The 13th Symposium on Accelerator Science and Technology, Suita, Osak£1, Japan, October 2001

HOM-DAMPE D STRUCTURE OF THE ARES CAVITY
Tatsuya KAGEYAMA and Yasunao TAKEUCHI
KEK, High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, 305 JAPANbeam experiments carried out in the TRISTAN accumulation
ring (AR) in 1996.

Abstract
The ARES cavity, developed against difficulties associatedwith heavy beam loading conditions in the high-luminosity
electron-positron collider KEKB, is a three-cavity system
operated in the Tt/2 mode, in which an accelerating cavity isresonantly coupled with a high-Q energy storage cavity via
a coupling cavity equipped with a parasitic mode damper on
the 0 and n: modes. Furthermore, to reduce the HOM
impedance, the accelerating cavity itself needs to be a
HOM-damped structure fairly compatible with this ARESscheme. This paper briefly reviews major features of theHOM-damped structure of the ARES cavity system andreports the recent operational status.

1 INTRODUCTION
The ARES cavity is a three-cavity system [1] developed tosolve difficulties such as couled-bunch instabilities due tothe accelerating mode under heavy beam loading conditionsin KEKB. In the ARES system, an accelerating cavity isresonantly coupled with an energy storage cavity operated
in the TE013 mode via a coupling cavity. The couplng cavity
is equipped with an antenna-type coupler to damp theparasitic 0 and n: modes emerging at both sides of the Tt/2
accelerating mode.
Needless to say, the accelerating cavity itself must be a
HOM-damped structure against coupled-bunch instabilities
due to HOMs also limiting the performance of KEKB.
Furthermore, the HOM-damped structure needs to be
structurally and electromagnetically compatible with theARES scheme. That is an inevitable boundary condition,i.e. two coupling apertures at both sides of the accelerating
cavity: one toward the coupling cavity, and the other towarda half-cell coupling cavity for the Tt/2-mode termination
keeping the accelerating field symmetrical with respect tothe vertical mid plane including the beam axis.

2 HOM-DAMPED STRUCTURE
Figure 1 schematically shows front and side views of theHOM-damped structure for the ARES cavity system.
Currently, the number of ARES cavities operated in thedouble-ring collider KEKB is 26 in total: 16 for the lowenergy ring (LER), and 10 for the high-energy ring (HER).
The recent operational status ofthe ARES cavities is reported
in a companion paper [2]. The design of the production
cavity shown in Fig. I is based on a conceptual demonstrator
named ARES96 [3], which had been successfully verified
in a series of stringent high-power tests and high-current

2.2 Accelerating Cavity
The accelerating cavity is made mainly of OFC (OxygenFree Copper) parts brazed stepwise in a vacuum furnace.
The accelerating cell was designed as simple as possible
from the viewpoint of structural stability in thermal
deformation rather than increasing the shunt impedance,
leading to a simple design, i.e. a pillbox-type cavity with
nosecones of 10 mm high. Compensating for thermal
detuning, a movable tuning plunger with a diameter of 70
mm and a travel of 60 mm is installed in the upper right port,
as seen in the front view of Fig. I. The coupling cavity andthe half-cell coupling cavity of the n:/2-mode terminator arebrazed directly to both sides of the accelerating cavity.
Through a rectangular aperture of 120 mmx 160 mm, eachcoupling cavity is coupled with the accelerating cavity.

2.2 Rectangular HOM waveguides
Four straight rectangular waveguides are brazed directly tothe upper and lower sides of the accelerating cavity in orderto damp monopole HOMs and dipole ones deflecting thebeam in the vertical direction. The waveguide width is 240mm, giving a cutofffrequency of625 MHz for the dominant
TE10 wave. The extracted HOM power is guided through an
E-bend waveguide in the horizontal direction and is finallydissipated in two bullet-shape sintered SiC ceramic absorbers(55 mm in diameter and 400 mm in length including atapered section) at the end of each waveguide. The SiC
absorbers are loaded through circular ports at the end plate,where metal o-rings are used for vacuum seal. Each absorberis directly cooled by water (- 4 Llmin.) flowing in a circularbore with a diameter of 12 mm, into which a alumina
ceramic pipe with an outside diameter of 8 mm and an inside
diameter of 5 mm is inserted to form a cooling-water circuit.
The power capability per absorber was verified up to 3.3
kW, using a HOM-load test bench with a L-band CW
klystron. Details of the HOM loads for the ARES cavity
were reported in Ref. [4].

