Proceedings of LINAC 2004, Lübeck, Germany

SKIP - A PULSE COMPRESSOR FOR SUPERKEKB
T. Sugimura, T. Kamitani, K. Yokoyama, K. Kakihara, M. Ikeda and S. Ohsawa
KEK, 1-1, Oho, Tsukuba, Ibaraki, Japanneeded. We considered using the cavity in the TE0,3,8mode adopted in the LIPS cavities [5]. The Q factor ofabout 150,000 is expected in TE0,3,8-mode cavity. Thereby,an energy multiplication factor of the same as SLED for
S-band can be obtained.

Abstract
A C-band RF pulse compressor “SKIP”, which stand forSuperKEKB Injector Pulse compressor, has beendeveloped for the SuperKEKB project aiming luminosity
upgrade of the present KEK-B factory. The design of thecompressor using TE0,3,8 mode cylindrical cavity is basedon the “LIPS” used in the LEP injector S-band linac.
Detailed dimensions of the cavity have been optimized
for C-band (5712 MHz) with low power models. In thehigh power test of the pulse compressor, a peak output
power of 200 MW is achieved with the input RF power of43MW in 2 µsec duration.

DESIGNAs already stated, the cavity in the TE0,3,8 mode wasadopted. Considering a simple cylindrical cavity, there are
two adjustable parameters such as diameter and height.
For the first step, a scale down model of LIPS cavity isconsidered. The diameter of 232.82 mm is as almost sameas the present S-Band SLED type pulse compressor in
KEKB injector linac. It is necessary to investigate theresonance modes of a cylindrical cavity with such form.
The mode which should be most careful of is the TM1,3,8mode which is degenerating with the TE0,3,8 mode. It isalso necessary to take care about the TE12,1,1 mode. Inorder to detune these modes, there is a groove on the baseplate as shown in a Fig1. In the numerical computation byMAFIA, these modes are detuned, as shown in the Table1.
Table 1: Frequency shift by groove

INTRODUCTIONKEKB attained the highest luminosity (1.3×1034cm-2s-1)
in the world. SuperKEKB, an upgrade of KEKB whosetarget luminosity is 1-5 ×1035 cm-2s-1 is underconsideration [1]. In the SuperKEKB project, energy
exchange of beams has an important role. In order toescape the influence of electron clouds, the energy of anelectron beam is lowered to 3.5 GeV from 8 GeV, and theenergy of a positron beam is raised from 3.5 GeV to 8GeV. Although it is easy to lower the energy of anelectronic beam, it is not easy to raise the energy of apositron beam. A positron is a secondary particle, andafter generating it, the space in which a positron beam isaccelerated is restricted. One of solutions is to double anacceleration field. Thus the C-band accelerator module
which has double acceleration field has been developed[2]. In the summer of 2003, an accelerating structure wasinstalled into the beam line and beam acceleration has
been performed since. The acceleration gain with theaccelerating structure is 40 MeV/m, feeding power from aklystron into one 1m-long structure. In the complete
composition of accelerator module, one klystron feeds
two 2m-long structures. Since peak power runs shortwith the present 50 MW class klystron in such a case, anRF pulse compressor is essential.
Modified RF pulse compressors of SLED are used in
the sections of S-band [3]. The half-scale model of S-bandstructure was adopted in the design of an acceleratingstructure [4]. In the design of a pulse compressor, when
the scale down model of S-band is considered, sufficient
Q factor is not acquired. Then, another design plan isMode
TM1,3,8
TE12,1,1TE0,3,8∆F from TE0,3,8 (MHz)-12.723
11.0730An energy multiplication factor M is given by [6][M = γe −Ta / Tc 1 − (1 − g )1+ν][g (1 + ν )] − (α − 1),−1where Ta is the filling time of the accelerating structure, Tc
is the filling time of the cavity, α=2β/(1+β) andν=Ta/Tc[ln(1-g)]. The group velocity vg varies linearlywith the position z according to vg(z)= vg0(1-gz/L), wherevg0 is a group velocity at position z=0, L is the length of
the accelerating structure and g is the quantity about thegradient of a group velocity vg variation along thestructure.
For the case of C-band accelerator,characteristic parameters of RF are given in Table 2.
Using these values, relation between coupling factor βand energy multiplication factor is represented in Fig. 2.
Table 2: Main RF parameter
FrequencyQ0TagL
Full RF pulse width
Pulse width before
phase inversion

φ232.82
φ212.8φ87.6
3mm

F0(MHz)5684.525
5708.321
5697.248

9mm

Figure 1: Sectional view of the groove

15712 MHz
130000350 nsec
0.6
1.924
2.00 µsec
1.65 µsec

Proceedings of LINAC 2004, Lübeck, Germany

inside the cavity in the radial and longitudinal directions,
it turns out that the number of nodes of the resonance “e”
is consistent with the TE0,3,8 mode as expected.

1.85

M

1.84
1.83

length of coupling hole
6mm
8mm
12mm
target

1.82
1.81
100

1.80
5678910mm

10β1βFigure 2: Energy multiplication factor as afunction of coupling factor β.

0.1

10.00.01
108.0
6.015
Diameter of hole [mm]20βFigure 4: Calculated coupling factor β as afunction of diameter of coupling hole.

