TWO-BUNCH OPERATION OF THE KEKB LINAC FOR DOUBLING THE
POSITRON INJECTION RATE TO THE KEKB RING
Y.Ogawa, A. Enomoto, Y.Funakoshi, K. Furukawa, T. Honda, N. Iida, N. Kamikubota, T.Kamitani,
E. Kikutani, M. Kikuchi, H. Kobayashi, H. Koiso, T. Matsumoto, Y. Ohnishi, S. Ohsawa, K. Oide,T. Suwada, KEK, Tsukuba, Japan

Abstract
A high-intensity, two-bunch acceleration scheme in the
primary electron beam for positron production has been
introduced at the KEKB linac for doubling the positron
injection rate to the KEKB ring. The KEKB linac has
until now injected single-bunch positron beams with aninjection rate of about 1.4 mA/s (50 Hz) into the KEKB
ring, requiring several minutes of topping-off time.
Therefore, two-bunch operation could eventually halve
the injection time, resulting in an efficient accumulationof the integrated luminosity of collisions. The results oftwo-bunch operation tests at the linac are reported.

1 INTRODUCTIONKEKB (KEK B Factory [1,2]) has been accumulating
integrated luminosity with an unprecedented velocity
since the Belle-detector roll-in in May, 1999. By July
2001, the total integrated luminosity had reached over 30
pb-1, being comparable with that of PEPII in SLAC.
Although the accelerator performance, itself, has alreadysurpassed that of PEPII in various aspects, such as in thepeak luminosity, the daily integrated luminosity, etc.,
more efficient accumulation of the integrated luminosity
would be required in order to contribute to physics
experiments [3]. In practice, the peak luminosity must be
primarily raised by increasing the stored currents as wellas elaborating collision-tuning, while the injection timeshould also be minimized. In Fig. 1, the daily KEKB
performance is shown, indicating about twenty-times

Figure 1: Daily snap shot of the KEKB ring.

injection in one day with a few minutes of each injection
time, which corresponds to more than two hours forinjection per day. Since the present injection time ismainly determined by that of a relatively low-intensity
positron beam, it is quite natural to increase the positron
intensity in the linac for minimizing the injection time.
The single-bunch intensity of the positron beam in theKEKB linac, however, has almost amounted to themaximum value, simply because the charge of a highintensity, single-bunched primary electron beam for
positron production has almost approached the beam
blow-up threshold due to single-bunch transverse wakefield effects. Therefore, we have adopted a two-bunch
acceleration scheme for doubling the positron intensity [4,5]. The present status of two-bunch acceleration as wellas various experiences concerning the stable accelerationof high-intensity two-bunched beams is reported.

2 TWO-BUNCH ACCELERATIONSCHEMEIn the KEKB linac, a high-intensity single-bunchedelectron beam is generated through a bunching section
comprising two subharmonc bunchers (SHB1: 114 MHzand SHB2: 571 MHz) and subsequent S-band bunchers(prebucher and buncher), giving a 10-nano-Coulomb,
single-bunch beam with a bunch length of 10 ps (FWHM).
The beam is accelerated to the positron production target
at an energy of 3.7 GeV. The generated positron beam isaccelerated to the end of the linac at an energy of 3.5 GeV
and directly injected into the KEKB ring. Major
constraints and issues for doubling the bunch number inthe KEKB linac are the following:• The bunch separation should be an integer multipleof a common period (96.29 ns) among the linac andring frequencies (Table 1).
• Two bunches must be accelerated during the same
rf pulse, signifying that the bunch separation should
be comparable with the width of the SLED gain
curve, in which the energy gain does not changevery much (Fig. 2).
• Beam loading compensation between two bunches
could be carried out by utilizing the SLED gaincurve (Fig. 2).
• Beam blow-up due to multi-bunched transverse
wake-field effects must be suppressed by fine andelaborate orbit tuning.

3 ACCELERATION OF A HIGHINTENSITY, TWO-BUNCHED
ELECTRON BEAM

Table 1: Various frequency relations.
Multiple
CommonFrequency
SHB1
SHB2
LinacRing

1

Frequency[MHz]
10.385454

Period[ns]
96.28911
5*11
5*5*11
7*7

114.240571.200
2856.000
508.887

8.754
1.751
0.350
1.965

1.00
Second Bunch

Loading Compensation
2.5 %

0.98
First Bunch
0.97

96.29 ns
0.96
0.95-100-50050

100

Gun Timing [ns]SLED Timing [ns]Figure 2: SLED gain curve.
The first constraint comes from complex relations
among the linac and ring frequencies (Table 1), giving themost severe requirement regarding the bunch separationin the linac. Taking account of the second constraint as
well, the bunch separation is uniquely determined to be
96.29 ns, corresponding to just the common period.
Figure 3 shows a block diagram of two-bunch generation
and acceleration.
Several issues concerning the bucket selection system
for the injection of two-bunch beams into the KEKB ringwith this bunch separation are described elsewhere [6].

