HIGH CURRENT AND HIGH POWER APPLICATION OF
SUPERCONDUCTING CAVITIES FOR KEKB
S. Mitsunobu, K. Akai, E. Ezura, T. Furuya, K. Hosoyama
KEK, High Energy Accelerator Research Organization
Y. Kijima
Mitsubishi Electric Co.
R. Noer
Carleton Collegeand T. Tajima
Los Alamos National Lab.
Abstract
KEKB is an asymmetric electron-positron collider at
8x3.5 GeV, which aims to provide electron-positron
collisions at a centre of mass energy of 10.58 GeV. For
the 8 GeV high energy ring, four superconducting
cavities have been used to accelerate high current ofmore than 500 mA. At the summer shutdown of 2000,
four more superconducting cavities were installed toincrease current up to 1.1 A. The cavity is single cell andhas a large diameter beam pipe to damp beam induced
higher order mode power. The operation results of thislarge application of superconducting cavities for highpower and high current will be shown.

1 INTRODUCTION
KEKB, an asymmetric energy double-ring electronpositron collider for B-physics, was commissioned inDecember 1998. Two types of heavily-damped cavities
are used in KEKB. A normal conducting cavity system(ARES) is used in both the low energy ring (LER) andthe high energy ring (HER); 4 superconducting cavities
were initially used in the HER. [1] Four additional
superconducting cavities were installed in the HER at

the summer shutdown of 2000. This increased the beam
current of the HER up to 650 mA. The current was not
limited by the superconducting cavities but rather by
HOM heating of vacuum components such as masks.
High current application of superconducting cavities
needs high field damped structure type cavities, highpower input couplers, HOM loads and powerful
cryogenic systems.

2 SUPERCONDUCTING CAVITY
Based on the long-term operation of 32
superconducting cavities in TRISTAN and the successful
beam test of the prototype cavity at the TRISTAN
Accumulation Ring (AR) reaching a maximum currentof 0.57 A in 1996, we started construction of 4
superconducting cavity modules for the KEKB HER.
Figure 1 shows the whole module. The superconducting
cavity has a single cell structure with large aperture
beam pipes on both ends so that higher order modes(HOM) can propagate out of the cavity and be damped
by ferrite absorbers bonded on the inner surface of thebeam pipe.

Fig. 1 Superconducting cavity of KEKBThe accelerating fields attainable in the cavities werefirst measured using a vertical cryostat. The results werehigher than 15 MV/m as shown in Fig. 2. The
accelerating fields decreased to 10-11 MV/m after full
assembling but exceed the necessary operating field of 68 MV/m.

power). The insulating material is two layers of 0.125
mm thick polyimide films, shown in Fig 3.

Fig 3 Bias type door knob transition for SC cavity.

3 HOM DAMPERS
Fig. 2 Performance of vertical test for recent for SCcavity.

2 KEKB INPUT COUPLER
The input coupler for the KEKB superconducting cavity
is almost the same design as that used for the TRISTAN
superconducting cavities. The 3 mm choke structure gap
was changed to 4 mm to reduce the field strength at theceramic disk. One of authors suggested the use of watercooling for the inner conductor of the TRISTAN coupler.
The calculated heat transfer by radiation is 0.6 W for theelectro-polished Cu inner conductor. This heat transfer issmall and is expected to have almost no effect on cavity
performance in the case of high fields of more than 10
MV/m for KEKB. The water cooling has enough cooling
capacity for up to 1 MW power transmition. The windowof the coupler is almost same as those in the 1MW
klystrons used at TRISTAN, which have a long lifetimeof more than 50000 hours. So the TRISTAN coupler canbe operated at 1 MW in principle but were used only up
to 200 kW in TRISTAN. So for higher power operation,
the KEKB couplers need more diagnostics for precise
RF processing and interlock operation.
Before cooling down the cavities, we condition the input
couplers up to 300 kW with perfect reflection condition,and up to 200 kW with dc bias voltages applied on theinner conductor up to +- 2kV.[2] The developed highpower door knob transition with capacitor for applying
bias voltage works well up to 300 kW perfect reflection
condition ( corresponding to 1.2 MW of forward

HOM dampers are made of ferrite and tested up to 5 kW
and 7 kW for the SBP and LBP, respectively, with a 508oMHz coaxial line, then baked at 150 C for about one
month to reduce the outgassing rate. Considering theimportance of making the amount of gas flowing into thecavity as small as possible for stable operation , weinstalled five 400 liter/s NEG pumps and one 300 liter/s
ion pump on the duct between each module.

4 CRYOGENIC SYSTEM
We are using the powerful cryogenic system originallyused for TRISTAN, with newly developed transfer lines
to reduce heat loss and to add more flexibility inconnecting with cavity modules.

5 BEAM OPERATION

The initial four cavities were operated stably withgradually increasing beam current . [1] Four newcavities were operated from start-up with high current of
more than 500 mA. The cavity vacuum pressure was-6
worse than 7x10 Pa ,about one order worse pressure
than the initial cavities.
We experienced rather a high trip rate of a few times per
week due to superconducting cavity breakdown.
Sometimes, processing with bias voltage of -700 volt
during cavity excitation was effective in improving thepoor vacuum condition . Trips did not occur for morethan two weeks after this processing. With the beam
current limited by heating of masks, the maximum

current reached is 650 mA for the HER withsuperconducting cavities. Two cavities have bud RFmonitoring cables, preventing feedback cavity frequency
tuning. So two cavities were detuned at liquid heliumtemperature. At the New Year shut down, the shorted
cables were replaced with new anchor type cables toprevent shorting between inner and outer conductors bycaused thermal cycling. Another cavity leaked at theindium seal at the LBP and is now being repaired.
The high beam current operation of these cavities
depends on the high power capabilities of our coupler380 kW at maximum, though 200-300 kW is sufficientfor normal operation.
The cavity performances have shown no degradation
during more than two years with high current operation.
Still higher current will be tried soon, after replacementof masks.

References[1] T. Tajima et al, PAC99 March29-April 2, 1999, NewYork City.
[2] Y. Kijima et al, This conference.