The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

R&D STATUS OF THE LINAC UPGRADE PLAN USING A C-BAND
SYSTEM FOR SUPERKEKBS.Fukuda, M. Akemono, M. Ikeda, T. Oogoe, T. Ohsawa, Y. Ogawa, K. Kakihara, H. Katagiri, T.
Kamitani, M. Sato, T. Shidara, A. Shirakawa, T. Sugimura, T. Suwada, T. Takenaka, K. Nakao, H.
Nakajima, K. Furukawa, H. Honma, T. Matsumoto, S. Michizono, Y. Yano and A. Enomoto,KEK, 1-Oho, Tsukuba, 305-0801 Japan.
BAbstractWe are developing a linac upgrade to increase thepositron energy from the current 3.5 GeV to 8 GeV for
SuperKEKB. In order to use the current infrastructure
effectively, a C-band system is being considered to beintroduced. One unit of a high-power RF source and an
accelerator system were successfully tested, and anaccelerator was installed in the beam line of the KEKB
linac to conduct a beam acceleration test. A brief
description of this project and the current R&D status arepresented in this paper.

Damping
Ring

E(e+)=1.0 GeV
Q(e+)=1.2nC

e+ target
C12345E(e+)=8.0 GeV,
Q(e+)=1.2 nC
HER

New C -band units

E(e–)=3.5 GeV,
Q(e–)=10 nC
to target
Q(e–)=5 nC
for Injection

LER
E(e–)=3.5GeV,Q(e–)=5 nC

Figure1 Proposed scheme using C-band systemcomplex similar to the current S-band system, as shownin Fig. 2. In order to make use of the existing
infrastructures, a modulator is changed to use a compact
inverter power supply, which charges to the existing
discharging circuits. This enables us to decrease themodulator size to one third of the existing one, as shownin Fig. 3, and to install 48 modulators in the existing
klystron gallery. Klystron sockets, such as pulse
transformers and oil tanks, are also planed to be reused.
The currently designed RF system has a 100-kW subbooster (SB) klystron that drives eight 50-MW klystrons
to suppress the cost for the individual driver amplifiers.
The accelerator guide is a half-scaled one of the current
S-band structures, which has a 40-MV/m acceleration
gradient; accelerator girders are also considered to bereused.
In order to obtain a 40-MV/m acceleration gradient, the40-MW output power from the klystron is multiplied by aSLED (a power multiplication of 3.4 at a pulse width of 2µs), and then fed to two 2-m long C-band accelerators,
which are scale-downed versions of the S-band structure
by one half. Assuming a filling time of 380 ns and an

The KEK b-factory (KEKB) has successfully achieveda goal luminosity of 1x1034 cm-2 s-1 and has been
accumulating valuable data of B-meson physics. Recently,
the SuperKEKB project, which aims to achieve a oneorder higher luminosity, is being considered. A required
major change for the linac in this project is switching thebeam energy, i.e., the positron energy from 3.5 to 8.0 GeV
and the electron energy from 8 to 3.5 GeV, in order tosuppress positron beam blow-up caused by the electroncloud in the ring. At the same time, positron and electron
currents are required to be increased up to 1.2 nC and 5nC, respectively. Among the several designs, it ispresently being considered to replace the last 3 sectors (24S-band klystron units) with the 6 sectors using the C-bandsystem (48 C-band klystron units) [1]. Positrons after theconversion target are accelerated up to 8 GeV using fortyeight SLEDs and 50-MW klystrons. This is schematicallyshown in Fig. 1. The RF system is considered to be a
MAIN DRIV E LINECountereGun

1.7-GeVJ-arc for e–INTRODUCTION

5712MHz

2856MHzCW
571 MHzA

Ref. S ignal
Generator(FMD-30)

C-band Waveguide Driving Line
SUB-BOOSTER 3-1

SUB-BOOSTER 2

10 MHz

MR

E/O

O/E

508 MHz114 MHz

57 1 MHz Optical Lines

TriggerSystemDriver30m

100m100m

SUB DRIVE LINE(39D)

Co ax ial Lines
20 m

40m
5m
SB-A

5me-

SB-B

80m
SB- C

IφA

SB-1

1 kW
Amp.
SystemElec tron G un

5 71
MH zPrebuncher
S HB2

IφA

SB3-2

50MW S-band
Kly stron

High Power
ATT /Pha se
Shi fter

11 4
M Hz
SHB1

SUB DR IV E LINE(WG)

30m

1 kW
Amp.
System

10kW
AMP.

