DEVELOPMENT OF A 114.24MHz SUB-HARMONIC BUNCHER FOR
THE KEKB INJECTOR LINAC
S. Yamaguchi, S. Ohsawa, M. Ikeda, A. Enomoto and Y. Igarashi*High Energy Accelerator Research Organization (KEK), Oho, Tsukuba, Ibaraki 305-0801 JAPAN* Mitsubishi Heavy Industries, Ltd., Nagoya Aerospace Systems,Oye, Minato, Nagoya, Aichi 455-8515 JAPAN
Abstract

1.5

A 114.24-MHz sub-harmonic buncher cavity was newly
developed to improve the single-bunch purity of theKEKB injector linac. The cavity was designed so as tohave a high shunt impedance and a low maximum surface
field at the acceleration gap. To reduce the probability ofrf breakdown, the inner wall of the cavity was electrolyticpolished and a vacuum pump-out was put at the end wallof the cavity. Copper was selected as the structural
material (it was copper-plated stainless steel for the oldcavity) in order to improve the water-cooling capability
and to reduce the resonant frequency shift by the gapdistance change when the input power increases. A cold
test showed a shunt impedance of 1.2 MOhm, which istwice as large as that for the old cavity. The design,
fabrication and results of low- and high-power tests and abeam-acceleration test are described.

1.5
1.4

1.3

EquivalentcircuitSUPERFISH

1.3

EquivalentcircuitSUPERFISH

1.2

old cavity

1.2

old cavity

1.4

The upgraded KEKB injector linac (8.0-GeV e- / 3.5GeV e+) is now at the stage of initial commissioning [1,2]. The pre-injector of this linac has to provide singlebunch beams of 10nC in 10ps to produce intense positron
beams. To satisfy this requirement, in this pre-injector,
two sub-harmonic bunchers (SHB), i.e., SHB1(114.24MHz) and SHB2 (571.2MHz) were introduced[3,4]. However, the bunch purity had not been
sufficiently good; the percentage of the main-bunch
current to the total current was only 80%. For this reason,
we have developed a new SHB1 cavity having a large
shunt impedance. Another problem of the old SHB1
cavity is low stability of the resonant frequency becauseof the gap-distance shift, due to poor cooling performance.
Redesigns of the structural material and cooling-pipe
configuration have been made.

2 DESIGN
According to the PARMELA simulation, it is possible
to reduce the amount of the satellite bunch to less than
1% if the shunt impedance of the SHB cavity is increasedby 1.7 times[4]. The dimensions of the new cavity wasdetermined for this goal on the condition that the cavity
can be install in the existing focusing coils (φ200mm).

R [MOhm]

R [MOhm]

1(a)

0.9
0.8
0.7

1.1
1(b)

0.9
0.8

a=32.5mm

0.7

b=75.5mm

0.6

0.6

0.5

0.5
1520253035404560

65707580859095b [mm]a [mm]

Figure 1: Dependence of shunt impedance on cavitydimensions; (a) inner radius, a ; (b) outer radius, b.
Figure 1 shows the dependence of the shunt impedance(R) on the inner radius (a) and the outer radius (b) of thecavity calculated by SUPERFISH and an equivalentcircuit model (quarter-wavelength-long short-circuitedantiresonant transmission line). This figure shows thatthe required shunt impedance can be obtained when thevalues of a and b are selected as 30mm and 90 mm,respectively; these values were 32.5mm and 75.5 mm,respectively, for the old cavity.
20015MV/m

180

Energy gain [keV]

1 INTRODUCTION

1.114MV/m
21kW 25kW 32kW 39kW 49kW
60kW

16kW

160
13kW

140

12MV/m
11kW11MV/m
9kW12011MV/m22kW 27kW 34kW

14kW

42kW

17kW
9.5MV/m
9.5MV/m

8.7MV/m

8.3MV/m

7.6MV/m

7.6MV/m

Pin=10kW100

Pin=20kW

6.7MV/m
6.9MV/m

6.1MV/m

Emax=10MV/m

80

5.4MV/m

Emax=12MV/m

4.9MV/m

6010203040a [mm]5060

Figure 2: Relation between the energy gain and the innerradius (a). The numbers beside each mark show themaximum electric field for Pin = constant or the required
source power for Emax = constant.
Figure 2 shows the relation between the energy gain of
SHB1 and the inner radius (a). When the input power isconstant, the smaller is a, the larger is the energy gain.
However, when the maximum surface field is constant,

558

1.6
1.5
1.4
1.3
1.2
1.1
1
0.9
0.8

18
17
16
15
14
13
12
11
10
9

R
Emax

010

20 30 40 50gap distance [mm]parts of the cavity were electrolytic-polished (20µm).

3LOW POWER TESTThe resonant frequency was tuned by adjusting thelength of the short nose (d L ). The sensitivity of thedimensions on the resonant frequency is given in Table 1.
Table 1: Sensitivity of the dimensions on the resonant
frequency. Units of kHz/mm.
∂f/∂LR ∂f/∂LL ∂f/∂dR ∂f/∂dRSUPERFISH 174.180210.73 36.95
Measurement
200.036.3
The results of a low-power test are given in Table 2. The
ratio of the measured Q 0 to the calculated Q 0 (Q 0m/Q 0c )has increased from 0.75 for the old cavity to 0.93. Thismay be due to an improvement in the electrical contactbecause end-plates were connected using bolts and nuts inthe old cavity but are connected by electron-beam weldingin the new cavity.

