1. 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

2. 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

3. 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

4. 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

5. 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 12.77 MV/m Max surface electric field * 31.12 MV/m 30.16 A/mMax surface magnetic field * 58.78 A/m * for 1 MeV energy gain, f=952 MHz, bore radius 70 mm

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

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

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

9. 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

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

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

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

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

14. S. N. “ Cavities for Super B-Factory” 14 of 38 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

15. S. N. “ Cavities for Super B-Factory” 15 of 38 Trapped TM 11 modes Zero mode  mode TM11 Cut-off 1.6621 GHz “MAFIA” calculations

16. S. N. “ Cavities for Super B-Factory” 16 of 38 Trapped TE 11 modes Zero mode  mode TE11 Cut-off 798.55 MHz “MAFIA” calculations

17. 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

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

19. S. N. “ Cavities for Super B-Factory” 19 of 38 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. S. N. “ Cavities for Super B-Factory” 20 of 38 Resistive-Wall Wake (bunch lengthening) Power SS: 39.44 MW Al: 7.88 MW Cu: 6.06 MW SR: 22.3 MW

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

22. S. N. “ Cavities for Super B-Factory” 22 of 38 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. S. N. “ Cavities for Super B-Factory” 23 of 38 Bunch length in the ring IP cavities 1 3 2

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

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

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

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

28. 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

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

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

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

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

33. 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

34. S. N. “ Cavities for Super B-Factory” 34 of 38 IP  1.816 mm

35. S. N. “ Cavities for Super B-Factory” 35 of 38  2.085 mm Before cavities

36. S. N. “ Cavities for Super B-Factory” 36 of 38  2.085 mm after cavities

37. S. N. “ Cavities for Super B-Factory” 37 of 38 IP  1.816 mm IP Cavities

38. 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.