100 kW C-band
Klystron

60kW S-band
Klystron

15m

SLED

S B4-1

SB4-2

SB5-1

SB5-2

5 0MW C -band
Klystron
SL ED

Ta rge t
ACC

ACC Guide
Bu ncher

Sector A

Figure 2

Secto r B

Sector C

Sector 1e+ e-Electron to LERACC

Sector 2Sector 3-1

Sector 3-2 Sector 4-1 Sector 4-2 Sector 5-1 Sector 5-2

Positron/Damping Ring

Block diagram of C-band System for SuperKEKB upgrade

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

50-MW klystron and driving system

Present Configuration ofModulator and Klystron
Klyst ron
Modulator
Modulator

Inverter

40MW
Klystron

Modulator

40MW
Klystron

6 00
60 0RF an d Coil P/S
C ont rol

560
56 0

Control

Modulator
60 0Control

8 00

Klystron

Proposed Configuration

BeamFigure 3 Comparison between the old and newmodulator (left) and RF feeding scheme (right)
attenuation constant of 0.703, a peak power of about 59
MW at the entrance of the accelerator guide and anaverage acceleration gradient of 40 MV/m. have beenobtained. The SLED is considered to be a CERN-LIPS
type one, which uses the TE 038 mode. This is almost thesame size as the current S-band SLED and it is expected
to utilize the manufacturing experience effectively.

C-BAND R&D STATUS
C band R&D started in the spring of 2002. The shortterm goal was to install an accelerator guide in the EKEB
linac of #4-4 (where no accelerator was installed) during
the shutdown period of the summer 2003 and a beam test
would be conducted in the autumn of 2003. We started bypurchasing C-band components, since we had nocomponents of the C-band frequency. Several important
items for this project were designing and manufacturing
the C-band accelerator, a SiC dummy load, a waveguidevacuum flange, the construction of an RF system
including the development of an SB klystron, a highpower RF window and a compact modulator. Thosecomponents were successfully completed by the summerof 2003. A high-power test of the system, and processingof the accelerator guide were also completed, and theguide was installed to the beam line of the KEKB linac onschedule.

Compact Modulator
A compact modulator was a key issue in order to install
all of the required units in the existing klystron gallery. A
proto-type was manufactured using a dc inverter power
supply, which charged the PFN. In the PFN, two circuitswith 14 inductances (1.55 nH) and 14 capacitances (15.5µF) each were placed in parallel. The designed flattop
was 2µ s and the characteristic impedance of the PFN was
5 ohms. A pulse transformer of having a 1:15 step-up ratio
gave a 350 kV pulse at maximum to the high-power
klystron. Though we faced many problems, which caused
the IGBT to break down at the first stage, it has beenstably operating for more than 1000 hours up to now.

A simple RF system was used in the test stand, which
comprised a 2856-MHz signal generator, a frequency
doubler, a pulse modulator, an 8W-transistor amplifier
and a SB klystron. The SB klystron was manufactured by
re-tuning a weather-satellite klystron. From the SBklystron, the output power was fed through a WR-5
waveguide to a Toshiba 50-MW klystron (E3746). Theklystron socket was used in the same way as the S-band
one, which was possible to use up to 350 kV.

High-Power RF Window
It was necessary to use the RF window to separate thevacuum system between the klystron and the accelerator
guide. Although two windows were equipped in the highpower klystron, because of the simplicity of the vacuumsystem, we developed a high-power RF window, which
was capable of being used at the 50-MW power level.
This was a mixed-mode window (TE 11+TM11) proposedby Kazakov. Using the mixed mode, the field decreased at
the ceramic surface and on the edge, where brazing wasconducted.

SiC dummy load
It was necessary to develop a dry load, which absorbed
the RF power at the exit of the accelerator guide. A SiCdummy load was developed with loading the SiC
cylindrical materials on the waveguide surface of the Eplane. Two SiC dummy loads set in parallel weresuccessfully tested, as described below.

Figure 4 Cut-away view of accelerator (top), disks andcylinders (bottom left) and coupler (bottom right).

