mm IP Cavities

S. N. “ Cavities for Super B-Factory” 38 of 38 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.