The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

OPERATION OF ARES CAVITIES FOR KEKB
T. Kageyama, K. Akai, K. Ebihara, E. Ezura, H. Mizuno, H. Nakanishi,
H. Sakai, Y. Takeuchi, and S. Yoshimoto
KEK, High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, 305 JAPAN
Abstract
The normal conducting RF system for the KEKB doublering collider contains: 8 RF stations with 8 klystrons and 16
ARES (Accelerator Resonantly coupled with EnergyStorage) cavities for the low-energy ring (LER); 5 RF
stations with 5 klystrons and l 0 ARES cavities for the highenergy ring (HER). The ARES cavity developed against
difficulties associated with heavy beam loading conditions
is a three-cavity system operated in the TC/2 mode, in which
an accelerating cavity is resonantly coupled with an energy
storage cavity via a coupling cavity equipped with a parasitic
mode damper on the 0 and 1t modes. On the whole, theARES cavities have performed well to support the highcurrent beams of both rings in spite of many troubles anddeformation rather than increasing the shunt impedance.
Details of the HOM-damped structure are found in acompanion paper [5].
In the horizontal direction, the coupling cavity is brazed
to one side of the accelerating cavity and coupled througha rectangular aperture of 120 mmx 160 mm. At the opposite
side, another half-cell coupling cavity is brazed for the 1t/2mode termination to restore the symmetry of the accelerating
cavity with respect to the mid vertical plane. Furthermore,
the coupling cavity is equipped with an antenna-type coupler[6] for damping the parasitic 0 and 1t modes. The extracted
RF power is guided downward through a coaxial line
toward a water-cooled dummy load.

2 OPERATIONAL STATUS

accidents.

1 SYSTEM OVERVIEW
Figure 1 schematically shows the ARES cavity system~ Thedesign is based on a conceptual demonstrator named
ARES96 [1] for a three-cavity system [2] proposed to solve
difficulties associated with heavy beam loading conditions.
Major RF parameters are listed in Table 1. The design
cavity voltage of 0.5 MV is generated with a wall power
dissipation of 150 kW in total, 60 kW and 90 kW inside theaccelerating and storage cavities, respectively, where thestored energy ratio ua: u, is 1 : 9.
The en~rgy _storage cavity operated in the TJ;m mode.
ts a large cy ltndncal steel structure with dimensions of 1070
mm in diameter and 1190 mm in axial length, whose innersurfaces are copper plated. The Q value of the TE modeis 1.65x 10\ which is 85% of theoretical assuming ~~~opper
electrical conductivity of 5.81 x Hf S/m. The circumferencesof both end plates are grooved in order to resolve thedegeneracy of the T~ 13 and TM 1u modes. The RF power isfed through an input coupler [3] attached to one of the twodrive ports at the middle level. The storage and coupling
cavities are electromagnetically coupled through a
rectangular aperture of 120 mmx 180mm, and mechanicallyconnected with rectangular flanges using bolts. Thin
stainless-steel lips at the flange connection are to be weldedfor vacuum seal in a final stage. For high-power testing
above the ground before installation or operation in earlycommissioning phases, a rubber gasket is to be temporarily
used instead.
. The accelerating cavity itself is a HOM-damped cavity
wtth four rectangular waveguides at the upper and lower
sides and with two grooved beam pipes [4] at both ends. The
accelerating cell structure was designed as simple as possible
from the viewpoint of structural stability in thermal

The commissioning of the HER was first started with 6
ARES cavities and 4 superconducting cavities in December
1998, and followed by the commissioning of the LER with
12 ARES cavities in January 1999. A typical operational RF
voltage in the early stage was 0.4 MV per ARES cavity,
80% of the design voltage. In the summer of 1999, thenumber of ARES cavities in the LER was increased from 12
to 16, and in the HER from 6 to 10. Furthermore, every
ARES cavity in the LER was vacuum-sealed by welding thethin metal lips at the flange connection after removing therubber gasket. On the other hand, rubber gaskets are stillused for the ARES cavities in the HER.
The real high luminosity operation was started in thefall of 1999. The beam currents of both rings were increased
stepwise, overcoming many difficulties and improving themachine performance. As a whole, the ARES cavities have
performed well in supporting the beam currents up to 950
rnA in the LER and up to 780 rnA in the HER in spite ofmany troubles and accidents stated later. In ordinary
operation for the Belle experiment, the maximum cavity
voltage was 0.43 MV, generating the total voltage of 6 MV
for the LER with 14 cavities at 7 RF stations out of the 8 RF
stations, one of which was not operated due to a cavity
vacuum trouble. The maximum RF power transferred with
one input coupler was about 260 kW at a beam current of
900 rnA in the LER.

