FROM TRISTAN TO B–FACTORY
Y. Kimura, KEK, Tsukuba-shi, Japan
Abstract
TRISTAN (1986–1995) is an electron-positron collider
for top quark search. The center of mass collision energy
was 50–64 GeV, then the highest in the world. To attainthe required accelerator performance, intensive innovative
R&D effort had to be made in such technologies as
superconducting RF and magnet, high power RFcomponents, vacuum, instrumentation, etc. B-factory,
KEKB is an asymmetric energy (8 GeV+3.5 GeV)
electron-positron collider for observation of CP violationin B-meson decays. The facility was constructed in 1998
by maximally utilizing the TRISTAN resources, and has
achieved the world highest luminosity of 2.11034cm-2s-1.
In 2001, the Belle experimental group successfully
observed the CP violation in B-meson decays as predictedby the Kobayashi–Maskawa theory, and this eventually
led the theory to winning the Nobel Prize in physics in2008.
OUTLINETRISTAN was the first Japanese electron-positron
collider, and built in 1981–1986. The collision energy
attained was 64 GeV, then the highest in the world.
National Laboratory for High Energy Physics (KEK)
was founded in 1971 as the first national laboratory forinter-university research, and a 12-GeV proton
synchrotron was built in 1971–1974 as its major researchfacility (KEK-PS). It must be noted that the KEK-PS
energy was initially proposed to be 40 Gev and compelled
to lower to 12 GeV to cope with the government's budget
cut to a quarter of the initial demand (about 30 BYen).
Intriguing is that the 40-GeV PS plan was realized as JPARC after almost 40 years, whereas the machine design
was considerably different than the other. Almost all of
the domestic high-energy physicists then were dissatisfied
with KEK-PS, and thought about higher energy world
machines to be built after KEK-PS. In such circumstances
Prof. T. Nishikawa, the head of the KEK Accelerator
Department, proposed the TRISTAN project in 1973. The
TRISTAN machine initially planned was a three-ringbeam collider to facilitate various types of beamcollisions: e+e-, e-p, and pp.
In the 1970s, the standard theory to describe theelementary particle was steadily established byexperiments, and the Kobayashi–Maskawa (K–M) theory
published in 1973 was widely noticed as a basis of thestandard theory. Hence the Japanese high energy physics
community rightfully hoped that the new machine after
KEK-PS should be the one to investigate the validity ofthe K–M theory and lead the theory to the Nobel Prizewinning.
The K–M theory states in brief that the CP violation canbe explained as the state mixing of six types of quarks.
Three quarks (u, d, s) were known in 1973, and forth andfifth quarks, c and b, were discovered in 1974 and 1977,
respectively. Then, to verify the K–M theory, experiments
were done to find the sixth quark, t, and to show that theCP violation emerges in accordance with the K–M
theory's prediction. In late 1970s, there were some-theoretical predictions that tt mass plausibly ranged from50 to 60 GeV/c2. And after thorough discussion it was
decided that TRISTAN should be an e+e- collider,reaching 30 GeV + 30 GeV and aim at finding t quark
though the KEK site did not seem to be large enough tobuild a realistic machine of this energy range.
The TRISTAN construction was completed in 1986 as-planned, and experiments to look for tt were carried out.
-But before long, it turned out that the tt mass was muchheavier than expected and in the energy range beyond
TRISTAN. Then some study groups were organized andinvestigated a possible construction of a B-factory for CP
violation experiments in parallel with the TRISTAN
project. What promoted this B-factory plan to a realistic
one was an idea of the asymmetric energy two-ring e+ecollider proposed in the late 1980s. In 1993, thegovernment accepted the KEK proposal that KEK began
the construction of an asymmetric energy B-factory (e+/e= 3.5 GeV/8 GeV) in 1994, provided that the TRISTAN
resources could be maximally utilized, and the TRISTAN
experimental program was completed by 1995.

Figure 1: Progress of the accelerator construction forparticle physics at KEK in 1971–2010
In order to investigate the CP violation, the KEKB
collider was required to achieve the luminosity as high as1034 cm-2s-1, or the stored beam currents exceeding
amperes, almost two to three hundred times of those
TRISTAN attained. And to reach this luminosity goal, theKEKB accelerator team had to overcome a lot ofdifficulties. But shortly after the completion of themachine in 1998, they could attain an unprecedentedly
high luminosity of 2.1  1034 cm-2s-1 and brought the Bfactory project to a great success. As a matter of fact, in
2001, the Belle experimental group succeeded inobserving the CP violation in B meson decay, and thiseventually led the K–M theory to winning the Nobel Prizein 2008.
Figure 1 summarizes the progress of the accelerator
construction for particle physics at KEK in 1971–2010.

