1. 2008/12/10 INFN R&D on Low Impedance Beam Chamber and Components Y. Suetsugu, for KEKB Vacuum Group Contents Introduction Beam Chamber Components Connection Flange Bellows and Gate Valve Movable Mask (Collimator) Summary & Future Plan

2. 2 2008/12/10 INFN Introduction Challenges of vacuum system for super KEKB: High current (LER: 9.4 A [~20 nC/bunch], HER: 4.2 A) Short bunch lengths (  z = 3 ~ 5 mm) Key issues: Intense SR power (~40 W/mm 2 max.) High photon density High gas load (~1x10 19 photons/m/s in average) Abundant photoelectrons  Electron cloud effect High impedance Likely to excite instabilities Intense HOM excitation  Heating of components

3. 3 2008/12/10 INFN Introduction Loss factors of major vacuum components: Loss factor: an indication of impedance Calculation for  z = 3 mm, present components Except for RF cavities (16~24 V/pC) Components Number Loss factor [V/pC] (  90) Resistive wall (Copper)3016 [m]6.9 Pumping port (Slots)18009.7 SR mask (5 mm)100010.2 Movable mask (Ver. 4)1619.2 Bellows (Finger-type)100011.0 Gate valve (Finger-type)300.3 Connection flange (O-ring)20001.6 BPM (Button-type)4001.1 Total 60.0

4. 4 2008/12/10 INFN 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 Beam [Beam Channel] [SR Channel] [NEG Strip] Beam duct with antechamber SR mask Pump

5. 5 2008/12/10 INFN 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. 6 2008/12/10 INFN 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. 7 2008/12/10 INFN 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. 8 2008/12/10 INFN 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) Gasket (Copper) Cu gasket Inside view Principle

9. 9 2008/12/10 INFN 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. 10 2008/12/10 INFN 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. 11 2008/12/10 INFN 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 Beam Finger type Comb type

12. 12 2008/12/10 INFN 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,  z ~7 mm  = 94 mm L = 160 mm Ver.0

13. 13 2008/12/10 INFN 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,  z ~7 mm

14. 14 2008/12/10 INFN Bellows and Gate valves Applied to gate valve for beam duct with antechamber Already installed into ring

15. 15 2008/12/10 INFN 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. 16 2008/12/10 INFN Movable mask RF properties Dielectric support Little interference with beam Small impedance Trapped mode Loss factor Bunch length [mm] Loss factor [V/pC] Ver.4 Ver.6

17. 17 2008/12/10 INFN Movable mask Trial models were tested with beam. Principle was proved: Temperature of bellows and SiC cooling water decreased. Bellows SiC cooling water Old type New type Old type New type

18. 18 2008/12/10 INFN 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 After TiN coating Mask head inside of waveguide Broken mask head

19. 19 2008/12/10 INFN Summary Loss factors of components (expected): Calculation for  z = 3 mm, antechamber Except for RF cavities (16~24 V/pC) Components Number Loss factor [V/pC] (  90) Resistive wall (Copper)3016 [m]6.9 Pumping port (screen)2200 [m]9.7  1.9E-03 SR mask (5 mm)100010.2  1.0E-05 Movable mask (Ver. 6)16 19.2  2.0 (?) Bellows (Comb)100011.0  4.0 Gate valve (Comb)300.3  0.1 Connection flange (MO)20001.6  1.0E-2 Taper401.6 BPM (Button)4001.1 Total 60.0  15.7

20. 20 2008/12/10 INFN 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. 21 2008/12/10 INFN Backup

22. 22 2008/12/10 INFN Backup Calculation model Pump slot and SR mask 5 mm

23. 23 2008/12/10 INFN Backup Calculation model Connection flange (O-ring)

24. 24 2008/12/10 INFN Backup Calculation model Bellows (finger-type) and Movable mask Ver.4

25. 25 2008/12/10 INFN Backup Measured loss factor of LER (Ieiri-san, KEK)

26. 26 2008/12/10 INFN Backup Pumping speed of 3-layer NEG strip (ST707) For N 2