3 TROUBLES AND ACCIDENTS
Since the commissioning of KEKB, we haveencountered many troubles and accidents with some ARES
cavities and with some accessory devices. The problems
can be roughly divided into two categories: infancy problems
especially with accessory devices emerging in a long term
operation, and cavity problems attributed to quality control-223-

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

issues usually incompatible with stringent cost goals in
production phase.
Major troubles and accidents are given below inchronological order:• A rubber gasket was burned due to RF power leakageatcavityNo.l oftheLERRFstation D8C(LER-D8C#1 inabbreviation) in April 1999. The gasket was used for
vacuum seal at the rectangular flange connection between
the storage and coupling cavities. The power leakage wasdue to a machining defect on the contacting surface at theflange of the storage cavity. In the summer of 1999, thestorage cavity was replaced with a new one and returned tothe manufacturer for repair.
• An intermittent vacuum pressure rise occurred at
LER-D8D#2 in October 1999. The trouble was due to achemical solution for copper plating, which had been trappedin welding defects at some pumping ports of the storage
cavity. Fortunately, the vacuum pressure was improved bybaking the pumping ports with electric heaters. The station
D8D recovered in January 2000.
• At LER-D7D#2, the doorknob-type transformer atthe entrance of the input coupler was burned and partiallymelted in January 2000. The trouble was due to defective
brazing between a thin copper dome part and a stiffenerring. Fortunately, the doorknob and other waveguide parts
were replaced without breaking the vacuum and the cavity
recovered in about 6 hours.
• In the fall of 2000, a thermal vacuum pressure rise
phenomenon suddenly started up at LER-D7C#2 when thecavity was fed with an RFpower of several ten kW. Finally,
this vacuum trouble brought the station to a shutdown from
November 2000 to July 2001. Recently, it was found thatthe trouble had been due to a piece of plastic stuff left in theaccelerating cavity.
• At HER-D5C#2, the input coupler window wascracked due to arcing in November 2000. The cause of thisaccident was an arc sensor which had not been attached toa proper view port. The input coupler was replaced with a
new one and the vacuum recovered in two days. However,the station was finally brought to a shutdown from November
2000 to July 2001 due to a leakage of cooling water to the
air at the lower end plate of the storage cavity, which wasfound while dealing with the coupler accident and wasdifficult to repair in situ.
• A sudden vacuum pressure rise occurred at HERD4C#2 in January 2001. A leak was located at a coolingwater circuit for the ceramic window of the parasitic modecoupler. After the coupler was replaced with a new one,another leak was found and located again at a cooling water
circuit for the input coupler window. Those leaks had been
probably caused by erosion-corrosion of copper walls.
Erosion-corrosion arises from a combination of chemical
attack and the physical abrasion as a consequence of thefluid motion. Aggressive waters do not allow a protective
film (oxides) to form inside the copper tube. Although theinvestigation is under way, the cooling water had been
probably made aggressive by molybdenum oxide comingfrom molybdenum wires exposed to the cooling water with
dissolved oxygen gas. Molybdenum wires are usually usedas backup support when brazing a metal sleeve to a ceramicwindow. After the accident, an anticorrosive agent wasadded to the cooling water for the couplers.
• At LER-D7C#2, the parasitic mode coupler wasburned in March 200 l, although the station was not operated.
The window was cracked due to thermal deformation. The
accident was initiated from a cooling water leakage inside
the 40-kW dummy load for the parasitic mode dampersystem. The water leakage led to the deterioration of theload performance, i.e. VSWR. Then, the beam-induced
power extracted through the parasitic mode coupler wasreflected at the load, and a standing wave was built up toheat the coupler. In order to avoid similar accidents, a
directional coupler used to monitor the extracted power isgoing to be converted to a bi-directional type, which enables
the real-time monitoring of the VSWR of the dummy load.
Fortunately, none of these troubles or accidents had
limited the beam current in the LER or HER. Through theoperation, we have improved the temperature monitor for
the ARES cavity system, especially for some high-power
accessory devices. For example, window arcing trips havebeen reduced by systematic maintenance based on thestatistics obtained by monitoring the doorknob temperaturefor every input coupler.

