Low R/Q Cavities for Super B-factory

1. Low R/Q Cavities for Super B-factory Sasha Novokhatski SLAC, Stanford University Accelerator Session April 20, 2005

2. Why Low R/Q Cavities for Super B? Because we need high currents to achieve super high luminosity

3. Low R/Q cavities are: • To damp multi-bunch instability • To avoid single-bunch instability • To decrease HOM power

4. 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

5. Varying cavity gap length cavity gap l/4 cavity gap l/2 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

6. Varying beam pipe radius “Wakefield” calculations

7. Wakefield spectrum “Wakefield” calculations

8. Electric Field Distribution Rb=70mm Rb=90mm Rb=110mm “SUPERFISH” calculations

9. Surface fields distribution**1 MeV energy gain. Electric field – Green.Magnetic field - pink “SUPERFISH” calculations

10. Cavity parameters

11. R/Q and HOM Power

12. Transient time factor and stored Energy

13. Maximum surface fields electric magnetic

14. Periodic Structure. Main mode couplingDistance between cavities 787.5 mm (5p) Rb=110mm p mode TM01 Cut-off 1.04276 GHz Coupling: 0.55/952=5.8E-04 Zero mode

15. Trapped TM11 modes p mode TM11 Cut-off 1.6621 GHz Zero mode “MAFIA” calculations

16. Trapped TE11 modes p mode TE11 Cut-off 798.55 MHz Zero mode “MAFIA” calculations

17. Checking single–bunch stability We need to know: • Wake potentials • Number of cavities • Total voltage • Momentum compaction

18. Cavity wake Potential Bunch shortening ?

19. 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

20. Resistive-Wall Wake (bunch lengthening) Power SS: 39.44 MW Al: 7.88 MW Cu: 6.06 MW SR: 22.3 MW

21. IP wake (large additional part) Power IP HOMs: 4.0 MW

22. 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

23. Bunch length in the ring IP 1 2 3 cavities

24. IP s=1.83 mm

25. Before cavities s=2.00 mm

26. after cavities s=2.00 mm

27. Cavities IP IP s=1.83 mm

28. More Voltage • 1.5 MV/cavity * 42 = 63 MV • Momentum compaction goes to 1.128E-03 to have the same bunchlength

29. IP s=1.77 mm

30. Before cavities s=2.02 mm

31. after cavities s=2.02 mm

32. Cavities IP IP s=1.77 mm

33. More focusing • We can increase momentum compaction more to bring bunch length to 1.8mm at IP

34. IP s=1.816 mm

35. Before cavities s=2.085 mm

36. after cavities s=2.085 mm

37. Cavities IP IP s=1.816 mm

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.