The TRISTAN accelerator system consists of a 2.5GeV e+,e- injector linac with a 0.25-GeV positron
generation linac, an 8-GeV accumulation ring (AR), and amain collider ring (MR). Figure 3 shows pictures of thoseaccelerators.

TRSTAN
Main parameter list of the TRISTAN e+e- collider isgiven in Table I, and the accelerator system configuration
are shown in Fig. 2.
Table I: Main paramter list of the e+e- collider
Circumference
3018 mNumber of electron and2+2
Positron bunches
Beam energy
25 – 32 GeV
Initial total beam current
14 mA
Nominal RF frequency
508.58 MHz
RF voltage
180 – 500 MV(APS/936 cells/310 MV,
SCC/160 cells/190 MV)
1.5 % - 2 %Emittance ratio (V/H)
Beam life time
3–5 hrBeta-functions at collision
0.04/1.0 m
Point (* V/  *H)
Beam sizes at collision
8/250 μm
point (*V/  *H)
Initial luminosity
4.51031 cm-2sec-1

Figure 2: Configuration of TRISTAN accelerator system.

Figure 3: Pictures of the TRISTAN accelerators.
It is well known that the optimally designed e+ecollider ring follows the rule to enlarge the diameter inproportion to the square of the beam energy. And theoptimum diameter of a 30-GeV ring is given to be about 3km. But the largest ring fit in the KEK site was estimatedto be about 1 km in diameter. Therefore the TRISTAN
ring had to be designed so that the huge radiation loss in
the relatively small ring could be compensated with anefficient high power RF accelerating system. To copewith this requirement, the ring was designed to haveunusually long straight sections (98 m  8) for RF cavity
installation, and a great deal of effort was concentrated indeveloping new high-power UHF accelerating
components, which included APS(alternating periodicstructure)-type normal conducting cavity system, world’s
first large-scale superconducting cavity system, klystrontubes with world’s highest continuous output power,
liquid He system with domestically highest cooling power,etc.
The APS cavity has an axially symmetric structure forsuppression of the beam induced deflecting field, and its
strong cell to cell coupling (large bandwidth > 1%) allows
a long multi-cell structure to save the installation spaceand reduce the number of RF components as couplers,
dummy loads, etc. The cavity is operated at 508.58 MHzand its main specifications are 23 M /m in shuntimpedance, 300 kW in maximum input power, and 1.6
MV/m in maximum accelerating field. In the TRISTANring, 104 APS 9-cell cavities were installed. The cavity isillustrated in Fig. 4.

control system, and versatile computer code SAD(Strategic Accelerator Design) for beam orbit analysis.
Figure 6 shows the all aluminum-alloy beam duct, 1.2MW klystron tube, and superconducting insertion
quadrupole magnet. Also shown in the Fig. 7 are thedisplays of the colliding beam behavior in the MR, theaccelerator control console, and one of the particle
detectors installed at the four beam collision regions.

Figure 4: TRISTAN APS cavity.
The R&D of the superconducting RF cavity at KEK
started as early as 1971, and made rapid progress
responding to its application for TRISTAN. The cavity
construction was carried out in 1981–1985. In this R&D
effort, various new technologies, which became thestandard ones for today's superconducting cavity
fabrication, were developed such as hydro forming of Nb
plates to half cavity cells, electron beam welding (EBW)
to form cavity cells, electropolishing of the inner cavity
surface, rinsing of cavity structure with pure H2O andH2O2, etc. Figure 5 shows the TRISTAN superconducting
2  5cell cavity and the number of the installed cavities
as a function of accelerating field.

Figure 5: TRISTAN superconducting cavity.
In addition to the RF system, TRISTAN, as the firstbeam collider in Japan, required the development of newtechnologies. Typical ones are all aluminum-alloy
vacuum system to achieve a vacuum pressure less than
10-8 Pa with radiation power deposit of 3 kW/m on thebeam duct surface, superconducting quadrupole magnets
for mini-beta insertion, which generated the field gradientas high as 70 T/m and enabled to make the beta-functions
at the beam collision points as low as 0.04 m verticallyand 1.0 m horizontally, wholly centralized computer

Figure 6: TRISTAN all aluminum-alloy beam duct, 1.2MW klystron tube, and superconducting insertion
quadrupole magnet.

