WECT006
BEAM FEEDBACK SYSTEMS AND BPM READ-OUT SYSTEMFOR THE TWO-BUNCH ACCELERATION AT THE KEKB LINAC
K. Furukawa , N. Kamikubota, T. Suwada
High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki, 305-0801, Japan
T. Obata
Mitsubishi Electric System Service, Tsukuba, Ibaraki, 305-0045, Japan
Abstract
In order to double the positron injection rate into the KEKBring, a two-bunch acceleration scheme has been studied atthe linac, in which bunches separated by 96 ns are accelerated in 50 Hz. In this scheme stabilization of the energyand orbit of each bunch is indispensable. Thus, the beamenergy and orbit feedback systems have been upgraded.
Since beam characteristics are acquired through beamposition monitors (BPM), their read-out system was improved to meet two-bunch requirements. Combined waveforms from BPM’s were adjusted with delay cables avoiding overlaps so as to enable the simultaneous measurements of the beam positions of two bunches.
The beam energies of two bunches were balanced by tuning the rf pulse timings, and the average energy was stabilized by adjusting the accelerating rf phases. The average
beam orbits were also stabilized. Slow feedback systems at
the injector section for charge and bunching stabilities arebeing planned as well. These systems were successfully
used in the test beams and will be employed during routine
operation.

1

INTRODUCTION

The electron/positron linac at KEK injects 8-GeV electronand 3.5-GeV positron beams into KEKB rings, where theCP-violation study is carried out. Since the efficiency ofthe experiment can be increased by shortening the injection
time, several mechanisms have been introduced to accomplish this[1, 2]. Especially, much effort has been made to
improve the positron injection time, since it is longer compared with that of electrons[3].
One of such effort is a two-bunch acceleration plan,
which has been studied and applied[4, 5]. In this scheme
two bunches of positrons are accelerated in one rf pulse,which is 50 Hz; they may double the injection rate. The
time space between two bunches, however, is restricted by
the rf frequencies of the linac and the rings, and the smallest space is 96.29 ns, since the common frequency is 10.38
MHz. Thus, a precise beam control and diagnosis are necessary.
 e-Mail: <kazuro.furukawa@kek.jp>

The beam diagnosis used so far has been made byemploying strip-line-type beam-position monitors (BPM),
wire scanners for transverse profiles and streak cameras forlongitudinal profiles. In order to maintain stable beams,
it is essential to have the beam instrumentations work forboth of the two bunches. The two-bunch read-out of BPMs
is especially important, because it is used in a number oforbit and energy feedback loops to stabilize the beams.

2 BPM AND READ-OUT SYSTEM
Along the 600-m linac, 90 BPMs are installed and theirsignals are transferred to one of 18 measurement stations.
Signals are delayed and combined so as not to overlap
each other, and are fed into a 5-Gs/s waveform digitizer(Sony-Tektronix TDS-680B/C)[6], as in Fig. 1. Although
the BPM signal is a fast bipolar, the readout precision is
optimized using the interpolation function of the digitizer.
All 18 digitizers are triggered by a single distributed signal,
which is synchronized with beam repetition and rf frequencies.
The waveform is read through the GPIB, and a signal from each electrode is analyzed with a predetermined
response function once per second by a VME computer(Force 68060). The response functions include 3rd-order
position-mapping functions, attenuation factors of various
components and position offsets against the center of thecorresponding quadrupole magnet derived from a beambased alignment.
Since the timing and amplitude ranges of BPM signals
are different depending on the beam modes and locations,the process is driven by a control database system[7].
The acquired beam positions at 18 stations are sent tocentral computers once per second and are served for various beam-energy and orbit feedback systems to maintainstable beam operation.

3

TWO-BUNCH OPERATIONThe BPM system was improved for two-bunch operation.

Oscilloscope (5GS/sec)
1st BPM
2ndCh.2
Right

Left
Right

Left

Ch.1

UpUp

Down
Down

Combiner
16ns Delay x 4

Combiner
16ns Delay x 4

Right
Left
Down
Up

1st BPM

2nd BPMBeamFigure 1: Signals from multiple BPMs arranged so thatcombined waveforms can be processed properly.

3.1

Improvements to BPM SystemAs written above, it is important to acquire the beam positions of two bunches along the linac simultaneously to
study the beams. In our instrumentation, signals from those
two bunches appear as two signals separated by 96.29 ns
on the waveform. Although it was sometimes necessary to
add more delay lines so as to avoid waveform overlapping,
there was no need to add any specific hardware to handle
such signals with small separations.
The calibration factors were re-examined since delaylines were added, and the beam-timing database for thesignal analysis was extended to accommodate two-bunch
information.
Processing functions/commands for BPMs on the central computers are also extended or added for two bunches,
while keeping the old functions as before for single-bunch
operations.
With these modifications, the BPM processing system
was extended for two-bunch operation without any performance loss in either precision and speed. It has been used
in beam operation since March, 2001.

