R & D STATUS OF C-BAND ACCELERATOR UNIT FOR SUPERKEKBT. Kamitani∗ , T. Suwada, for the KEK electron/positron Injector Linac andthe Linac Commissioning Group, KEK, Tsukuba, Japan
Abstract
In an extensive luminosity upgrade of the KEK-B factory, considered as a future plan, the injector linac has toincrease the positron acceleration energy from 3.5 GeV to8.0 GeV. In order to double the acceleration field (from 21to 42 MV/m), a design study of the C-band acceleratingunit has been started. This paper reports on the R & D status of the C-band components for a high-power test and fora beam-acceleration test at the KEKB injector linac.

ring (DR) is added for emittance reduction. The accelerator
units after the DR are to be upgraded with C-band components to achieve 8-GeV injection. Another upgrade scheme
using beam recirculation instead of the C-band units is also
considered, but it is not mentioned here.

Recently, the KEK b-factory (KEKB) has surpassed its
goal luminosity of 1×10 34 cm−2 sec−1 . Though it is theworld-highest luminosity ever achieved, an even higher luminosity (1035 − 1036 cm−2 sec−1 ) is desired for a deeper
understanding of a B-meson physics. The design consideration of this extensive upgrade, called ”SuperKEKB”[1], started in 2001, and R & D of the components are
now underway. One of the major changes in this upgrade
is the switch of the beam energies (from 8.0-GeV electrons and 3.5-GeV positrons to 3.5-GeV electrons and 8.0GeV positrons). Raising the beam energy suppresses thepositron beam blow-up caused by the electron-cloud inside
the vacuum chamber.
The present KEKB injector linac accelerates electronsup to 8 GeV with 55 accelerator units, including a few
stand-by units [2]. The electrons are injected directly into
the High Energy Ring. For positron injection, the electrons are accelerated up to 4 GeV in the former half ofthe linac (26 units), and hit a metal converter target to produce positrons. Those positrons are accelerated up to 3.5
GeV in the latter half (29 units), and are injected into theLow Energy Ring. For the SuperKEKB, positron acceleration energy is required to be increased from 3.5 GeV
to 8 GeV. The maximum energy gain by all of the present
positron accelerating units is not sufficient (4.8 GeV) for 8GeV injection. Since the positron intensity is proportional
to the primary electron energy, the number of positron acceleration units cannot be increased by changing the target
position at the cost of the units for the primary electron.
The simplest way to realize this energy upgrade is to increase the acceleration field by about twice. It requires afour-times higher rf input power, if the present S-band accelerating structures are used. It is not realistic based on
the limit of the capacity of the present klystron gallery andthe input electricity power. Instead, the possibility to use
C-band accelerating structures is considered. The upgrade
scheme presently considered is shown in Fig.1. A damping∗ takuya.kamitani@kek.jp

e- GunDamping
Ring

1.7-GeVJ-arc for e—E(e+)=1.0 GeV
Q(e+)=1.2nC

e+ target
CINTRODUCTIONAB

1E(e—)=3.5 GeV,
Q(e—)=10 nC
to targetQ(e—)=5 nC
for Injection

2345E(e+)=8.0 GeV,
Q(e+)=1.2 nC
HER

New C-band units
LER
E(e—)=3.5GeV,
Q(e—)=5 nC

Figure 1: upgraded Linac with C-band units

ACCELERATOR UNIT
The present S-band accelerator unit of the KEKB injector linac involves four 2-m accelerating structures fed by aklystron through a SLED-type rf-pulse compressor. The
average accelerating field gradient is 21 MV/m with 41
MW output power of 4 µs pulse duration from the klystronand with rf-pulse compression. The energy gain by a unit istypically 160 MeV. In the upgrade to the C-band, an S-bandunit is replaced by two identical sub-units; each is composed of two 2-m accelerating structures fed by a klystronthrough an rf compressor. The average accelerating field
gradient is expected to be 42 MV/m with 40 MW output
power of 2 µs duration from the C-band klystron and witha LIPS-type rf-pulse compressor [3]. The total energy gain
by two sub-units is 320 MeV.
Compared with the S-band unit, the rf power source isdivided into two. It is mainly because there is no C-band
klystron available which can supply the 80 MW power necessary for the four accelerating structures. Instead, two 50MW class klystrons are used. The modulator is also divided into two. A design comparison was made between
the single modulator feeding two klystrons and the twomodulators feeding a single klystron, respectively. In thecase of the single modulator, it becomes too large to re-use
the present components, like the capacitors, and requires
complete renewal of the design to fit in the present klystrongallery, and costs too much. Finally, we decided to adopt
a system composed of the two identical sub-units. It facilitates the design of the modulator and reusing the presentcomponents. What is more, by complete isolation of thesub-units, the operation is more flexible, since individual
tuning is possible.

