The Present Performance and Future Upgrade of the KEKB Electron Linac
K. Furukawa∗ ,the KEK Electron Linac Group and the Linac Commissioning Group,
High Energy Accelerator Research Organization (KEK),Tsukuba, Ibaraki, 305-0801, Japan
Abstract
The KEKB linac injects 8-GeV electrons and 3.5-GeV
positrons into the KEKB asymmetric collider rings designed for B-physics studies. KEKB has recorded thehighest luminosity records to which the linac contributeswith advanced operational stability. In order to furtherimprove the luminosity, new schemes of dual-bunch injection and continuous injection have been studied andadopted. It is also planned to upgrade KEKB for a luminosity of 1035 /cm2 /s with exchanged beam energies (3.5GeV electrons and 8-GeV positrons). The upgraded linac
is being designed and studied with a higher acceleration
gradient and others to realize such a super B factory.

1

INTRODUCTION

The performance of KEKB [1] has gradually improved,
and recently it has recorded an integrated luminosity of399/pb/day and a peak luminosity of 7.35 × 1033 /cm2 /s.
Such a high luminosity reinforces the achievement of important results in the study of CP-violation with the Belle
detector [2].
The injection rate from linac also had to be improved,
and it has been planned to adopt a higher beam quality
control and advanced operation modes, which include dualbunch injection and continuous injection.
With such a success in B-physics studies, it wasproposed to build an upgraded factory machine, SuperKEKB, with a 10-times higher design luminosity of1035 /cm2 /s [3]. This is the major challenge for the linac
because both the positron energy and the beam charge must
be upgraded. It is planned to study the upgrade possibilities
at first with a C-band high-gradient scheme [4].

2OPERATION STATUS

The linac has been operating very stably these days, andit seems that such difficulties as a discharge issue have been
overcome, which we experienced in the bunching section
and the positron target section [5].

2.1

Statistics

Table 1 gives the operation statistics during the past threeyears, where a machine down means that the machine was
not ready, including rf trips and short maintenance work
between injections; a beam loss means that the machine∗ e-Mail: <kazuro.furukawa@kek.jp>

Table 1: Linac operation time (hours).
Operation Time
Machine Down
Beam Loss

FY19997297
768 (10.5%)74 (1.0%)

FY20007203
601 (8.3%)
54 (0.8%)

FY20017239
385 (5.3%)
22 (0.3%)

Table 2: Four beam modes of the linac.
Energy Charge Injection Inj. TimeRing(nC)(GeV)(/day)(min.)KEKB e−1.28
8.0∼14/18∼1.5
KEKB e+ (1) .64×1
3.5
∼14∼8
KEKB e+ (2) .64×2
3.5∼18∼4PF e−0.2
2.51∼5PF-AR e−0.2
2.5/3.0
12∼4
KEKB e+ (1), (2) denote single- and dual-bunch operations.

was not available while an injection was being requested.
The linac injects beams into the Photon Factory (PF) andthe PF-AR as well as the KEKB rings. Table 2 gives thetypical beam mode parameters for four storage rings. The
beam modes are switched about fifty times a day, without
any beam quality degradation. Recently, we started 3-GeV
injection into PF-AR instead of 2.5 GeV, which seems tohave cured the instability issues in the ring.

2.2

Beam and Machine Quality Control

Because of the characteristics of the factory machines,
especially at KEKB, continuous operation of the rings andthe linac through the year is crucial. Thus, in order to maintain the beam quality, we routinely monitor several machine and beam parameters. Here are some examples ofsuch parameters:• Number of rf-trips and the reason for each klystron(every week).
• rf phasing at each klystron (every other week).
• Twiss parameters and matching condition Bmag (every day).
• Energy spread (every day).
If those parameters are not optimal, machine tuning is carried immediately. For example, if a matching condition
parameter Bmag for a certain location is large, beam optics
matching is carried out using a Twiss-parameter measurement by wire scanners. Figure 1 shows that the number ofFigure 1: Number of rf trips per year.
rf trips per year is decreasing rapidly. The reason is mainly
because we gradually began to choose the optimum highvoltage values for the klystrons, besides quality improvements of the klystrons and pulse modulators.

2.3

Dual-Bunch Acceleration

During beam injection to the KEKB rings, an experimentcannot be performed. Most of the injection time is used forpositrons, since the stored current is larger and the lifetimeis shorter in the ring.
Thus, in order to double the positron injection efficiency
dual-bunch acceleration had been studied [6] and necessary
equipment had to be installed [7, 8]. During this year we
tried this injection scheme for more than a one-month period, and were successful.
However, because of the common frequency between thelinac and the ring, bunch selection in the ring is restricted.
That led to heating of the beam pipes in the rings, and also
affected the luminosity tuning. While we may need somemore studies concerning those issues, the luminosity increased by 4 to 8%.

