RD on Low Impedance Beam Chamber and Components

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 2008/12/10 INFN

Introduction n Challenges of vacuum system for super KEKB: High current (LER: 9. 4 A [~20 n. C/bunch], HER: 4. 2 A) l Short bunch lengths (sz = 3 ~ 5 mm) l n Key issues: Intense SR power (~40 W/mm 2 max. ) l High photon density • High gas load (~1 x 1019 photons/m/s in average) • Abundant photoelectrons Electron cloud effect l High impedance • Likely to excite instabilities • Intense HOM excitation Heating of components l 2008/12/10 INFN 2

Introduction n Loss factors of major vacuum components: l l l Loss factor: an indication of impedance Calculation for sz = 3 mm, present components Except for RF cavities (16~24 V/p. C) Number Loss factor [V/p. C] (f 90) 3016 [m] 6. 9 Pumping port (Slots) 1800 9. 7 SR mask (5 mm) 1000 10. 2 16 19. 2 1000 11. 0 30 0. 3 Connection flange (O-ring) 2000 1. 6 BPM (Button-type) 400 1. 1 　Components Resistive wall (Copper) Movable mask (Ver. 4) Bellows (Finger-type) Gate valve (Finger-type) Total 　 2008/12/10 INFN 60. 0 3

Beam chamber n Beam duct with antechambers Antechambers = additional side chambers • Required to dilute the power density of SR • Copper (OFC), which has high thermal strength. l Effective to reduce impedance and photoelectrons in the beam channel • Pump ports and SR masks are in the antechamber. l Beam duct with antechamber [NEG Strip] Beam SR [SR Channel] [Beam Channel]2008/12/10 INFN Pump SR mask 4

Beam chamber n Manufacturing Copper duct with antechamber is without historical precedent. l Two methods were tried. l l Cold drawing method seems to have higher reliability for mass production. 2008/12/10 INFN 5

Beam chamber n For straight (wiggler) section l l l n BPMs Bellows Pump ports Installed into KEKB positron ring l No serious problem up to 1. 7 A (1. 1 m. A/bunch =11 n. C/bunch) 2008/12/10 INFN 6

Beam chamber n For arc section, trial modes has just manufactured. l l l Beam chamber for B-magnet NEG pump for narrow antechamber Will be installed into KEKB positron ring next spring SR 2008/12/10 INFN 7

Connection Flange n n Huge number High impedance MO-type flange (DESY S-band accelerator type) l l l Provides smooth inner surface RF shield = Vacuum seal Applicable to complicated aperture Flange (SS 316) Principle Gasket (Copper) Cu gasket Inside view 2008/12/10 INFN 8

Connection Flange n Applied to beam chamber and bellows l l l Reasonable fastening torque No serious problem up to 1. 7 A (1. 1 m. A/bunch=11 n. C/bunch) Heating due to Joule loss at SS was observed. Cooling of flange Bellows Beam chamber 2008/12/10 INFN 9

Connection Flange n R&D of copper-alloy (Cr. Zr. Cu) flange is undergoing l l l High heat and electrical conductivity Promising results were obtained. Will be applied to the next test chamber Copper alloy flange (under test) 2008/12/10 INFN 10

Bellows and Gate valves n Usual finger-type RF shield is hard to withstand high current. n Comb-type RF-shield has been developed. Low impedance High thermal strength Applicable to complicated aperture l Small flexibility l Expensive l l l Finger type Beam 2008/12/10 INFN Comb type Beam 11

Bellows and Gate valves Start with a circular bellows first, continue with race-track one. n Temperature rise decreases down to 1/6. n No problem up to 1. 7 A n l 1. 1 m. A/bunch, sz~7 mm f = 94 mm L = 160 mm Ver. 0 2008/12/10 INFN 12

Bellows and Gate valves n Applied to bellows for beam duct with antechamber l n Several bellows have been already installed into KEKB No problem up to 1. 6 A l 1. 0 m. A/bunch, sz~7 mm 2008/12/10 INFN 13

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

Movable mask A big impedance source in the ring n Stealth type [Ver. 6] has been developed. n l l Metal head with a ceramic support Recent trial model: Graphite + Diamond 2008/12/10 INFN 15

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

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

Movable mask n Problems: still not applicable to high current l n Improvement is underway l n Diamond support was broken at 1. 3 A (0. 8 m. A/bunch) Grade up of diamond, Ti. N coating on diamond. Bench test using vacuum wave-guide is planed. Before Ti. N coating Mask head inside of waveguide Broken mask head After Ti. N coating 2008/12/10 INFN 18

Summary n Loss factors of components (expected): Calculation for sz = 3 mm, antechamber l Except for RF cavities (16~24 V/p. C) l 　Components Number Loss factor [V/p. C] (f 90) Resistive wall (Copper) 3016 [m] 6. 9 Pumping port (screen) 2200 [m] 9. 7 1. 9 E-03 1000 10. 2 1. 0 E-05 16 19. 2　 2. 0 (? ) 1000 11. 0 4. 0 30 0. 3 0. 1 2000 1. 6 1. 0 E-2 Taper 40 1. 6 BPM (Button) 400 1. 1 SR mask (5 mm) Movable mask (Ver. 6) Bellows (Comb) Gate valve (Comb) Connection flange (MO) Total 　 2008/12/10 INFN 60. 0 15. 7 19

Summary n High impedance of vacuum components is a serious problem for Super KEKB n Beam duct and various components with low impedance have been developed. l Flange, bellows and gate valves. l Movable mask should be improved. The loss factors reduced down to approximately 1/4, although still high. n HOM absorbers may be required around IP. n 2008/12/10 INFN 20

Backup 2008/12/10 INFN 21

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

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

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

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

Backup n Pumping speed of 3 -layer NEG strip (ST 707) n For N 2 2008/12/10 INFN 26