RF SOURCEACCELERATING STRUCTURE

The present S-band unit uses a 50-MW class klystron(Mitsubishi PV3050 or Toshiba E3730), which supplies 41
MW output power of 4.0 µs rf pulse duration. In the R& D work for the C-band linear collider, a 50-MW classklystron was developed by T. Shintake and H. Matsumoto[4] and is commercially available (Toshiba E3746). It is
operated at 5712 MHz with a beam of 358 kV voltage and320 A current in pulse repetition of 50 Hz. It needs 240 Wof drive power and delivers an output power of 53 MW in
2.5 µs duration. In the C-band upgrade of the KEKB linac,
this klystron is the first candidate. A klystron of this typehas already been fabricated and used in a high-power test
in a test stand since December 2002 and will be used in abeam acceleration test scheduled from this September.
The present line-type modulator produces a 4.0 µs long(flat-top) 306 kV high-voltage pulse for the klystron at arepetition rate of 50 Hz. In the upgrade to the C-band unit,
two modulators for the two klystrons must be installed in
the same space of the klystron gallery, as shown in Fig.2.
To make the modulator more compact, the present highvoltage dc power supply and the charging circuit will be replaced by a compact inverter-type power supply. To generate a flat high-voltage pulse of 4.0 µs duration, two pararell
pulse-forming-network (PFN) circuits, each composed of
20 capacitors (0.0146 or 0.0155 µF) and 20 inductors (0.67µH), are used in the present modulator. In the C-band modulator, a pair of PFN circuits (14 capacitors and 14 inductors) is used to generate a pulse of 2.0 µs duration. Most of
the components in the S-band modulators, like PFN capacitors, thyratrons, cabinets, pulse transformers and oil-tanks
can be reused, which contributes to reduce the cost of theupgrade.

The accelerating structure used in the present S-bandunit is a 2-m long, disk-loaded, traveling-wave structure
operated at 30.0 deg C. The structure is composed of 54
regular cells and two (input, output) coupler cells, andthe phase advance per cell is 2/3π. The iris diameter isdecreased linearly cell-by-cell to achieve a quasi-constantfield gradient. The typical size of the iris diameter is 24.275- 20.300 mm from the entrance to the exit. A total of fivevariant types of structures which have different iris sizes
are used to distribute the deflecting mode of the cavities
and prevent a beam break up from occuring during longpulse operation. As shown in Fig.3, the first prototype Cband 1-m accelerating structure is designed as a scale-down
model of the present S-band 2-m structure, which has thelargest iris diameter within the five variants. The dimen-4800
600

Modulator

1500

Present
Configuration

Klystron600

850

RF & CoilP/S

560
480

Invertor

850

1500

Configuration
fora C-band unit

Modulator
Klystron

600

1200

800

Control

Figure 2: Modulator/Klystron layout
In the present rf drive system, an rf signal of 2856 MHz
is transmitted from the main drive system to each subbooster through phase-stabilized optical fibers. In the subbooster, the signal is amplified by a 60-kW klystron anddistributed to eight klystrons. In the sub-booster of theC-band unit, the frequency is doubled with a multiplier
to 5712 MHz. Concerning the sub-booster klystron, instead of designing a brand-new one, an available 200 kW
klystron operated in 5.3 GHz for radar (Mitsubishi PV5101) is modified for 5712 MHz operation.

Figure 3: 1-m long C-band Accelerating structuresions of the regular cells can be scaled precisely to half thatof the S-band structure. They are already processed and thephase advances in respective cell are adjusted to 120 degree
within an error of 2 degrees with the nodal-shift measurement. As has been done for the S-band structure, the disks
and the cylinders processed individually are formed into an
integral structure by an electroplating technique. Unlike
the regular cells, the coupler dimensions (the iris width andthe coupler cell diameter) have to be modified because thedimension of the wave-guide, which is connected to thecoupler, is not scaled. The coupler dimensions are roughly
optimized with the MAFIA-3D code to achieve the proper
coupling strength and the minimum reflection, and further
finely adjusted by milling and lathering step-by-step withthe feedback from the measurement of the rf property. Fabrication of the couplers for high-power operation and electroplating of the regular cells are in progress. The couplers
and the regular cells are to be connected later by electronbeam welding, and will be finished by the end of June.