Figure 7: Displays of the colliding beam behavior in MR,
the TRISTAN accelerator control console, and one of theparticle detectors installed at the four beam collision
regions.
B-FACTORYThe KEKB accelerator is an asymmetric energy, tworing, e+e- collider, and its main parameters are listed inTable II. B-mesons to study the CP violation are
produced by such reaction as e-(8 GeV) + e+(3.5 eV) -4s(10.58 GeV/c2)  B + B. As the branching ratio of
the CP violating decay is so small, the KEKB luminosity
has to be as high as 1034 cm-2s-1, and the stored beam
currents exceeding one ampere are required. Then theinjector linac was remodelled to be able to accelerate e-and e+ beams to 8 GeV and 3.5 GeV, respectively, and thelinac beams were injected directly to the main e- and e+colliding rings without using an accumulation ring like
TRISTAN. Figure 8 shows the configurations of theKEKB accelerator system and the remodeled linac, andthe pictures of those accelerators.
Table II: Main parameters of the KEKB ElectronPositron Collider
PositronElectronRing
Ring
Circumference (m)3016Luminosity (cm-2s-1)
2.11034
Crossing angle (mrad)
 11Integrated luminosity
1.5/day, 8.4/week,(fb-1)
30.2/monthBeam energy (GeV)3.58.0
Beam current (A)1.61.2
Number of bunches
1584
Harmonic number5120
RF frequency (MHz)508.8
Total RF voltage (MV)8.0
13.0
120/0.59
120/0.59
Beta’s at IP * H/ *V(cm)Vertical beam size at IP
0.94
0.94
*V (μm)
Beam-beam parameter
0.127/0.129
0.102/0.090
H/ V
Beam life time (min.@133 @ 1.6
100 @ 1.2A)
Injection rate (nC/s)
50
50–100ring beam collider. In KEKB a scheme is adopted tocollide the two beams at a horizontal angle of + 11mrad.
Though this finite angle crossing might cause a synchrobetatron resonance type beam instability and a slight lossof luminosity, this allows the beam separation without
bending elements which become the sources oftroublesome synchrotron radiation.
The magnet lattice of the KEKB rings is designed tohave a 2.5 unit cell lattice with paired non-interleaved
sextupoles for chromaticity correction. And local
correction sextupole systems are adopted to compensate
for chromaticity arising from the strong final focus
normal conducting and superconducting quadrupole
magnets to make the beta-functions at the collision point
as low asH/V= 120 cm/5.9 mm. It is found that thislattice design makes the KEKB ring invulnerable to beam
instabilities.
New types of RF accelerating cavities were also
developed to eliminate serious frequency-detuning effectof the accelerating cell caused by very strong current
beams. One is a normal conducting ARES (Accelerator
Resonantly Coupled with Energy Storage) and the other isa single-cell large aperture superconducting cavity.
Figures 9 and 10 illustrate the ARES cavity and KEKB
superconducting cavity.

Figure 9: ARES cavity.

Figure 8: Configurations of the KEKB accelerator systemand the remodeled linac, and the pictures of thoseaccelerators.
In designing the KEKB rings, some measures weretaken to achieve such unprecedentedly high luminosity.
Several effective ones are as follows.
A special beam orbit structure is required to collide andseparate two beams in the interaction region of the two-Figure 10: KEKB superconducting cavity.

The KEKB rings were completed in 1998, and long
before the beam commissioning encountered very serious
beam instability in the e+ ring, which was caused byelectron cloud generated by synchrotron radiation at thebeam duct surface. But after a bitter struggle against thisinstability, it was finally found that it could be suppressedby installing solenoid coils in every empty space of thering to generate a few tens of Gauss axial magnetic fieldand sweep out secondary electrons as illustrated in Fig. 11.
Then the KEKB luminosity was rapidly improved andexceeded the design goal of 1  1034 cm-2s-1 in 2003.
KEKB recorded the world’s highest peak luminosity of2.1  1034 cm-2s-1 in 2009. Evolution of the KEKB
luminosity is shown in Fig. 12.

Figure 12: Evolution of the KEKB luminosity.
Now KEKB has been shut down, and is going to be
remodeled to SuperKEKB of 801034 cm-2s-1 in peakluminosity for better understanding of the CP violationand looking for phenomena beyond the standard theory.
SuperKEKB is envisaged to start operation in 2014.

Figure 11: Suppression of the electron-cloud instability by
installation of solenoid coils in the e+ ring.