JAERI-Conf 95-021

1p-4
Present Status of the KEKB ProjectShin-ichi Kurokawa
National Laboratory for High Energy Physics, KEK
1-1 Oho, Tsukuba, lbaraki, 305 Japan

ABSTRACTKEKB (KEK B-Factory) is an 8 x 3.5 GeV, two-ring,
electron-positron collider aiming at producing copious Bmesons for detecting CP-violation effect at bottom quarks.
To achieve a luminosity of 1034 cm-2s-l required byexperiments, the rings should store 1.1-A electrons and2.6-A positrons. These large currents are distributed into
5000 bunches. The large currents and the large numberof bunches excite strong coupled-bunch instabilities,
which should be avoided by adopting special accelerating
cavities and strong bunch-by-bunch feedback systems.
Electrons and positrons collide at a finite angle of ± 11
mrad at an interaction point, which BELLE detector
surrounds. The construction of KEKB started in 1994 andit will be commissioned in JFY1998.

replacing klystrons with high-power ones, andcompressing RF pulses by SLEDs. We can also increase
the energy of electrons impinging on the positron
production target from 250 MeV to 4 GeV, thus
multiplying positron intensity by 16. The injection timeof positrons to LER is estimated to be 900 sec. A newbypass tunnel of 130 m for transport lines between thelinac and KEKB rings will be constructed in JFY1996
and 1997.
II. BASIC DESIGN

The main parameters of the KEKB accelerators are
given in Table 1. HER and LER have the same
circumferences, ernittances, and the .B functions at IP.
The large current, the large number of bunches, small
bunch spacing, the small value of .B function at IP andfinite-angle crossing of beams are the salient features ofI. INTRODUCTION
KEKB.
We adopt a noninterleaved sextupole chromaticity
Two rings of the KEKB (3.5 GeV low-energy ring, correction scheme to increase transverse dynamicLER, for positrons, and 8 GeV high-energy ring, HER, apertures necessary for injection and longitudinalfor electrons) will be installed in the existing TRISTAN dynamic aperture necessary for making
Touschek
tunnel of 3 km circumference and the infrastructure of lifetime sufficiently long[2].
TRISTAN will be maximally utilized. Taking advantageof the large tunnel size of TRISTAN, two rings will be set
side by side; unnecessary vertical bending of trajectories
that may increase the vertical emittance of the beams isminimized.
Figure 1 illustrates the arrangement of two rings.
KEKB has only one interaction point, IP, at Tsukuba
experimental hall, where electron and positron beams
collide at a finite angle of ± 11 mrad. BELLE detector
will be installed at IP. The straight section at Fuji is used
for injection from the linac and also for installing RF
cavities of LER. Cavities of HER will be installed instraight sections at Nikko and Oho. These straightsections are also reserved for wigglers for LER, which
reduce the longitudinal damping time of LER from 43
msec to the value of HER, 23 msec. In ~rder to make thecircumference of the two rings equal, a cross-over should
be made at Fuji experimental hall where two rings passeach other.
To facilitate full-energy injection into the KEKB
rings, the present 2.5-GeV electron linac will be
The upgrade is done by
upgraded to 8 GeV[l].
combining the main linac with .the positron production
linac, increasing the number of accelerating structures, Fig. 1 Configuration of KEKB accelerator system.
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JAERI-Conf