3.2

Operation Software with BPM

Most of the operation software which utilize the BPM
information was extended to meet both single- and twobunch operations. One of such examples is Fig. 2, whichmeasures the beam energies of two bunches by correlation
between a steering-magnet field and the beam-position response at the bunching section.

Figure 2: Software panel to measure the beam energies
of two bunches with a steering magnet (BX A1 B8) anda BPM (SP A1 C5). After adjusting the timing of the electron gun, they became almost the same energy 15.5 MeV
and 15.8 MeV.

3.3

Two Bunch Controls

In order to accelerate the beams properly, the beam characteristics of two bunches need to be adjusted so as to bethe same. For example, in order to adjust the beam-energy
differences, we change the beam timing and rf pulse timing. The beam timing can be changed by 10-ps steps[4]and the rf pulse timing can be changed by 1.75-ns steps ateach sector independently. Most of other parameters in thelinac are not sensitive against time separation of 96.29 ns.
With such adjustments, the 10-nC primary electron
bunches are accelerated up to 3.7 GeV and positrons aregenerated as shown in Fig. 3.

3.4

BEAM FEEDBACK LOOPSThe beam feedback loops in the linac for energy and orbit
stabilization[2] were also extended to control two-bunch
beams. Since we don’t have many mechanisms to control two bunches independently, most feedback loops weremodified to use positions derived from the charge-weighted
averages of two bunches. With these changes, those loops
can maintain the average orbit and energy. In software,
only the monitoring function was extended to read the average positions if two bunches are accelerated. For positron
injection, about 20 beam feedback loops are used, and they

5REFERENCES[1] K. Furukawa et al., “Beam Switching and Beam Feedback
Systems at KEKB Linac”, Proc. of LINAC2000, Monterey,USA., 2000, p.633.
[2] K. Furukawa et al., “Energy Feedback Systems at KEKB Injector Linac”, Proc. of ICALEPCS99, Trieste, Italy, 1999,p.248.
[3] K. Furukawa et al., “Towards Reliable Acceleration ofHigh-Energy and High-Intensity Electron Beams”, Proc. ofLINAC2000, Monterey, USA., 2000, p.630.
[4] S. Ohsawa et al., “Increase of Positrons by High-intensity
Two-bunch Acceleration Scheme at the KEKB Linac”, to be
published in Proc. of PAC2001, Chicago, USA., 2001.

Figure 3: Beam orbit and charge of electrons (left) andpositrons (right) along the 600-m linac. Two slightly different lines indicate the first and second bunches.
Read Beam Position (x and δx)
at Reference Points
BPM
BPM

Beam
Fluctuation

Klystron

BeamKlystron––> Convert into Timings
for rf Energy Doublers
Beam Positionsof 2-Bunches
BPMs

Timing Knob– Beam Fluctuation––> Convert into Energy
Difference
Average (Integration)
x Gain – Offset
Energy Difference Feedback

Figure 4: Energy-difference feedback loop at the J-arc.

are all extended for two bunches.
While normal energy and orbit feedback loops use
charge-weighted average positions, feedback loops to minimize the energy differences use the position difference between two bunches, as shown in Fig. 4.
Although the energy difference does not change frequently, such loops stabilize the beam over the long term.

4SUMMARY

The data-acquisition system for the linac BPMs was upgraded to provide beam positions in two-bunch operation
without losing any original features. Along with improvements of the streak camera and wire scanner systems, it
has still been indispensable to study and operate on linacbeams. The system is also used by many operation software programs, including beam-energy and orbit feedbacksystems.

[5] Y. Ogawa et al., “Two-Bunch Operation of the KEKB Linac
for Doubling the Positron Injection Rate to the KEKB Ring”,
to be published in Proc. of APAC2001, Beijing, China, 2001.
[6] T. Suwada et al., “Stripline-Type Beam-Position-Monitor
System for Single-Bunch Electron/Positron Beams”, Nucl.
Instr. and Meth. A440 (2000) 307.
[7] N. Kamikubota et al., “Data Acquisition of BeamPosition Monitors for the KEKB Injector-Linac”, Proc. of
ICALEPCS99, Trieste, Italy, 1999, p.217.