Superconducting Cavities forSuper-KEKB
S. Mitsunobu

Fig.3 Power transferred to the beam and beam current.

Summary of KEKB SCC Operation• Max. Current
1100ｍA• Max. Power for Beam
３８０ｋW
• HOM Power10ｋW• Operation Voltage
１．４‐２．０ ＭＶ• Average Trip rate is once per month for goodvacuum condition.
• D10C Cavity reeked at Indium joint. Baking
Temperature should decrease to 50 ℃.

KEKB SC  Super-KEKB SC• Cavity Field 1.4-2.0MV OK• Input Coupler 380kw  460kW OK(test)• HOM Power 10 kW  60kW to be solved• Large Gate Valve D=220mm to be solved• Add 4 Cavities• Larger Sigma z should be studied

Dedicated 1 MW klystron system for coupler
test bench
Testing high power coupler, at D10 area
new 1MW klystron power supply andklystron system have been constructed.
One of limitation of high power operation
for KEKB coupler is heating at capacitor forbias voltage. An air duct is added near thiscapacitor to increase cooling power.
Couplers could operate more than 500 kW.

New 1MWCap of Doorknob

Air DuctHOM Damper• Cooling limit (4.0m/sec )60kW• Inner Surface Temperature Limit ２００ ℃• KEKB 80kw with 4A at 3mm BL(2.5pC)• D=220 mm Beam Pipe 56 kW(4A,3mm)• KWKB could be operate (4A, 4MM, 1.8pC)
P=k（σz）・I**2/frev/Nb

D10 HOM Power at 1.053A

Layout of HOM damper test benchAcoustic monitor attachedA SBP HOM damper have been tested up to 5.8 kW .

Higher Order Beam Multipactoringand Single Side Beam Multipactoring• High current application of superconducting cavity, someelectron energy depositions affect to not only cavityperformance but also cryogenic load of refrigerators. Thethreshold current formula for tow sides beam
multipactoring derived by O. Grobner is easily extended
to higher order beam multipactoring and single side beam
multipactoring. Estimations of the threshold current of
positive and negative charge beams in the field free beamtube are given for high current application or small bore
beam tube application such as KEKB and TESLA.

Higher Order Beam Multipactoring•••••••••••••

Second order beam multipactoring
Nb2nd= (1-1/2**0.5)rp2 /re/Lb~ 0.292 Nb1st ~1/3 Nb1stFor KEKB LER or positron for HER
LER beam pipe and coaxial HOM damper of Crabcavity 2.5 A (NbLER=1.3x1011 )rp= 0.05 m
Lb= 2.4 m
Nb1st=3.6x1011 > NbLER
Nb2nd= 1.05x1011 < NbLERKEKB HER positron case for SCC (Charge switching)rp =0.105
Np2nd= 4.7x1011 > 5.72x1010 (1.1A )Single side multipactoring•••
•
•
••
•

For secondary electron yield higher than one and finite initial velocity
Nb=rp*|rp+-a|/2*re*Lba : vi*Lb/cvi : initial velocity of secondary electron(~5eV)+ : positive charge beam
- : negative charge beama < 2*rp• For KEKB superconducting cavity• rp= 0.105 m•
Lb= 2.4 m•
a= 0.0104 m• re=2.82x10-15 m• Nb= 0.105(0.105-0.0104)/re/2.4/2=7.3x1011•>5.73x1010 (1.1 A)• No single side multipactoring for KEKB SC.

Summary• KEKB SC will be used with small modificationfor Super-KEKB.
• Coupler already tested more than 500kW(800kW
in short time),beam test will be done.
• HOM damper is most important issue for SuperKEKB SC.
• If possible 4mm bunch length is help full reducing
HOM problems and saving cost. This option isimportant for increasing integral luminosity before
completion of super-KEKB.