Proceedings of the Second Asian Particle Accelerator Conference, Beijing, China, 2001

THE KEKB BUCKET SELECTION SYSTEM - RECENT PROGRESS AND
THE PLAN IN THE NEAR FUTURE
E. Kikutani, Y. Ogawa, M. Tobiyama, KEK, Tsukuba, Japan
Abstract

Table 1: Machine parameters concerning the bucket selection system

The KEKB bucket selection system, which enables us torealize a variety of bunch fill-patterns in the KEKB rings,
has been working well since the start of the commissioning of KEKB in Dec. 1998. Although the hardware ofthe system has not changed since the start of the machineoperation, its software is continuously evolving, in order tomeet the requirements from the view-point of more sophisticated machine operation. In this paper, we will describe
recent progress in the software, as well as the plan for thenear future, upgrading it to be able to handle two bunches
accelerated in one pulse of the injector linac. With this upgrade, the injection time of the positron beam into the low
energy ring will remarkably be shortened.

1

rf frequency
harmonic number of the rings
repetition rate of the injection
minimum bunch spacing

509MHz512050Hz
2ns

tightly synchronized, and which bucket is occupied by theinjected bunch is determined by the relative timing between
the revolution signal of the ring and a trigger signal for thelinac gun. By controlling this timing (or more concretely
by giving a suitable delay to the gun-trigger-signal), we canfill the bunch into a desired bucket.

INTRODUCTION

The KEKB bucket selection system consists of threeparts; (1) a part that actually sets the delay explained above,(2) a part which communicates with accelerator operators,and (3) a bunch-current monitor part. The bunch-current
monitor is indispensable to precise control of the relative
bunch-currents over the buckets. These three parts are connected with a high-speed dedicated communication system
based on optical fiber cables, since the three parts are installed in separate buildings. Figure 1 schematically explains the connection of the three parts of the system.

The KEKB collider at KEK in Japan is a B-meson factory
consisting of a 3.5 GeV positron-ring and an 8 GeV electron ring with their injector. The injector is actually a linaccomplex which can provide the collider with both electron
and positron beams. At the end of 1998, commissioningof KEKB started and the machine performance has been
progressing since then, and the peak luminosity reached
4.5 x1033 cm -2 s-1 in July 2001. In order to obtain highluminosity, the number of bunches stored in the rings ischosen to be very large. At present, the number of bunches
per ring is 1153 in daily physics-experiment operation, andthe number will be increased to make the luminosity still
higher. The fill pattern, that is, which bucket is filled withbeam and which not, is one of important parameters for operating storage rings under such a high-current/multi-bunch
condition. Then, a sophisticated fill-pattern-managing system or a bucket selection system is required.
The KEKB bucket selection system has been working
well since the start of commissioning. Machine parameters concerning the bucket selection are summarized in theTable 1. While the hardware system has not been changed
since then, the software has been continuously evolving tomeet operational requirements. In the previous paper[1]published in late 1999, we described the system architecture and experiences up to that time. In the present paper,
we review the system and discuss the recent progresses aswell as the plan for the near future.

Linac building

Control building

bunch current
monitors

Fuji building

optical cable forshared memory

Shared Memory(VME board)
VME computer

control network

2

delay modules

A BRIEF REVIEW OF THE KEKBBUCKET SELECTION SYSTEM

Figure 1: Three parts of the KEKB bucket selection systemand their connection.

