PEP Super-B High Power RF

1. PEP Super-B High Power RF Peter McIntosh SLAC Super-B Factory Workshop in Hawaii20-22 April 2005University of Hawaii

2. Outline • RF Requirements • Cavity Limitations • Voltage • Power • Klystrons • 1.2 MW • 2.4 MW • Circulators • HVPS System • System Configurations • Conclusions

3. RF Requirements • 3 cavity solutions being investigated: • R/Q = 5, 15 and 30 W (see S Novokhatski’s talk). • The RF power required for L = 7e35 and 1e36 varies as a function of cavity option as the R/Q impacts primarily the HOM losses: • As R/Q goes up  cavity HOM losses go up! • The R/Q also impacts the cryogenic losses which affect the Total AC power required: • As R/Q goes up  cavity cryogenic losses go down! • For the cavity options being investigated, the net difference in Total AC power is almost zero! • Assuming the cavity to be used lies somewhere between 5 – 30 W we can see that ……

4. RF and AC Power (5W)

5. Increased Reduced RF and AC Power (30W)

6. RF and AC Power Summary • To define the number of cavities required, have assumed that 1 MW can be supplied to each RF cavity (see later). • For L = 7e35 using R/Q = 5 W cavity: • LER = 21.7 MW • HER = 16.2 MW • For L = 7e35 using R/Q = 30 W cavity: • LER = 22.1 MW • HER = 16.2 MW • For L = 1e36 using R/Q = 5 W cavity: • LER = 39.8 MW • HER = 24.9 MW • For L = 1e36 using R/Q = 30 W cavity: • LER = 42.0 MW • HER = 25.0 MW • Cavity HOM losses increase by 2.2 MW in the LER at 1e36. • Total AC cryogenic power however reduces considerably for the 30 W cavity by 50% for both luminosity options compared to the 5 W cavity. • Net AC power difference is comparable (to within 2%) for each cavity option at each luminosity.

7. R/Q=15W Solution

8. Cavity Limitations - Voltage • Practical achievable voltage/cell depends upon: • Cavity Qo • Niobium purity • Cryogenic operating temperature • Cryogenic load • For the R/Q = 5, 15 and 30  cavities: • Required voltage per cell Vc = 1.25 MV, requiring Qo = 3e9, 1e9 and 1e9 respectively. • For feedback stability R/Q = 5 W preferable  lowest detuning (seeD. Teytelman’s talk) • For cryogenic reasons R/Q = 30 W preferable (see later). • Number of cavities required is the same for each @ L =7e35. • At L = 1e36, the cavity HOM losses in the LER require more RF cavities (2) at R/Q = 30 W. • What cavity voltage can we expect to reach ….

9. Voltage for R/Q = 5  Cavity

10. Voltage for R/Q = 30  Cavity

11. Voltage for R/Q = 15  Cavity

12. Cavity Epk and Hpk Parameters S Novokhatski

13. Voltage Overhead (for 30W) Theoretical Quench Limit for Nb (Hpk = 1700 Oe or 135.281 kA/m) Field Emission Onset (Epk > 10 MV/m)

14. Voltage Overhead (for 5W) Theoretical Quench Limit for Nb (Hpk = 1700 Oe or 135.281 kA/m) Field Emission Onset (Epk > 10 MV/m)

15. Voltage Overhead (for 15W) Theoretical Quench Limit for Nb (Hpk = 1700 Oe or 135.281 kA/m) Field Emission Onset (Epk > 10 MV/m)

16. Cavity Limitations - Power • To minimize the number of RF cavities per ring: • Based on what has been achieved at ~ 500 MHz for both KEK-B and CESR: • 1 MW total RF input power per cavity has been chosen! • Cavity will employ dual RF feeds, each providing up to 500 kW. • RF breakdown investigations need to be performed to identify a system that can meet this power requirement at 952 MHz. • Coaxial coupler arrangement  more compact. • Is this power level realistically achievable?

