LEP3 RF System: gradient and power considerations

1. LEP3 RF System: gradient and power considerations Andy Butterworth BE/RF Thanks to R. Calaga, E. Ciapala

2. Outline • Introduction • RF voltage and limits on cavity gradient • Beam power, input couplers and choice of frequency • Higher order modes • Conclusions

3. Choice of RF system For a given application, the parameters of a SC RF system depend on a number of factors: • Desired gradient • beam energy for e- storage ring (SR loss/turn) • available space • available cryogenic cooling capacity (limit gradient, highest possible Q0) • Beam power • beam current, synchrotron radiation power • power per input coupler (Pbeam vs. total no. of couplers, choice of Qext) • available RF power sources (amplifiers, RF distribution)

4. LEP3: Collider and injector rings Collider ring: • 12 GV total RF voltage • High gradient required (space limitation, cost) • High SR power (100 MW) • Reuse of LHC cryogenics plants sufficient? Injector ring: • 9 GV total RF voltage • High gradient as above • Low beam current & SR power (3.5 MW) TLEP-H • 6 GV total RF voltage • Gradient negotiable (cost, no space limitation…?) • High SR power (100 MW)

5. Potential options

6. Cryomodule layout • Approx. cavity length is similar • ILC cryomodule can be used for both frequencies R. Calaga

7. Gradients: 1300 MHz • ILC cavity performance requirements: • 35 MV/m, Q0 > 0.8 x 1010 vertical test (bare cavity) • 31.5 MV/m, Q0 > 1.0 x 1010 in cryomodule (mounted) Test results for eight 1.3 GHz 9-cell TESLA cavities achieving the ILC specification (DESY) (mounted in cryomodule) BCP + EP

8. Cavity gradient yield (ILC) J. Ozelis, SRF2011

9. High gradient R&D (ILC) • Ongoing R&D in new techniques • e.g. Large grain niobium cavities • Steady progress in gradients over time (but lots of scatter) Large-grain 9-cell cavities at DESY D. Reschke et al. SRF2011

10. Gradients: 700 MHz • BNL 5-cell 704 MHz test cavity (A. Burill, AP Note 376, 2010) LHeC CDR design value for ERL 2.5 x 1010 @ 20MV/m • R.Rimmer, ADS Workshop, JLab748 MHz Cavity Test BCP only • First cavities, lots of room for improvement • Measurement after only BCP surface treatment (no EP cf. TESLA cavities) BCP only Courtesy of R. Calaga

11. LHC cryogenic plant capacity

12. Cryogenic heat load cf. LHC cryoplant capacity @ 1.9K of 2.4 or 2.1 kW per sector Heat load per cavity =

13. Injector ring • Repeat the above exercise for the injector ring… • Cryo capacity not for free for 2-ring design…

14. Power required per cavity • Total SR power = 100 kW @ 120 GeV • Do any power couplers exist with these specifications?

15. CW input couplers for SC cavities S. Belomestnykh, Cornell, SRF2007

16. Not surprising… • Physical size and hence power handling decrease with frequency • Thermal design • cooling of room temperature parts • cryogenic load at 2K • Multipacting… R. Calaga

17. CW input couplers for ERLs • Injectors: high power, low Qext , low gradient • Main linacs: low power, high Qext , high gradient H. Sakai, KEK, SRF2011

18. V. Vescherevitch, ERL’09c

19. V. Vescherevitch, ERL’09

20. For main Linac, Qext: 3 x 107 V. Vescherevitch, ERL’09

21. Injector ring • Assuming a top-up intensity of 7% of collider maximum • Seems to be within reach of current CW coupler technology

22. TLEP-H • Total RF voltage: 6000 MV  half as many cavities as LEP3 • SR power = 100 MW as for LEP3 • Power per cavity 2x that for LEP3 • Similar cavity powers as LHeC ring-ring option • Solution with shorter cavities or double couplers • cf. LHeC?

23. Example: LHeC CDR ring-ring option • 560 MV total RF voltage, 100 mA beam current, 60 GeV  S.R. power losses 43.7 MW • Consider 5-cell 721 MHz cavities • gradient > 20 MV/m • 27 cavities would produce the required voltage • but with 1.6 MW of power per cavity  beyond reach of current coupler technology! • Use 2-cell cavities with the same geometry • Use more cavities (112) at lower gradient (11.9 MV/m)  390 kW per cavity • Use 2 input couplers per cavity  195 kW per coupler  still high but achievable

24. Power couplers: conclusion Collider ring: • Currently no input couplers @ 1.3 GHz with sufficient power capacity (~200 kW) • Some designs for ERL get close but still around 50 kW • Easier with lower frequency (700MHz?) • Consider a dual-coupler design (cf. LHeC)? Injector ring: • Low power, probably within the capability of current CW coupler designs TLEP-H: • With cavities at high gradient, cavity powers are extremely high • look for lower gradients/shorter cavities/multiple couplers cf. LHeC?

25. Higher order mode power • Cavity loss factors R. Calaga Average PHOM = k||.Qbunch.Ibeam For Ib=14mA, Qbunch = 155 nC • 700MHz: k|| = 2.64 V/pC, PHOM ~ 5.7 kW • 1.3 GHz: k|| = 8.19 V/pC, PHOM ~ 17.8 kW  to remove from the cavity at 2K!

26. HOM damping summary Antenna / loop HOM couplers Waveguide HOM dampers RF absorbing materials Beamline HOM loads LEP3 1.3 GHz 14 17,800 TLEP-H 700MHz 24 19,700 After M. Liepe, SRF2011

27. IOT & klystron efficiency

28. Summary: frequency choice • Advantages 700 MHz • Synergy SPL, ESS, JLAB, eRHIC • Smaller HOM power • Smaller Heat load • Power couplers easier • IOT and SSPA amplifiers available • Advantages 1300 MHz • Synergy ILC, X‐FEL • Cavity smaller • Larger R/Q • Smaller RF power (assuming same Qext) • Less Nb material needed

29. Conclusions • Limitations for the collider ring are mainly linked to the high beam power • 1.3 GHz TESLA/ILC cavities are now a mature technology and have good gradient performance and consistently high Q0 > 1.5 x 1010 @ 20 MV/m • However, power couplers need > an order of magnitude increase in CW power handling  R&D • 700 MHz cavity developments are in an earlier stage of maturity than TESLA but look promising and may be better suited to high power CW application  R&D needed on input couplers • High HOM powers to remove from 2K cavity  R&D needed on HOM couplers/absorbers • TLEP-H: low RF voltage but high beam power  Lower gradients, more/shorter cavities, multiple power couplers  R&D