GunSHB1SHB2

PBBuncher

114 MHz 571 MHz 2856 MHz
1155275
96.29 ns
10.4 MHz1

Two high-voltage pulses with a time interval of 96.29
ns are produced by combining the outputs of two
independent grid pulsers with a pulse width of about 1 - 2
ns each, in which both the amplitude and pulse timing areadjustable. They are applied to an electron gun with anacceleration voltage of 200 kV, and immediately fed into
two SHBs and an S-band bunching section, generating ahigh-intensity two-bunched beam with a charge of 10nano Coulomb and a bunch length of 10 ps (FWHM),
respectively. The main concern in the high-intensity, twobunch generation is how to accommodate both pulses withthe same bunching conditions by adjusting the pulse
height and the relative time delay of the grid pulsers:• The S-band pre-buncher and buncher comprisetravelling-type accelerator sections with rf-filling
times of 8 ns and 48 ns, respectively, which arequite short compared with the bunch separation of
96.29 ns. This means that there is no beam-loading
effect due to multi-bunch longitudinal wake-fields.
Single-bunch longitudinal wake-field effects,
however, could cause an energy difference between
two bunches unless the intensities of the two pulses
are the same. Therefore, we must assure the same
bunching conditions for two bunches by equalizing
both charges.
• Vary the relative time delay of two pulses andacquire an optimum value so that the energies of
two bunches at the exit of the buncher are
equalized with each other. The energy is measuredby utilizing beam-position monitors and steering
magnets (Fig. 4).
The observations of the bunch profiles at the exit of thebunching section showed that the bunch profiles for thetwo bunches are almost identical, confirming the abovetuning procedures.
18.5

First Second
KEKB Ring
96.29 ns
Linac/Target
508.9 MHz
2856 MHz
Every 49 bucket
275

Figure 3: Two-bunch acceleration scheme.

18.0

SLEDBunch Energy [MeV]

Relative Energy

0.99

3.1 Two-Bunch Generation

First Bunch
Second Bunch

17.5
17.0
16.5
16.0
15.5
15.0
1.62 1.64 1.66 1.68 1.70 1.72 1.74 1.76 1.78
Gun Delay of Second Bunch [ns]Figure 4: Energy equalization at the buncher.

4 CHARACTERISTICS OF TWOBUNCHED POSITRON BEAMS

3.2 Two-Bunch Acceleration
After exiting the bunching section, two bunches areaccelerated in the regular accelerator sections, where themulti-bunch beam-loading effects are the main concerns.
The energies of both bunches must be equalized by thefollowing methods, depending on with or without SLED(beam-loading compensation):• For the case without SLED, put the first bunch into
the accelerator sections during the rf-filling time soas to gain a lower energy than that of the secondbunch, thus compensating for the beam-loading
effects.
• For the case with SLED, equalize the energies of
the two bunches by adjusting the SLED timing, asmentioned in section 2.
The energy equalization is verified by measuring theenergy difference of the two bunches, for instance, at themiddle of the J-arc section, where the dispersion function
is finite, or by observing the phase difference of betatronoscillations for two bunches.

Positrons are produced at a tungsten target, on which
high-intensity, primary electron beams impinge. Passing
through the positron capture section comprising a pulsecoil, a solenoid coil and high-field accelerator sections,the positron beam is accelerated to the end of the linac atan energy of 3.5 GeV. The energy spread is almost
halved by an energy compression system (ECS) installedat the end of the linac, thus minimizing the beam loss dueto the energy tail along the beam-transport line to theKEKB ring. We could accelerate two-bunched positron
beams with a charge of about 0.5 nano Coulomb having agood beam quality; the observed energy spread for twobunches is almost identical and the measured twiss
parameters are also quite similar to each other. We havealso succeeded to inject two-bunched positron beams into
the KEKB ring in special filling patterns [6] as well as inleading them to the end of the beam-transport line.

5 CONCLUSIONS
3.3 Orbit and Optics of Two Bunches
When the energies for the two bunches become definite
along the linac, orbit and optics corrections are required
for two-bunch beam transport without any beam loss.
Since the beam characteristics of the two bunches at theexit of the bunching section are the same, optics
corrections and matching only for either the first bunch or
the second bunch should be sufficient, in principle. On
the other hand, orbit corrections might be carried out foreach bunch by introducing a new algorithm, for instance,a method involving the average minimum of two-bunchorbits.
This is because the transverse wake-field
generated at the first bunch due to a misalignment of theaccelerator sections and quadrupole magnets might
deteriorate the second bunch, leading to beam blow-up.
The observed beam orbits for two bunches are quite
similar (Fig. 5), indicating that suppression of thetransverse wake-field effects may be satisfactory.

e-Targete+Figure 5: Orbit and charge of two bunches measured
by bpm. The dense line indicates the first bunch.

Two-bunch operation tests of the KEKB linac for
doubling the positron injection rate to the ring have beensuccessfully carried out, giving promising results
concerning possible normal operation. We expect toobtain stable operation of two-bunch injection into the
KEKB ring in FY2001.

REFERENCES[1] K. Oide, “Status of KEKB B-Factory”, HEACC2001,Tsukuba, March 2001.
[2] S-I. Kurokawa, “B-Factory Commissioning and FirstResults”, PAC’01, Chicago, June 2001.
[3] K. Abe et al., “Observation of Large CP Violation inthe Neutral B Meson System”, Phys. Rev. Lett. 87,091802 (2001).
[4] Y. Ogawa et al., “High-Intensity, Two-Bunch
Acceleration for Doubling Positron Production at the
KEKB Linac”, HEACC2001, Tsukuba, March 2001.
[5] S. Ohsawa et al., “Increase of Positrons by HighIntensity Two-Bunch Acceleration Scheme at theKEKB Linac”, PAC’01, Chicago, June 2001.
[6] E. Kikutani et al., “The KEKB Bucket Selection
System – Recent Progresses and a Plan in the Near
Future”, in these proceedings.