4 CONCLUSION
On the whole, the ARES cavities have functioned well so
far to support the high current beams for the LER and HER
while we have encountered many troubles and accidents.
With operational experience accumulated through thosedifficulties, we have made great progress and are confidentof the performance and growth potential of the ARES cavity
system toward high-luminosity frontiers beyond 4.4~ I 0 33
cm- 2 s- 1 explored with the KEKB collider.

ACKNOWLEDGMENT
The authors are grateful to Profs. S. Kurokawa, K. Oide andY. Yamazaki for their continuous encouragement.

REFERENCES[I] T. Kageyama eta!., "Development of High-Power
ARES Cavities", Proc. ofPAC97.
[2] Y. Yamazaki and T. Kageyama, Part. Accel. 44 107(1994).
[3] F. Naito et al., "The Input Coupler for the KEKB ARESCavity", Proc. of APAC98.
[4] T. Kageyama, "Grooved Beam Pipe for Damping Dipole
Modes in RF Cavities", Proc. of the 8th Symposium onAccelerator Science and Technology, pl16, 1991.
[5] T. Kageyama andY. Takeuchi, "HOM-damped structureof the ARES cavity", this conference.
[6] F. Naito et al., "Coupling Cavity Damper for the ARESCavity", Proc. ofPAC97.

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The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

Table I: Major RF parameters of the ARES cavity.

!RFUa: Us
RIQ
Q= 508.887 MHz= 1:9= 15 Q
= 1.1 X 10 t
5Pc = 150 kW per ARES Cavity
generating
Vc =0.5 MV (KEKB Design)

t: in high-power operation with Pc = 150 kW

Two SiC bullets per Waveguide----------~~~~~IlJFio - -Eight SiC tiles per Groove

Figure I: A schematic drawing of the ARES cavity system.
Accelerating Cavity with four rectangular HOM waveguides (HWG's) for damping monopole HOM'sand dipole HOM's deflecting the beam in the vertical direction, and with two Grooved Beam Pipes (GBP's)at both end plates for damping dipole HOM's deflecting the beam in the horizontal direction.
CC: Coupling Cavity functions as the keystone of the ARES cavity system, and is equipped witha Parasitic Mode Coupler (PMC) to damp the 0 and 1t modes.
PMC: Parasitic Mode Coupler for reducing the impedances of the parasitic 0 and 1t modes at both sidesof the 7tf2 accelerating mode. Both modes are to be damped down toQL "'100.
CF: Connecting Flange
The Storage Cavity (SC) and the Coupling Cavity (CC) are mechanically connected with rectangularflanges using bolts. Thin stainless-steel lips at the flange connection are to be welded for vacuum seal.
GBP: Grooved Beam Pipe selectively lowers the cutoff frequency of the TE 11 wave and damps dipole HOM'sof the accelerating cavity. Each groove has 8 SiC ceramic tiles brazed to a water-cooled copper plate.
HCC: Half-cell Coupling Cavity restores the symmetry of the accelerating cavity (A C) with respect to the midvertical plane including the beam axis.
HWG: HOM Waveguide is 240 mm wide and 28 mm high. Two bullet-shape sintered SiC ceramic absorbers
are inserted from the end of each waveguide.
Storage Cavity is a cylindrical steel cavity with electro-plated copper surfaces, and operated in the T~ 13
SC:
mode with Q11 165000.
Supporting Structure allows the storage cavity (SC) x- and y-parallel motions in the horizontal plane,SS:and the pitch-, roll- and yaw-motions with respect to the connecting flange (CF) direction.
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