2.3 Grooved Beam Pipes
Grooving the inner wall of a circular beam pipe canselectively lower the cutoff frequency of the T~ 1 wave,
which couples with dipole modes in the cavity. That is theGrooved Beam Pipe (GBP) method [5], which can heavily
damp dipole modes without sacrificing the acceleratingmode. As seen in Fig. 1, the beam pipe with an inside
diameter of 150 mm attached to each end plate of the-226-

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

accelerating cavity has two deep grooves at the upper andlower sides in order to damp dipole modes deflecting thebeam in the horizontal direction. The groove dimensions
are 30 mm in width and 95 mm in depth, lowering the cutoff
frequency of the TE11 wave below 650 MHz. In eachgroove, there are eight SiC tiles arranged in a line, where theextracted HOM power is dissipated. Each SiC tile is brazed
to a water-cooled copper plate with a copper compliant
layer between. The GBP HOM load was also tested up to 0.5
kW per groove at the test bench mentioned earlier.

3HIGH-CURRENT OPERATIONThe commissioning of the HER was first started with 6
ARES cavities and 4 superconducting cavities [5] inDecember 1998, and followed by the commissioning of theLER with 12 ARES cavities in January 1999. In the summer
shutdown of 1999, the number of ARES cavities in the LER
was increased from 12 to 16, and in the HER from 6 to 10.
The real high luminosity operation for the Belle experiment
was started in the fall of 1999. The beam currents of bothrings were increased stepwise: overcoming many difficulties,
for example, with movable masks; and improving themachine performance, for example, with solenoid windingsin the LER. As a whole, the ARES cavities have performed
well in supporting the beam currents up to 950 rnA in theLERand up to 780mA in the HER while we have encounteredmany troubles and accidents, some of which are reported in
a companion paper [2). However, focusing on the HOMdamped structure alone, we have not encountered anytroubles so far.
For each of the 10 ARES cavities installed at the LER
RF section D7, the HOM power dissipated in the fourbullet-shape SiC absorbers at the downstream side wasobtained from the temperature rise of the cooling waterflowing through those absorbers in series and the flow rateadjusted in advance. Figure 2 shows the data obtained forevery cavity as a function of the LER beam current in arange from 600 rnA to 900 rnA. The bunch pattern was asingle train of 1152 bunches with 4-bucket spacing followedby a gap of 512 vacant buckets. The mirror symmetry of thecavity structure with respect to the beam direction assuresthat twice the power in Fig. 2 is roughly equal to the total
HOM power dissipated in the four rectangular waveguidesof each cavity, which amounts to about 1.6 kW at 900 rnA.
Each curve drawn in Fig. 2 is a fit to the data for each cavity,assuming a quadratic dependence of the HOM power on thebeam current. Figure 3 shows the coefficient of the quadraticterm obtained from data fitting for every cavity. It can besaid that there is a slight tendency for the cavities at bothends of the RF section to have smaller HOM power
dissipations. It may be due to higher order modes above thebeam pipe cutoff frequency. At each end of the RF section,
the inside diameter of the beam pipe is tapered down from150 mm to 96 mm. Before each tapered section, a cylindrical
SiC duct with an inside diameter of 150 mm and an effective
length of 240 mm is installed as a broadband terminator toabsorb outgoing HOM power and as an isolator to absorb
incoming HOM power from the arc section outside.
Also, the HOM power dissipated at the SiC tiles in thegrooved beam pipe at the down-stream side of each cavity
was obtained in a similar way stated above. Figure 4 shows
the data obtained for every cavity at the LER RF section D7.
Again; twice the power in Fig. 4 is roughly equal to the total
HOM power dissipated in both grooved beam pipes of eachcavity. Figure 5 shows the coefficient of the quadratic term
for every fitted curve in Fig. 4. Comparing Fig. 5 with Fig.
3, then, there can be seen a clear tendency of nonuniformity
in distribution of the HOM power dissipations along the RFsection. However, we need further investigation to confirm
this tendency. That is because unfortunately the two cavities
at the RF station D7C were not operated due to a vacuum
trouble when these data were taken. An irregular boundary
condition, where the storage cavity is not properly tuned tothe accelerating cavity in the TC/2 mode, deforms theaccelerating field and may introduce a dipole component
around the beam axis.