Emax [MV/m]

R [MOhm]

the energy gain is maximum at a = 40mm. Finally we set
the value of a at 30mm because the power source
available at present is limited to 10kW and beam handlingwould be difficult if the value of a is too small.
The distance of the acceleration gap was determined as
being 40 mm (it was 18mm for the old cavity) because
the longer is the gap distance, the lower is the maximumsurface field. Though the shunt impedance also decreases
for a wider gap distance, it is only a few percent (seeFigure 3).

Table 2: Results of low-power tests.
measurement
Q0Q0QcQL
βQ0m/Q0c
75236989 6741 3433 1.04
0.93

60

SUPERFISH

Figure 3: Shunt impedance, maximum electric field vsgap distance.

The shunt impedance was obtained from R/Q values
measured by Slater's bead perturbation method [5]. Figure
5 shows the electric-field distribution on the beam axis
near to the acceleration gap.
1.14mm

16mm0.98mm

0.8Ez [a. u.]

As the structural material, copper-plated stainless steel
has been used for the old cavity. Also, only the outerconductor has been water-cooled. Consequently, thecooling performance is not good, which causes a change
in the resonant frequency when the input power ischanged. In the new cavity, copper (oxygen free copper)
is selected as the structural material, and the innerconductor and both end plates are water-cooled.
The calculated value of the shunt impedance for the
final dimensions (see Figure 4) is 1.28MΩ, which is 1.7times that for the old cavity (0.77MΩ).
As an rf input coupler, a loop antenna of coaxial
waveguide (20D) has been selected instead of that ofcoaxial line, which is used in the old cavity, because it
has a high possibility of rf breakdown. A copper rod(φ24mm) is used as the tuner. According to a MAFIA
calculation, a frequency change of 200kHz can be obtainedby a stroke of 40mm. The tuner is remote-controlledusing an ultrasonic motor
An ion pump (10l/s) is put at the end plate (see Figure4) to improve the vacuum level of the cavity. All innerSUPERFISH

0.70.60.50.40.3

gap

0.20.10020

φ198
φ6060z [mm]80100120

Figure 5: Distribution of the axial electric field measured
by the bead-perturbation method.

EBWEBW40

vacuum port

coupler

coupler
φ180

φ26

φ40

tuner

d =42.5tuner

L=706.84

monitorFigure 4: Cross-sectional view of the new Sub-harmonic buncher cavity.

559

R [MOhm]

Aluminum spheres of diameter 4, 6, and 8mm were usedas perturbaters. When these values are extrapolated to thelimit of zero volume, the value of shunt impedance of
1.15MOhm was obtained (see Figure 6).

1.9
1.7
1.5
1.3
1.1
0.9
0.7
0.5(a)

y = 0.0024x + 1.1509

0100200
V [m3](10-9)

Figure 6: Relation between volume of bead and shunt
impedance.

4(b)

300

HIGH POWER TESTElectric discharges by multipactoring have been
observed during the initial stage of rf conditioning; it took
ten days to reach an input power of 10kW without
focusing coils and two days after the addition of coils.
During rf conditioning, no change in the resonant
frequency was observed. The pulse shapes for an input
power of 10kW are shown in Figure 7. The time constantcalculated from this pulse shape is about 10µ s. The
theoretical value is 9.6µs ( = 2QL/ω).

Figure 8: Shape of 12nC single bunch. (a) old SHB; (b)new SHB.

6 SUMMARY
In order to improve the bunch purity of the KEKB
injector linac, a new 114.24MHz SHB cavity wasdesigned, fabricated and tested. As a result, the shunt
impedance was increased twice as large as that for the oldcavity (70% came from the geometry and 20% from anincrease of Q 0(measured)/Q 0(theoretical), and a bunch
purity of almost 100% was obtained for a 12nC singlebunch beam. Good stability of the resonant frequency wasobtained by improving the cooling performance.

7 ACKNOWLEDGEMENTS
The authors wish to thank H. Hayano for his valuablecomments on the design of the SHB cavity. They
acknowledge Y. Morozumi for his help in fabricating theinput coupler.

8 REFERENCES

Figure 7: Photograph of rf pulses. Top trace, Reflection;
Middle trace, Input; Bottom trace, Transmission. Inputpower, 10kW; 10µs per division.

5BEAM ACCELERATION TEST

After installation of the SHB cavity on the beam line, abeam-acceleration test was performed. The beam charge
was 12nC at the gun output. Figure 8 shows the bunchshape observed by a streak camera [6]. The bunch purity
was improved from 80% to almost 100%.

[1] A. Enomoto et al., “Construction of the 8-Gev e- /3.5-GeV e+ Injector Linac for KEKB,” theseProceedings.
[2] Y. Ogawa, “Commissioning of the KEKB linac,”these Proceedings.
[3] J-Y Choi et al., “Bunching system of the KEKB
Linac," Proceedings of LINAC96, CERN, August26-30, 1996, 340-342 (1996).
[4] S. Ohsawa et al., “Pre-injector of the KEKB 8-GeV
Linac," these Proceedings.
[5] J. C. Slater, “Microwave Electronics," D.Van
Nostrand Company, Princeton, 1950.
[6] Y. Ogawa et al., “New Streak-Camera System for theKEKB Linac," Proceedings of APAC 98, KEK,
March 23-27, 1998, to be published.

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