4.0
2.0

100.0304050607080

β20

Plunger position [mm]Figure 3: Measured coupling factor β as afunction of position of short plane when thediameter of hole is 18.8 mm.

D=18.8 mm

beta=6.9

6.08.01
5.0

7.0

9.0

Length of Coupling Hole [mm]

COLD MODEL

Figure 5: Measured coupling factor β as afunction of length of coupling hole when thediameter of hole is 18.8 mm.

In order to determine the dimensions of the compressor,
a cold model was manufactured and RF measurement wasperformed. Using the cold model, cavity length wasdetermined so that the resonant frequency becomes 5712
MHz. +1mm of cavity length is equivalent to -8.7 MHz
on frequency.
The cavity is coupled to the waveguide through twoholes on the sidewall of waveguide. They are half wave
length away from each other. Figure 3 shows a relation
between β and position of short plane. The position of ashort plane is optimized in order to maximize β and isabout 3/4 wave length, which corresponds to the plunger
position of 61.3 mm, off the one of coupling holes.
Figure 4 shows relations between β and Diameter ofcoupling hole computed by MAFIA. Three coupling platewith different thickness were prepared based on therelations, and each β and Q factor was measured withchanging a diameter of the holes. Figure 5 shows arelation between the length of coupling hole and β.
Although the tendency of an actual measurement result
suited the calculation result, some absolute values hadshifted. The diameter and length were determined bycold-model measurements to be 18.8mm and 6mm,respectively. Under these conditions, unloaded Q factor is148000. Figure 6 shows that there are 8 resonances near
5712 MHz. By bead perturbation measurement of a field

5.0

S11 (dB)

0.0

abc

d efgh

5750

5800

-5.0

TE0,3,8-10.0
-15.0
-20.0
5600

5650

5700

Frequency [M Hz]

Figure 6: Reflection from the cavity (S11). Thereare 8 resonances near 5712 MHz.

HOT VERSION
The first product is manufactured based on thedimensions determined with the cold model. However,unloaded Q value and β were smaller than those of thecold model. It is expected to come from the difference in2

Proceedings of LINAC 2004, Lübeck, Germany

the processing method of a coupling plate. Although
coupling plate of the cold model was processed with alathe, the plate of the hot version was processed with amilling cutter. As a result, the roughness of the surface ofa coupling plate differed, and unloaded Q value and β fell.
Therefore, the length of holes was reduced to 5.35 mm
from 6 mm in order to achieve sufficient β.
Results of RF measurement for hot version of cavities arelisted in Table 3. A 3dB-hybrid coupler is attached to twocavities and it is completed. Figure 7 shows a whole view of theRF pulse compressor. Figure 8 shows the 3dB-hybrid coupler
for the pulse compressor.

Table 3: Measured parameter of Cavities(30 centigrade, N2 gas)

Cavity A
Cavity B
Cavity A,B with
3dB coupler

Q0136000
137000β6.71
6.79

133000

6.59

Figure 8: Whole view of 3dB-hybrid coupler (left) andwaveguide for low power RF measurement (right).

CH1
CH2

After an RF processing of about 170 hours, the peakoutput power attained 200 MW at a repetition rate of 50
pps with the pulse duration of 2 µsec. Figure 9 showssome forms of RF pulses. RF power at the exit of a
klystron and the exit of the pulse compressor was measured,and it turns out that the ratio of the peak output power toinput power is 4.7.
This RF pulse compressor is named “SKIP”(SuperKEKB Injector Pulse compressor). SKIP is now
installed into the RF system of C-band accelerator moduleof injector linac for KEKB, PF (Photon Factory) and PFAR (Advanced Ring). It will be tested for a year under thereal operation. The energy multiplication factor M will bemeasured in the beam acceleration test.

CH3TIME

400 nsec/div

Figure 9: Form of input signal (CH1), VSWR
(CH2) and output signal of SKIP (CH3). They
are plotted in arbitral units in vertical axis.
The peak of CH3 exceeds 200 MW.

ACKNOWLEDGEMENT
The authors would like to give their sincere thanks toMitsubishi Heavy Industries, LTD. for their powerfulcooperation.

REFERENCES[1] J. W. Flanagan, Y. Ohnishi, et al., Letter of Intent for
KEK Super B Factory, Part III: Accelerator Design,
KEK Report 04-4
[2] S. Michizono, et al., “KEKB INJECTOR LINAC
AND UPGRADE FOR SUPERKEKB”, thisconference.
[3] I. Sato, et al, “design report on PF Injector linac
Upgrade for KEKB”, KEK Report 95-18, in Japanese[4] T. Kamitani, et al., “Development of C-band
accelerating section for SuperKEKB”, this conference.
[5] A. Fiebig, et al., “Design considerations, construction
and performance of a SLED-type radiofrequency
pulse compressor using very high Q cylindricalcavities”, CERN/PS 87-45(RF) March, 1987
[6] Z. D. Farkas, et al., “SLED: A method of doubling
SLAC’s energy” SLAC-PUB-1453, June, 1974

Figure 7: Whole view of the pulse
compressor “SKIP” at test stand.

SUMMARYA C-band pulse compressor SKIP with TE0,3,8-mode
cylindrical cavities is developed. It achieves a peak output
power of 200 MW with the input RF power of 43 MW
and the repetition rate of 50 pps.

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