4CONCLUSION

Focusing on the HOM-damped structure alone, the AREScavities have functioned well without any troubles to support
the high current beams stably in both LER and HER.
Furthermore, the HOM loads have ample margins for highcurrent operation. These two facts make us confident of theperformance and growth potential of the HOM-damped
structure for the ARES cavity system toward high-luminosity
frontiers beyond 4.49x 1033 cm-2 s·' explored with the KEKB
collider.

ACKNOWLEDGMENT
The authors would like to thank all the staff members of theKEKB RF group led by Prof. E. Ezura for their support
indispensable to stable operation of the ARES cavities.
They are also grateful to Profs. S. Kurokawa, K. Oide andY. Yamazaki for their continuous encouragement.

REFERENCES[1] Y. Yamazaki and T. Kageyama, Part. Accel.44 107(1994)[2] T. Kageyama et al., "Operation of ARES Cavities for
KEKB", this conference.
[3] T. Kageyama et al., "Development of High-Power
ARES Cavities", Proc. ofPAC97.
[4] Y. Takeuchi et al., "HOM Absorber for the ARES
Cavity", Proc. of PAC97.
[5] T. Kageyama, "A Design of Beam Duct Cross-Section
for Damping Dipole Modes in RF Cavities", Proc. ofthe 15th Linear Accelerator Meeting in Japan, p79,1990, and "Grooved Beam Pipe for Damping Dipole
Modes in RF Cavities", Proc. of the 8th Symposium onAccelerator Science and Technology, p116, 1991.

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Energy Storage Cavity

Accelerating Cavity
Two Bullet-shaped SiC Absorbers per rectangular HOM Waveguide

Tuner Port

CouplingCavity

I

Tuner Port
Half-Cell
Coupling Cavity~··

IOOOmm/7777777777..

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Figure 1: Front and side views of the HOM-damped structure for the ARES cavity system~ 0.80<J)(!;,

s: 0.60.

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lii 0.4>:0&.

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~ 0.15a..

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0:c0.10.05

4000600

800

200

1000

400

600

800

1000

e+ beam current (rnA)

e+ beam current (rnA)

Figure 2: For every ARES cavity at the RF section 07 in the LER, theHOM power dissipated in the four bullet-shape SiC absorbers at thedown-stream side is plotted as a function of the beam current.

Figure 4: For every ARES cavity at the RF section 07 in the LER, theHOM power dissipated at the SiC tiles in the grooved beam pipe atthe down-stream side is plotted as a function of the beam current.

~

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<(1 1o·6E~

a 1o·7~6 1 o·7(J~ 310"""~-c8"'0c.91~Ql7"':::> 210"

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2 10"7·8""iii80(J410"70cavity Designation-- Beam Direction -->110"70A1 A2 81 B2 C1 C2 01 02 E1 E2cavity Designation- Beam Direction ··>Figure 3: A graph showing the coefficient of the quadratic term of thebeam current for every cavity, obtained from fitting the HOM power
data in Fig. 2.

Figure 5: A graph showing the coefficient of the quadratic term of thebeam current for every cavity, obtained from fitting the HOM power
data in Fig. 4.

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