Proceedings of the 2001 Particle Accelerator Conference, Chicago

INCREASE OF POSITRONS BY A HIGH-INTENSITY TWO-BUNCHACCELERATION SCHEME AT THE KEKB LINAC
Satoshi Ohsawa, Atsushi Enomoto, Eiji Kikutani, Kazuro Furukawa, Naoko Iida, Mitsuo Ikeda,
Norihiko Kamikubota, Takuya Kamitani, Hitoshi Kobayashi, Haruyo Koiso, Toshihiro Matsumoto,
Yujiro Ogawa, Yukiyoshi Ohnishi, Katsunobu Oide, and Tsuyoshi Suwada, KEK, Tsukuba, JapanAbstract
As the accumulation current of positrons increases inthe KEKB ring, the injection time is becoming longer. It
will thus be one of the most important issues affecting theaccumulation of the integrated luminosity. As one of thesteps, we introduced a high-intensity two-bunch
acceleration scheme at the KEKB linac to intensify
positrons by means of doubling the primary electrons. We
recently obtained test results of 0.54 nC for the first bunchand 0.49 nC for the second bunch at the linac end. This
scheme increased the positron intensity by nearly 65%.
Since the linac frequency is not a harmonic number of theLER frequency, the best time interval between twobunches is 96.29 ns, corresponding to 49 LER-buckets.
Even with this limitation, it is undoubtedly a very usefulscheme for increasing the positron injection rate. The
beam test results are described.

1 INTRODUCTION
From the beginning of the KEKB project, we have only
been accelerating S-band single bunch beams at theKEKB linac for injection into the KEKB rings. The
reasons why we have used the single bunches are as
following: 1) Since wake field effects produced byproceeding bunches would be strong in the case of highintensity multi bunches, it is easy to avoid the effects forstable operation. 2) Since the bunch separation is 350ps inthe linac, the s-band multi bunches do not satisfy theKEKB requirement that the beams should be injected
within a time range of 30ps against the ring RF buckets.
As the LER accumulation current is increasing, thesubject of intensifying positrons is becoming more
important. We have thus introduced a two-bunch schemein order to double the total charge of the primary electron
beams for increasing positrons. In order to satisfy thesecond condition, the bunch separation was determined soas to be 96.29 ns, which is the period of the common
frequency (10.3 MHz) of both the linac and the KEKBrings. The separation is conveniently sufficiently longer
than the RF filling times of the prebuncher and thebunching section of the buncher, which are 8 ns and 48
ns, respectively. Therefore, it is expected that in thebunching process the second bunch would not be affectedby the wake fields produced by the first bunch in theprebuncher and the buncher because the RF waves are
replaced

0-7803-7191-7/01/$10.00 ©2001 IEEE.

before the second bunch arrives at the devices, which areboth of travelling wave type.
An energy difference occurs between two bunches dueto the time-varying SLED pulse and beam loading.
Energy compensation can be carried out by means ofoptimising the SLED pulse timing against the twobunches in order to equalize the two-bunch energies [1].
We experimentally examined the feasibility of the twobunch scheme. The results are very satisfactory, whichshows that the two-bunch scheme is promising to improve
the positron injection rate.

2 TWO-BUNCH BEAM GENERATION
2.1 Double-Pulse Production for the Gun
First of all, a new grid-pulser system for the electron
gun has been developed that independently produces twopulses to execute the two-bunch scheme [2]. The system
mainly consists of a pair of same grid pulsers with timedelay units and four combiners that were newly
developed to combine high-voltage, short pulses of 1 kVand 1 ns. It can produce single pulses for single-bunch
operation as well as double pulses for two-bunch
operation. The bunch-selector system gives inhibit signals
for the time-delay modules to choose any pulses. It willautomatically determine the operation mode; namely, thesingle-bunch or two-bunch modes will be selected rapidly
at injection into the LER.

Figure 1: Typical example of double grid pulses(upper), and the electron beam from the guncorresponding to the grid pulses (lower). The pulse3284

Proceedings of the 2001 Particle Accelerator Conference, Chicago
heights and timings were changed as free parameters, anddetermined so that the two-bunch charges and orbits
coincide with each other. Therefore, the peak intensities
are not the same, and the time interval is different from96.29 ns, which is the period of the common frequency.
The time-delay module has two functions: 1) it
produces stabilized trigger signals synchronized with an
RF wave of 571.2MHz, and 2) it changes the trigger timedelay with steps of 1.75 ns over a wide range of morethan 100 ms. Each delay module roughly determines thepulse timing independently. Any trigger pulses can beinhibited at the delay modules with the signals from thepulse selector system.
The grid pulser has the following two functions: 1) afine timing control that continuously changes within arange of 3ns, and 2) a peak voltage control for output
pulses. These values are controlled with DC signals(0~10V) from a PLC. For stable operation, the pulse
timings are, externally controlled with a feedback systemthat adjust the fine timings. Four output ports of the coaxial type are used in order to make the source impedancelow.
Trigger

Delay

PulserInhibit

Delay

PulserCombiner~96.3nseach other. Since a grid pulser has four output ports, fourcombiners are used.
The monitor signals of the pulses are transported
through an optical fiber to a digital oscilloscope on theground level. The pulse timings are measured relatively
with respect to an RF wave of 571MHz, and are used in
the digital feedback system to stabilize the pulse timings.
Double electron pulses from the gun are compressed
into two bunches in the bunching section of the preinjector by two sub-harmonic bunchers (SHBs) operated
at frequencies of 114.24 MHz and 571.2 MHz, and by aprebuncher and a buncher operated at 2856 MHz.