Since KEKB has no damping/accumulation rings, thebeams are directly injected from the linac to the rings. At
injection time, the rf systems of the linac and the rings are669

Proceedings of the Second Asian Particle Accelerator Conference, Beijing, China, 2001

3
3.1

RECENT PROGRESSES

4

Generation of arbitrary fill-patterns
4.1

The operator-communication part provides us with a toolfor generating standard fill patterns. The term “standard”
means that the fill pattern is made up of a number of bunchtrains with equal spacing, where the number of the trains
should be a measure of the harmonic number of the rings,
5120. An operator can specify (a)the number of the trains,(b) the number of bunches per train, and (c) the bunch spacing. Then the system generates the pattern corresponding to the specified parameters and loads it to the hardware pattern-table when requested. In ordinary physicsexperiment operation, we use this pattern generator system.
Besides this standard fill-pattern generator, a new patterngeneration system is developed. An operator can input anarbitrary fill pattern via an ascii-based fill-pattern file. The
data in the file is in 5120-line, 2-column format, as shownin Fig. 2. This “read-in pattern” system is very useful inoperating the rings in the machine-study mode.

TWO-BUNCH ACCELERATION INTHE INJECTOR LINAC
Motivation and the principle

In the usual operation of the injector linac, only one bunchis accelerated within one linac-pulse. If multiple bunches
are accelerated in one pulse, injection time will be shortend, and as a result, the accumulation rate of the luminosity
will be effectively increased. This is the motivation of themulti-bunch acceleration in the linac, particularly for thepositron beam.
However, the realization of this idea is not a simple task.
The complexity comes from the relation between the rf frequencies of the linac and the rings. The rf frequency of thelinac is 2856MHz and that of the rings is 509MHz. Since
these frequencies have the common divisor of 10.38MHz,
they are made by multiplying this common frequency, bymultipliers 275(55 x 5) and 49 for the linac and the rings,respectively. Figure 3 shows this frequency relationshipand the basic idea of the bucket selection.
x5

x55

0
1
234..
.

100
0.80..
.

5119

0to the linac

10.38MHzx 49509MHz

to the ringsx49
TD4s50Hz or 1/nclock1/5120
revolution source

in2kHzout

Figure 2: An example of the fill-pattern file. The first column is the bucket id and the second column indicates therelative bunch intensity as explained in the following.

3.2

S-bandsynchro.

gun, kicker
etc.

Figure 3: Basic idea of bucket selection. The rf signals of
the linac and the rings are made by multiplying the common frequency, 10.38MHz, by 275 and 49, respectively.
The linac-gun is triggered by the timing signal delayed by
the TD4V module appeared in this figure.

Intensional uneven filling

Usually, bunch-currents of all the filled buckets are controlled to be the same among them, since the evenness of
the “fill shape” is essential for the stable operation of therings. To ensure evenness, the bucket selection system isequipped with the bunch-current equalizing (BCE) function. When the BCE switch is on, the system checks out
the relative bunch currents and sends the linac beam into
the bucket that has the smallest bunch current.
Sometimes, however, we are required to fill bunches intentionally un-evenly, for beam-studies. Due to these nonstandard fill-patterns, unusual power spectra arise in eachpart of the rings. This can be helpful for us to identify
impedance-sources by carefully checking the response ofthe beam. To use this option we can use the fill-patternfile we described above. We put the figure 1 in the second
column for the standard bunch current. On the other hand,when we want to store a smaller current, 90% of the otherbuckets for example in some specific bucket, we must put
the figure 0.9 in the 2nd column of the corresponding line.

From the frequency relationship, we find that two beambunches can sit on the same rf phase only if they are separated by 49 times of the one cycle of the ring rf-frequency(~ 98ns) or its multiples. That is, if we inject beam from thelinac to the ring with best injection efficiency, two bunches
should be positioned, for example at bucket #1 and bucket#50, in the ring. This is a very tight constraint for the choiceof the fill patterns.