17. KEK-B (fRF = 508 MHz): Biased coaxial coupler Operate typically up to 350 kW For Super-KEKB hope to reach 500 kW Tested up to 800 kW (through) CESR (fRF = 500 MHz): Aperture waveguide coupled Operate typically up to 300 kW Operated up to 360 kW (through) Cavity Input Couplers

18. Klystrons – 1.2 MW • SLAC already produces 1.2 MW tubes at 476 MHz for PEP-II. • Each powered by a 2.5 MVA DC HVPS. • Tube operates at 83 kV and 24 A with perveance of 1.004. • Maintaining these beam parameters for Super-B @ 952 MHz would enable the same HVPS system to be used. • Scale the cavity frequencies, drift tube spacing, gap lengths, drift pipe and beam radii. • Magnetic field increases by factor of 2  existing 476 MHz tube focus coil adequate.

19. 1.2 MW Klystron – Small Signal

20. 1.2 MW Klystron – Large Signal

21. 1.2 MW Klystron Specification Gun Accelerating Cavities 140.0 RF Output (WR975) Collector (Full power)

22. Klystrons – 2.4 MW • Doubling in RF power means that the existing 2.5 MVA HVPS can no longer be used  now need a 4 MVA HVPS. • Beam power characteristics increase up to 125 kV and 29.2 A with drop in perveance to 0.6607. • Higher beam voltage increases cavity spacing and gap lengths  accelerating section ~ 20% longer than the 1.2 MW tube. • Magnetic field comparable to that of the 1.2 MW tube. • Thermal loading of the output circuit requires more detailed investigation. • Suspect will most likely require a dual output to minimize thermal loading at the RF windows.

23. 2.4 MW Klystron – Large Signal

24. 2.4 MW Klystron Specification Gun Accelerating Cavities 160.0 RF Output (WR975) Collector (Full power) * Needs further optimization

25. Klystron Option Footprints 1.2 MW @ 476 MHz 83 kV and 24 A Perveance = 1.004 210.07 1.2 MW @ 952 MHz 83 kV and 24 A Perveance = 1.004 140.0 2.4 MW @ 952 MHz 125 kV and 29.2 A Perveance = 0.6607 160.0

26. 2.4 MW AFT Circulator Layout

27. 1.7% 1 dry load, 1 water load Full Reflection! Klystron would see 2.4 kW in beam abort x 4 increase c.f. 1.2 MW 476 MHz unit Circulators Spec

28. HVPS • Originally designed for a depressed collector klystron. • Existing 2.5 MVA HVPS has a primary SCR-controlled rectifier operating at the existing site-wide distribution voltage of 12.47kV: • control provides for fast voltage adjustment and fault protection. • Rectifier configuration prevents the dump of filter capacitor stored energy into the klystron in the event of a klystron arc. • 12.47kV enters the circuit breaker and manual load disconnect switch and provides a safety lock and tag disconnect for maintenance. • Remote turn-on and turn-off is by a full, fault-rated vacuum breaker used as a contactor. • A 12-pulse rectifier reduces power line harmonic distortion to industrial standards.

29. PEP-II/SPEAR3 2.5 MVA HVPS

30. Super-B HVPS Options • 1.2 MW Klystron: • Existing 2.5 MVA HVPS system compatible. • No development overhead. • 2.4 MW Klystron: • Same 2.5 MVA HVPS design, with larger transformers to reach 4 MVA: • Applicable transformers are commercially available. • Higher voltage required (125 kV): • Makes HV connections more difficult/expensive. • Anticipate a 20 – 30% size and cost increase over the existing 2.5 MVA unit.

31. System Configuration 1 1.2 MW Klystron Single 952 MHz RF Cavity 1.2 MW Circulator WR975 Waveguide

32. 2.4 MW Klystron Dual 952 MHz RF Cavities 2.4 MW Circulator WR975 Waveguide System Configuration 2

33. 1.2 MW Circulator Dual 952 MHz RF Cavities 2.4 MW Klystron 1.2 MW Circulator System Configuration 3

34. Conclusions • RF requirements for L=7e35 and L=1e36 identified  need up to 190 MW site AC power! • Low R/Q cavities needed for stability control. • Cavity voltage and RF power limits identified  how far can we push these?!? • High power klystrons (> 1 MW) at 952 MHz look to be achievable. • High power circulators appear to be available from industry. • HVPS systems for Super-PEPII klystrons are available now at 1.2 MW, but require development at 2.4 MW. Watch this space!

35. Thank You

36. RF Parameters Summary