2.2 Two-bunch Beam Generation
In the SHBs, which are single cavities of resonancewith high-quality factors, the wake fields induced in thecavities do not damp effectively during the bunch timeinterval of 96 ns. The amplitudes of the wake fields due toa beam of 13nC/pulse are, however, still rather smaller
than those of the fundamental RF fields, as is shown inFig. 5. Therefore, the SHBs still retain the bunching
function, although the induced fields deform the bunchshapes.
Although beam acceleration was successful, the bunch
profiles observed with a streak camera had rather large
tails. Further investigations and fine-tuning are requiredin order to improve the shapes, if necessary, with moresophisticated beam diagnostic tools. We could expect
further increases of positrons.

Figure 2: New grid pulser system for the two-bunch
acceleration scheme.

114.24 MHz 571.2 MHz
GunPulser No.1

SHB1

SHB2

2856 MHz

Prebuncher

Bunch

Pulse monitor

Figure 4: Main components of the bunching section.
Combiner
Grrid/CathodePulser No.2

~ 96.3ns

Figure 3: Combiners make double pulses from pairs ofindependent pulses. The pulse heights and timings arefree parameters.
A pulse combiner was introduced to make double thepulses from pairs of single pulses that are produced byindependent pulsers, and simultaneously to prevent any
interference between the pulsers in order not to damage

Figure 5: Typical RF wave in the first SHB (114.24
MHz). The wake fields induced by the 13-nC beam aresuperposed on the fundamental RF wave. Since the beam
repetition rate is different from the RF, deformation due tothe wake fields is clearly seen.

3285Proceedings of the 2001 Particle Accelerator Conference, Chicago
In two-bunch acceleration, first of all, almost all of theparameters were fixed for the first bunch to accelerate itwell. We then proceeded to the next step to coincide thesecond-bunch energy, orbit and charge to the first ones
along the linac. We matched the second bunch bychanging the gun-beam intensity and timing that wereindependent parameters in the region following thebuncher. We then optimised the pulse timings of theSLED RF waves against the two bunches in order tocoincide the bunch energies. This procedure was also
useful to reduce an orbit difference between the twobunches.

Figure 6: Bunch shapes of the 1st bunch (upper) and the2nd bunch (lower), which were measured with a streak
camera located at the end of the pre-injector.

3 TWO-BUNCH ACCELERATIONRESULTS

Figure 7: 2-bunch beam orbits and charges along thewhole linac. A positron target is located at the center. The
blue/green points show values of the 1st/2nd bunch.

4 CONCLUSIONS

We have achieved the successful acceleration of highintensity, two bunches with charges of 7.8nC and 8.0nC to
a positron target with a small loss, which produced
positron bunches with charges of 0.54nC and 0.49nC at
the end of the linac, respectively. The sum of the positrons
reached to 165% of the single bunch charge that actually
injected into the ring during the two-bunch acceleration
test.
Figure 7 shows a typical example of two bunches. The
horizontal and vertical orbit displacements ( dx and dy )and the bunch charges are shown. The blue points show
the values of the first bunch, and the green ones show thesecond bunch. The data of the two bunches are shown on
the same corresponding frames.
Table 1: Beam charges and the enhancement factors
against a single bunch during normal operation.
Position of
1st bunch
2nd bunch Enhancement
measurement charge
chargefactorElectron@ buncher
9.54 nC
9.59 nC
179%@ target7.79 nC
8.02 nC
190%
Positron@ target
0.88 nC
0.90 nC170%@ linac end
0.539 nC
0.486 nC
165%

We have succeeded to increase positrons by a factor of
65% by means of introducing a high-intensity, two-bunchacceleration scheme at the KEKB linac. This method
opened a way to the short injection of positrons. Twobunches of positron beams with charges of 0.54nC and
0.49nC were successfully accelerated to the end of thelinac. Their behaviour was surprisingly similar along thelinac despite undesirable bunch profiles with rather large
tails. When the bunch shapes were improved with further
careful fine-tuning, nearly twice as many positrons could
be expected.

5 ACKNOWLEGIMENTS
We would like to express our special appreciation toProf. Motohiro Kihara for his encouragement and support.
We also wish to express our thanks to the members of theKEKB linac and machine operators for their continuous
efforts to maintain the linac in good condition, which isimportant for this improvement.

6 REFERENCES[1] Y. Ogawa et al., "High-Intensity, Two-Beam
Acceleration for Doubling Positron Production at the
KEKB Linac, " HEAC'01, Tsukuba, March 2001.
[2] S. Ohsawa et al., "Two-Bunch Beam Generation at
KEKB Linac," HEAC'01, Tsukuba, March 2001.

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