4.2

Considerations on the fill-pattern

At present time we operate the rings with a bunch spacingof 4 rf buckets. However, it is clear that we can not use thisfill-pattern due to the constraint. From many experiments
we have learned that the 5 rf-buckets-spacing is harmful
because of a heating-up problem at the interaction-region
chamber installed inside the physics detector. We have also
learned that the 3-bucket-spacing introduces a very strong
coupled-bunch instability, and that the luminosity is not670

Proceedings of the Second Asian Particle Accelerator Conference, Beijing, China, 2001

4.4

increased as we expected even though the number of thebunches (and also the total current) is increased. Thus, inthe 2-bunch acceleration scheme, we must use a fill-pattern
whose spectrum is similar to the 4-bucket spacing and notsimilar to the 3-bucket nor 5-bucket spacing.
Following this principle, we listed three candidates ofthe fill-pattern for the two-bunch acceleration scheme:

Trial of the two-bunch acceleration

After the fill-pattern investigation, we tried actual twobunch acceleration and injection into the both rings. The
experiment was performed just before the summer shutdown of 2001. The fill pattern 1 (explained in Section 4.2)
was used for the experiment. Figure 4 shows the bunchcurrent distribution in this experiment.

1. Basically the bunch-spacing is 4 rf-buckets. But every
98 th rf-bucket, there is an irregular spacing (5 or 1),
2. Basically the bunch-spacing is 4 rf-buckets. But every
49 bucket, there is an irregular spacing (5 or 1) ,3. The third fill-pattern is made up with repetitions of a
train of 7 bucket-length each of which is divided into
3 and 4 spacing as show below. The number 7 is goodfor our situation where the buckets of two linac bunchis separated by 7 x 7 buckets.
1001000 1001000

1001000...

Prior to the actual two-bunch acceleration, we stored thebeam with these three patterns and compared the performance, such as the injection efficiency, the luminosity andso forth. The experiment was done with usual one-bunch
acceleration of the linac and is purely for the fill-pattern
study. The results from the experiment are as follows:

Figure 4: The filled pattern with the two-bunch acceleration
mode. We successfully accelerated the bunches and stored
them in the ring. In the pattern, we see an irregular pattern
due to the imbalance of the first and second bunches in one
linac-pulse. After careful adjustment, this problem will besolved.

* The fill patterns 1 and 2 are almost the same as theusual 4-bucket spacing operation.
* The fill pattern 3 is worse than the former 2 patterns,
particularly, the injection efficiency for this fill pattern
is about 1/3 of the usual pattern. More injection tuning might increase the efficiency, but we did not try it
because it would take a very long time.

4.35SUMMARYThe KEKB bucket selection system has been working wellsince the start of the commissioning at the end of 1998.
The software of the system has been continuously evolving to meet the operational requirements. An arbitrary fillpattern with individual intensity control was newly devised.
In addition, the software system is now reconstructed to incorporate the two-bunch positron-bunch acceleration in thelinac. In the summer 2001, we performed the experimentof the two-bunch acceleration and injection into the rings.

Injection procedure

Up to the present time, only one bunch in a pulse is accelerated and the BCE switch is almost always ON from thestart to the end of the injection. However, the BCE systemcompares the bunch-currents bucket to bucket, not bucketpair to bucket-pair where bunches in each of pair are distanced by 49 rf-buckets. Hence, the BCE does not function
correctly unless the intensity of the linac bunches are completely the same. In addition, even if the intensity of linacbunches are exactly controlled to be equal, problems willremain. For example, when we top-off the beam, the remaining beam-current is not regularly distributed over thebuckets, in general.
Considering this variety of cases, we find it is reasonable to divide the injection procedure into two steps. In
the first step, the two-bunch acceleration is used and theinjection rate is tuned as high as possible. In this step
the BCE is switched off. After the beam-intensity reaches
some fraction of the full-current, 80%-90% depending on
various conditions, the injection comes to the second step,
where the BCE switch is turned on and acceleration modeis changed from two-bunch to one-bunch.

6REFERENCES[1] E. Kikutani et al., “Bucket selector system for KEKB”, Proc.
ICALEPCS99, Trieste, October 1999.
[2] Y. Ogawa et al., “High-Intensity, Two-bunch Acceleration forDoubling Positron Production at the KEKB Linac” to be appeared in the Proc. of the 18-th International Conf. on Hih
energy Accelerators (HEACC2001), Tsukuba, 2001.

671