In this scheme sextupoles are arranged in pairs: twosextupoles in a pair are Jt phase-advance apart in bothhorizontal and vertical planes. Sextupole pairs are notinterleaved. This scheme cancels out the geometricaberrations caused by the sextupoles effectively since thethe transfer matrix between sextupoles is -I.
One unit cell of the adopted lattice has a phaseadvance of 2.5Jt and includes two pairs of sextupoles, SFand SD. The addition of extra rr12 phase advance over 2Jt
cell enables effective correction of chromatic kicks andsignificantly improves the dynamic apertures. In theadopted lattice the momentum compaction factor can bechanged from -1 X 1Q-4 to 4 X 1Q-4 and the emittancefrom 50% to 200% of the nominal value. This flexibility
makes a strong tool to tune the machine.
In the IP straight section of LER, a local chromaticity
correction scheme is adopted to correct the large verticalchromaticity produced by the final focus quadrupole
magnets close to IP.
We adopt a finite-angle crossing scheme of ± 11
mrad. In this scheme, parasitic collision is not a concern
Table 1
Ring
EnergyCircumference
Luminosity
Crossing angle
Tune shifts
Beta function at IP
Beam current
Natural bunch length
Energy spread
Bunch spacing
Particles/bunch
Emittance
Synchrotron tune
Betatron tune
Momentumcompaction factor
Energy loss/tum
RFvoltage
RF frequency
Harmonic number
Longitudinal
damping time
Total beam power
Radiation power
HOM power
Bending radius
Length of bending
rna nett without wigglers
tt with wigglers

Eeven though every bucket is filled with beam; separation
dipole magnets that would be necessary for a head-on
collision are no longer necessary. The horizontal widthof the beam pipe at IP is minimized in the finite-angle
crossing where no synchrotron-light fans are produced byseparation dipole magnets; smaller beam pipe improves
the vertex point resolution and permits efficient use of theWe use superconducting final-focus
luminosity.
quadrupole magnets in order to have a flexibility oftuning.
By computer simulation we found that although thefinite-angle crossing somewhat reduces usable areas inthe Vx-Vy plane due to synchro-betatron resonances, a fairamount of areas in the vx-Vy plane is still free fromreduction of luminosity, if we make the vs(synchrotronWe have also started antune) smaller than 0.02[3].
R&D work on superconducting crab cavities in order toprepare unpredictable beam-beam effects due to thisfinite-angle crossing. Crab cavities tilt the bunches andmake them collide head-on at the interaction point.