Development of an Accelerator Guide
Though the basic design was assumed to use a 2m-longaccelerator guide, a 1-m long C-band accelerator guide,
which was half scale of the current S-band one, wasmanufactured as a prototype. This structure had anelectric property of the last half section of the 2-m longguide, and 2a was varied from 12.44 to 10.41 mm. The
filling time was 234 ns and the attenuation constant was0.434. With the condition of no SLED and 40-MW output
power from the klystron, an acceleration gradient of about
39 MV/m was expected. The design was made using theHFSS and MAFIA-3D code. Disks and cylinders weremanufactured at KEK and MHI (Mitsubishi Heavy

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

Figure 5 Birds-eye view of test hall (left) and waveguidesystem in test stand (right)
Industry) and electroplating of the regular cells andelectron beam welding of the couplers were done at MHI.
Fig. 4 shows a drawing and photos of the cell and coupler.

Test Bench and Test Results
A C-band test station was set in the accelerator
assembly hall next to the S-band klystron test hall in theKEKB linac, considering that the distance from the RF
source to the shielding room was short. In this stand, amodulator test was performed in February, 2003, and a Cband klystron test was conducted up to 42.5 kV of Es(PFN voltage) with a pulse repetition of 50 pps and apulse width of 2 µ s. An output power of 43 MW wasmeasured with two water-loads. Then, the klystron output
characteristics were measured. We also adjusted the pulse
flat top by changing the PFN inductance. Tentatively,
1.3% flatness was obtained. Another adjustment led to aflatness corresponding to a phase variation of 2.6 degrees.
Fig. 5 shows photos of the test hall. Fig. 6 shows variouswaveforms. A resonant ring test of an RF window up to a
forward travelling wave of 160 MW was performed in
March, 2003, which confirmed the high-power capabilityof a mixed-mode RF window. A high-power test of SiC
dummy load up to 43 MW was successfully conducted in
June, 2003 [2].

Figure 7 Resonant ring test and wave forms. Second
waveform shows the forward travelling wave.
saved. Conditioning up to the 43.7 MW (corresponding to42 MV/m) with 50 pps repetition and a 500 ns pulse
width (this corresponds to the SLED power width) was
successfully achieved with reasonable fault rate. Before
installing to the beam line, phase shift measurements wasperformed to confirm no serious damage to the guide. Fig.
8 shows the processing history as a function of the pulse
shot numbers. This accelerator guide was installed to thebeam line of KEKB linac in September.

SUMMARY
The first test of the C-band accelerator unit, including
the RF driving system, klystron, waveguide components
and 1m- long accelerator guide, was successfully
performed for the linac upgrade of SuperKEKB. We
installed an accelerator guide to the KEKB linac beamline, and are now waiting for beam acceleration.
Conditioning of the system in the klystron gallery isscheduled from October 1, 2003, and an evaluation ofbeam acceleration will be conducted. At the same time,
we will prepare a new test stand to test the next
accelerator guide and an RF compressor. We have a plan
to install two 2m-long accelerator guides fed by a 50-MW
klystron or four 2m-long accelerator guides fed by two50-MW klystrons in the #4-4 station of the klystrongallery. This will give us important C-band operation
experience as well as an energy contribution to the KEKB
operation.

RF Power

Es

Figure 6 Wave forms; Voltage pulse (top), Phase(second), current (third) and output rf(bottom).
Horizontal scale is 1 µs/div.
The high-power processing of a 1m-long accelerator
guide was conducted from July to August of 2003. Thisprocessing was performed under a pattern determined by
the program. Interlocks of the internal pressure and theVSWR were used to process the accelerator guide
properly. Data, such as the processing history, the darkcurrent and waveforms of the reflected power, wereautomatically saved to memory to be used for an theanalysis of this processing. A vibration sensor to detect
the arcing position in the guide was set and data was also

Figure 8 Processing history of accelerator guide witha function of shot numbers of rf pulse.

REFERENCES[ 1] T. Kamitani et. al., “R&D Status of C-band Accelerator unit forSuperKEKB”, PAC2003, Portland, Oregon, USA, May 12-16, 2003
[ 2] S. Fukuda et al., “C-band Plan of KEK electron/positrons linac forSuperB (in Japanese)”, Proc. of the 28th Linac Meeting in Japan,Tokai, Japan, July 30-August 1, 2003