R&D on Low Impedance Beam Chamber and Components

1. R&D onLow Impedance Beam Chamber and Components • Contents • Introduction • Beam Chamber • Components • Connection Flange • Bellows and Gate Valve • Movable Mask (Collimator) • Summary & Future Plan Y. Suetsugu, for KEKB Vacuum Group

2. Introduction • Challenges of vacuum system for super KEKB: • High current (LER: 9.4 A [~20 nC/bunch], HER: 4.2 A) • Short bunch lengths (sz = 3 ~ 5 mm) • Key issues: • Intense SR power (~40 W/mm2 max.) • High photon density • High gas load (~1x1019 photons/m/s in average) • Abundant photoelectrons  Electron cloud effect • High impedance • Likely to excite instabilities • Intense HOM excitation  Heating of components

3. Introduction • Loss factors of major vacuum components: • Loss factor: an indication of impedance • Calculation for sz = 3 mm, present components • Except for RF cavities(16~24 V/pC)

4. Beam chamber • Beam duct with antechambers • Antechambers = additional side chambers • Required to dilute the power density of SR • Copper (OFC), which has high thermal strength. • Effective to reduce impedance and photoelectrons in the beam channel • Pump ports and SR masks are in the antechamber. SR mask Pump Beam duct with antechamber [NEG Strip] Beam [SR Channel] SR [Beam Channel]

5. Beam chamber • Manufacturing • Copper duct with antechamber is without historical precedent. • Two methods were tried. • Cold drawing method seems to have higher reliability for mass production.

6. Beam chamber • For straight (wiggler) section • BPMs • Bellows • Pump ports • Installed into KEKB positron ring • No serious problem up to 1.7 A (1.1 mA/bunch =11 nC/bunch)

7. Beam chamber • For arc section, trial modes has just manufactured. • Beam chamber for B-magnet • NEG pump for narrow antechamber • Will be installed into KEKB positron ring next spring SR

8. Connection Flange • Huge number High impedance • MO-type flange (DESY S-band accelerator type) • Provides smooth inner surface • RF shield = Vacuum seal • Applicable to complicated aperture Flange (SS316) Principle Gasket (Copper) Inside view Cu gasket

9. Connection Flange • Applied to beam chamber and bellows • Reasonable fastening torque • No serious problem up to 1.7 A (1.1 mA/bunch=11 nC/bunch) • Heating due to Joule loss at SS was observed. Cooling of flange Bellows Beam chamber

10. Connection Flange • R&D of copper-alloy (CrZrCu) flange is undergoing • High heat and electrical conductivity • Promising results were obtained. • Will be applied to the next test chamber Copper alloy flange (under test)

11. Bellows and Gate valves • Usual finger-type RF shield is hard to withstand high current. • Comb-type RF-shield has been developed. • Low impedance • High thermal strength • Applicable to complicated aperture • Small flexibility • Expensive Comb type Finger type Beam Beam

12. Bellows and Gate valves • Start with a circular bellows first, continue with race-track one. • Temperature rise decreases down to 1/6. • No problem up to 1.7 A • 1.1 mA/bunch, sz~7 mm f = 94 mm L = 160 mm Ver.0

13. Bellows and Gate valves • Applied to bellows for beam duct with antechamber • Several bellows have been already installed into KEKB • No problem up to 1.6 A • 1.0 mA/bunch, sz~7 mm

14. Bellows and Gate valves • Applied to gate valve for beam duct with antechamber • Already installed into ring

15. Movable mask • A big impedance source in the ring • Stealth type [Ver.6] has been developed. • Metal head with a ceramic support • Recent trial model: Graphite + Diamond

16. Movable mask • RF properties • Dielectric support Little interference with beam Small impedance Loss factor Ver.4 Loss factor [V/pC] Ver.6 Bunch length [mm] Trapped mode

17. Movable mask • Trial models were tested with beam. • Principle was proved: Temperature of bellows and SiC cooling water decreased. SiC cooling water Old type Old type Bellows New type New type

18. Movable mask • Problems:still not applicable to high current • Diamond support was broken at 1.3 A (0.8 mA/bunch) • Improvement is underway • Grade up of diamond, TiN coating on diamond. • Bench test using vacuum wave-guide is planed. Before TiN coating Mask head inside of waveguide Broken mask head After TiN coating

19. Summary • Loss factors of components (expected): • Calculation for sz = 3 mm, antechamber • Except for RF cavities (16~24 V/pC)

20. Summary • High impedance of vacuum components is a serious problem for Super KEKB • Beam duct and various components with low impedance have been developed. • Flange, bellows and gate valves. • Movable mask should be improved. • The loss factors reduced down to approximately 1/4, although still high. • HOM absorbers may be required around IP.

21. Backup

22. Backup • Calculation model • Pump slot and SR mask 5 mm

23. Backup • Calculation model • Connection flange (O-ring)

24. Backup • Calculation model • Bellows (finger-type) and Movable mask Ver.4

25. Backup • Measured loss factor of LER (Ieiri-san, KEK)

26. Backup • Pumping speed of 3-layer NEG strip (ST707) • For N2