RF COMPONENTS
In place of the WR-284 (WRJ-3) standard wave-guide(inner dimension; 72.1 mm × 34.0 mm) used in the S-band
unit, the WR-187 (WRJ-5) standard wave-guide (inner dimension; 47.55 mm × 22.15 mm) is adopted for the Cband unit, considering the power loss and the contamination of the higher-order mode. An Merdinian-type flange
for the wave-guide is designed and used at the test stand.
As an rf window to separate the vacuum of the accelerating structure and that of the region close to the klystron,

a pillbox-type window is already available, which has beendeveloped as a component of the C-band klystron. However, its maximum transmission rf power is 25 MW. A new
type of an rf window, which can transmit 50 MW, is being
developed to simplify the system in the C-band unit. Unlike
the pill-box type window, it uses a mixture of two modes(TE11 and TM11) to generate a traveling-wave in the ceramic of the window. It lowers the electric field strength
at the edge of the ceramic and reduces the electron emission, which results in a breakdown of the ceramic. With
the high-power test performed using a resonant ring at thetest stand, the new rf window has been confirmed to workwithout any problems at 160 MW rf power and a 2 µs duration at 50 Hz repetition.
A 3-dB high-power rf divider of the C-band has been
designed by simulations using the HFSS code to determine
the approximate dimensions. It does not have a button inside, which is often used for a fine adjustment to reduce
the reflection, but sometimes causes discharging. Instead,the inner dimension is finely optimized by measuring therf transmission properties of 16 candidate low-power test
modules. Modules for high-power operation are under fabrication.
A high-power dummy load to fit for the WR-187 waveguide is being designed. Inside the wave-guide of thedummy load, cylindrical SiC blocks of 12 mm diameterstand in line with the gradually growing heights to absorb
the rf-power uniformly. The heights and the positions ofthe blocks are optimized using the HFSS code. They arefurther optimized to achieve the least reflection by an rfmeasurement using a low-power test module, in which theblocks are changeable and the positions are variable. The
modules for high-power operation are being fabricated.
The rf pulse compressor for the C-band is still in thestage of conceptual design. Since the SLED-type cavity
using TE015 mode [2] does not give a sufficient Q-factor
and field multiplication at a frequency of C-band, an LIPStype cavity using the TE038 mode [3] is being considered.
A low-power prototype cavity will be fabricated this summer to optimize the coupling hole dimensions.

connected to the modulator as a proper load. After conditioning, a 43 MW output power of 2.0 µs duration from theklystron at 50 Hz repetition was achieved in April.
The accelerating structure, the 3-dB high-power divider
and the SiC dummy loads will be installed in the test standin June. Their rf-conditioning and a high-power test will
be performed in July and August. In mid of August, thecomponents of the C-band accelerator unit will be moved
from the test stand to the location of the ”4-4” unit in theKEKB linac, which is temporarily vacant. The energy gain
and the average acceleration field will be evaluated by measuring the beam energies of the electrons and the positrons
in the beam study from September.

TEST STAND AND BEAM
ACCELERATION STUDY[3] ”Design Consideration, Construction and Performance of aSLED Type Radiofrequency Pulse Compressor using very
high Q Cylindrical Cavities” By A. Fiebig, R. Hohbach, P.
Marchand, J.O. Pearce (CERN). CERN-PS/87-45, Mar 1987.
3pp. Presented at 1987 Particle Accelerator Conf., Wash.,
D.C., Mar 16-19, 1987.

For rf-conditioning and high-power test of the C-band
components, setting up of a test-stand started in December
2002. The test stand has been used for conditioning theS-band accelerating structure, which is to replace the damaged structure after long use in the linac. The stand has also
been used for a high acceleration field study of the S-bandstructure, which recorded a field of 40 MV/m with 41 MW
supplied into a single 2m-long structure through a SLEDtype rf compressor cavity. The stand has been re-arranged
for a C-band study as shown in Fig.4. A unit test of themodulator, began in January 2003 with an coaxial dummyload. Initial tuning and the trouble shooting of the modulator has been performed. Later, a C-band klystron has been

Figure 4: C-band High-power Test Stand

REFERENCES[1] ”Expression of Interest in A High Luminosity Upgrade of theKEKB Collider and the Belle Detector”, By I. Abe, et al.,January 2002
http://www-kekb.kek.jp/SuperKEKB/Document/EoI.pdf[2] ”The KEKB injector linac”, By I. Abe, et al., Nuclear Instruments and Methods in Physics Research Section A, Volume499, Issue 1, 21 February 2003, Pages 167-190[4] ”Development of C-band 50 MW pulse klystron for e+ e- linear collider”, By T. Shintake, N. Akasaka, H. Matsumoto,
Y. Ohkubo, H. Yonezawa, KEK Preprint 97-47, Presented
at 1997 Particle Accelerator Conference, Vancouver, BC,Canada, May 12-16, 1997