2.4

Continuous Injection

As the stored beam currents in the rings decrease between injections, the luminosity decreases accordingly. In
two rings the positron beam reduction is larger since thelifetime is shorter. Thus, we started to study the continuous injection of the positrons during physics experiments
after careful tests against detector noises. In this scheme,
positrons are injected continuously at 5 to 10 Hz instead ofa maximum rate of 50 Hz. As shown in Fig. 2, the positron
current was kept almost constant and the reduction of theluminosity was relaxed. A beam-current reduction before
each injection was caused by a PF-AR injection.
Although a part of the detector needs some more preparation against injection noise and heating, the integrated luminosity may increase by 20% with this scheme as a roughestimate. However, if we inject positrons in this scheme,
the measurements and corrections to maintain the beam
quality may become a issue. This is discussed later.

Figure 2: Continuous injection to keep the positron beamconstant.
After those successful adoptions of both dual-bunch injection and continuous-injection schemes, we will further
utilize them in the coming operation periods.

3

UPGRADE FOR SUPERKEKB

Based on the success of the KEKB, the new project SuperKEKB was proposed to study the B-meson system parameters precisely and to search for new physics, such as
the super-symmetry. The new machine should provide aten-times higher luminosity, 1035 /cm2 /s, while the design luminosity of KEKB is 1034 /cm2 /s. In the new design a factor of ten is achieved by squeezing the beta function at the interaction region and by increasing the stored
beam currents in the rings, both of which should provide afactor of 3.2.
However, because of the electron cloud in the beam pipeof the positron ring, the luminosity may be limited. Thus,in order to cure this issue, energy exchange is planned,
where the energies of the electrons and positrons should be
3.5 GeV and 8 GeV. A positron injection energy of 8 GeV
is the major challenge at the linac as well as the increaseof the injection rates. The machine design is being carriedout in two schemes, one of which we have already started
development.

3.1

High-Gradient C-Band Scheme

Since the linac site is limited, the energy upgrade of thepositron beam from 3.5 GeV to 8 GeV needs restructuringof the linac. After investigating several possibilities, two
options are being considered. The first option is to employ
C-band structures to double the energy gain at a part of thelinac and a damping ring to lower the emittance, as shownin Fig. 3.
A damping ring is necessary not only for the smaller
aperture of the C-band structure, but also to relax the design criteria of the improved interaction region at the ring.
It will be installed after the positron capture section, where• Fast actuators, like phase shifters, to be scanned in themeasurement.
• Fast and synchronous data-acquisition system.
• Fast kicker.
• Timing system modifications.

Figure 3: Energy upgrade with high-gradient structures asa first option.
the positron energy is 1 GeV.
C-band structures will be installed in the 24 units after
the damping ring. Currently, each unit contains a 40-MW
klystron, a SLED pulse compressor and four 2-m long Sband structures, which provide an energy gain of 160 MeV(21 MeV/m). It will be replaced by two 40-MW C-bandklystrons, two rf-pulse compressors and four C-band structures, with which the energy gain will be 320 MeV per
unit (42 MeV/m). The maximum positron energy will be
8.7 GeV.
The C-band technology will be based on developmentby the JLC C-band group [9]. However, the components
are being optimized and simplified for our needs, since
we don’t need multi bunches, and we want to simplify themaintenance. In the first design, the klystron will be a nonPPM type, the pulse compressor will be a LIPS type andthe accelerating structure will be just a scale-down of thecurrent S-band structure without HOM damping. Thosecomponents have been designed and are being fabricated.
Their high-power tests and beam tests will be carried in
2003.

3.2

Recirculation Scheme

Another scheme being considered utilizes a recirculation in which generated low-energy positron will be kept
in a damping ring for the next rf pulse and will be recirculated from the head of the linac. There can be several
variations in this scheme. Although it does not require any
new accelerating components, very different beams have tobe accelerated in the same pulse and several bypass lines
and kickers have to be installed. Thus, the operation andmaintenance will be complicated.

3.3

Beam Measurement

Since continuous injection was adopted, and it will be
more important in SuperKEKB, the beam quality control
will be impossible, which is currently performed during 1hour periods between injections. Thus, a non-destructive
beam measurement would be crucial. To that end, a stealthbunch measurement is considered, in which a bunch between injection bunches will be used to measure the characteristics and will be removed by a kicker at the end of thetransport line. This scheme requires the following components to be developed:

Some of these items are being developed for other purposes, like a fast beam-feedback system [10]. The development will be continued, since beam-quality control is indispensable.

4SUMMARY

The KEKB linac has been stably operated and has contributed much to improve the KEKB luminosity with its
improved operation, which includes beam quality control and advanced injection modes, dual-bunch injection
and continuous injection. On the other hand, in order tomeet the injection specifications of the newly proposed SuperKEKB project, several upgrade schemes are considered
and designed. The development has started for one of theschemes, which is an upgrade with a higher gradient by Cband structures. Beam quality control for the future is alsoplanned with a better beam measurement scheme.

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