Main Parameters ofKEKB

cexazabSB±110.4
0.59
3.3 X 1010

m1.4 X 1010

1.8 x w-8;3.6 x 10-10
0.01 ~ 0.02
45.52/45.08

m47.52/43.08
1 X 10-4 ~ 2 X 10-4

0.81t/1.5tt
5 ~ 10

3.5
10 ~20MeVMVMHz508.887
5120

fRF
hAem

6.7 X 10-4

7.1 X IQ-4

ap

uovc

m

1.1

2.6

E/Ey

vsvjvycm-2s-1
mrad

0.039/0.052
0.33/0.01~AGeV
m3016.26
1 X 1034

L13*/13*
I yHER
8.0

LER
3.5

T.E

43t/23tt

23ms

PB
PRPHOM

2.7t/4.5tt
2.1t/4.0tt
0.57
16.3
0.915

4.0
3.8
0.15
104.5
5.86

MWMWMWpIs-21-mm

JAERI-Conf

III. HARDWARE SYSTEMThe RF cavity for the KEKB should have a structureby which higher-order modes (HOMs) in the cavity are
damped to the level where the growth times of thecoupled-bunch instabilities excited by HOMs becomecomparable to or longer than the damping time. The
fundamental mode of cavity also excites coupled-bunch
instabilities if the detuning frequency of the cavity due tobeam loading becomes comparable to or larger than therevolution frequency of the ring.
The cavity shouldhave enough stored energy in order to make the detuning
frequency much smaller than the revolution frequency.
We are now developing two types of cavities for theKEKB. One is a normalconducting cavity called ARES
and the other is a superconducting, single-cell, singlemode cavity.
T. Shintake showed that the amount of the detuning
frequency can be drastically decreased by attaching alarge volume, low-loss, energy-storage cell to an
accelerator cell[4]. On the basis of this proposal, a 3cell structure, where an accelerating cell and an energy
storage cell is connected via a coupling cell is proposedand called ARES[5]. In order to suppress HOMs, achoke-mode cavity [6] is used as the accelerating cell ofARES. The first prototype accelerating cell of ARES wasdelivered to KEK and successfully tested up to 110 kW
of wall dissipation which corresponds to a gap voltage of0.73 MV.
A superconducting cavity has a large stored energy
due to its high field gradient and is immune to the beamloading. The superconducting cavity for KEKB is asingle-cell cavity with two large-aperture beam pipes
attached to the cell[7]. HOMs propagate toward thebeam pipes, since their frequencies are above the cut-off
frequencies of the pipes. HOMs are absorbed by ferrite
HOM absorbers.
A full-size Nb model was constructed and tested in a
vertical cryostat.
The maximum accelerating fieldobtained was 14.4 MV/m with a Q value of 109[8]. The
prototype cavity for the AR beam study is underconstruction. Prototype HOM dampers made by HIP(hotisostatic press) method were successfully high-power
tested [9].
Feedback systems that can damp the coupled-bunch
oscillations of the beam with a bunch spacing of 2 ns arebeing developed[lO]. A 2-tap FIR digital filter system
works as the kernel of the signal processing unit. Thiskind of filter can be composed of memory chips andsimple CMOS logic ICs without relying on DSP chips.
By using 500 MHz ADC and DAC, two custom-made4-bit GaAs 1:16 500-MHz demultiplexers and two 4-bit
GaAs 16:1 500-MHz multiplexers, and having 16
parallel 2-tap FIR logic circuits, we can construct a
signal processing unit on a single board. Kickers andwideband amplifiers are now being developed.

95-021

We have decided to use Cu as material for vacuum
ducts by taking into account its low photodesorption
coefficient, high thermal conductivity, and self shielding
capability of X-rays. Cu ducts for LER are now underconstruction.

IV. SCHEDULE
Three-month long beam test is planned to be held in1996 by the use of TRISTAN AR. We plan to store
more than 500 rnA electron beam in AR with amultibunch mode at 2.5 GeV. To accumulate this highcurrent, the existing APS type RF cavities will beremoved temporally from the ring and an ARES cavityand a single-cell superconducting cavity for KEKB willbe installed. The transverse and longitudinal feedback
systems will be also installed and tested.
Main components of LER such as magnets andvacuum equipment are procured in JFY1995 and 1996,
whereas those for HER in JFY1996 and 1997.
TRISTAN will be terminated by the end of 1995 anddismantling of TRISTAN main ring will start fromJanuary 1996. By the end of 1996 the TRISTAN tunnel
will become ready for installation of magnets. We plan
to commission the KEKB with Belle detector within
JFY1998.
REFERENCES

A. Enomoto, et al., Proceedings of the 1994
International Linac Conference, p.l94.
[2] H. Koiso and K. Oide, Lattice Design in KEKB
Colliding Rings, to be published in Proceedings of1995 PAC, Dallas.
[3] K. Ohmi, K. Hirata, and N. Toge, A Study ofBeam-Beam Interactions at Finite Crossing
Angles for a B-Factory, to be published inProceedings of 1995 PAC, Dallas.
[4] T. Shintake, Part. Accel. 44 (1994) 131.
[5] Y. Yamazaki and T. Kageyama, Part. Accel. 44(1994) 107.
[6] T. Shintake, Jpn. J. Appl. Phys. Lett. 31 (1992)
1567.
[7] S. Mitsunobu, et al., A Prototype Superconducting
Cavity for KEK B-Factory, to be published inProceedings of 1995 PAC, Dallas.
[8] K. Asano, et al., Stable Performance ofSuperconducting Cavities for KEKB, to be
published in Proceedings of 1995 PAC, Dallas.
[9] T. Tajima, et al., Development of HOM Damper
for Superconducting Cavities for KEKB, to bepublished in Proceedings of 1995 PAC, Dallas.
[10] E. Kikutani, et al., Recent Progress in theDevelopment of the Bunch Feedback Systems forthe KEKB, to be published in Proceedings of 1995PAC, Dallas.

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