JAERI-Conf 95-021

3a3-1
DESIGN OF A BEAM-POSITION MONITOR CIRCUIT FOR THE KEKB INJECTOR

Y. Hosono* H. Kobayashi**, T. Suwada**, T. Urano**, A. Lazos*** Fac. Eng. Univ. of Tokyo, 7-3-1 HONGO, BUNKYO-KU, TOKYO 113, Japan** KEK, National Laboratory for High Energy Physics, 1-1 OHO, TSUKUBA-SHI,
IBARAKI-KEN 305, Japan
Abstract
The PF linac is presently being
upgraded for the KEKB project. In thisbeam-position
stripline-typeproject,monitors (BPMs) are to be installed atevery Q-magnet. The beam-displacement
signals are to be processed by BPM
circuits. The circuit must work with highstability under strong electromagnetic
interference. Two types of narrow-band
circuits have been investigated: onecomprises a band-pass filter of 120-MHz
center-frequency with a 3.2-MHz bandone
comprises
otherthewidth;
differentiating circuit and several-stage
integrating circuits. The design andperformance of BPM circuits as well as
the design of the BPM electrode arediscussed.
1.

filter, and the other consist of an RC
differentiating circuit and several-stage
integrating circuits. In this paper, thelatter one is mainly discussed as well as
the performance of the BPM electrodes.
2. BPM electrode
The BPM electrode is made of stainlessitssymmetry;steel with four-folddimensions are shown in fig. 1. All of theBPM electrodes are calibrated using aprecise test bench. A 0.5-mm wire is used
to simulate the beam line, and the BPM
electrode on a movable stage is scannedin the plane of ±5-mm square around thebeam line. The beam position is calibratedusing the following formula:
X= Ao + Alx + A2y+A3x2 + A4xy+ AsY2 +~x3 + ~x2y + Agxy2 + ~lv.-v3 and y = v.-v
42Introduction

where X = 1

The PF 2.5-GeV linac is being upgradedin order to meet the requirements ofKEKB project. The electron-beam energy
is to be increased from 2.5 GeV to 8 GeV,
and that of the positron beam from 2.5
GeV to 3.5 GeV. The bunch charge of thepositron beam is to be increased so as tobe 20-times as intense as the present one.
In order to realize this big increase of thepositron beam, the primary electron
beam must contain 10 nC/bunch. In orderto suppress the transverse wakefield due
to a high-current beam, the beam mustbe accelerated in the center axis of theaccelerating structures.
BPM electrodes will be installed atevery Q-magnet for measuring the beamdisplacement from the center axis. The
BPM circuits are to be used near to thewhich generate
klystron modulators,
very strong electromagnetic noise, andansuchinstablywork
must
narrow-band
Twoenvironment.
amplifiers have been studied for thispurpose. One comprises a band-pass~+~V2+~

We have measured all coefficients from
Ao to ~ for 16 MPM-electrodes whichhave been installed in the linac system.
In these coefficients, Ao is an offset ofthe electrode and A1 is often used forfirst-order approximation. Typical values
for Ao and A1 were several ten micronsAll
respectively.
mm,
8.59
andcoefficients from Ao to ~ will be stored
in the computer system for calibration
purposes.
3. Design of Circuits
The BPM circuits have four channels
which measure outputs from V1 to V4.
The linearity required in order tomeasure the beam position with a 0.1 mm
accuracy is 1%. This specification is very
hard, and none of the circuits describedlater satisfy this linearity. Further
improvements should thus be made. We-428-JAERI-Conf 95-021

also consider compensating the linearity
using computer software.
The circuits are to be installed undervery strong electromagnetic noise, thefrequency spectrum of which has beenstrongvery
isTheremeasured.
TwoMHz.
20
electromagnetic noise underawith
amplifiers
types of narrow-band
been
haveMHzpass band over 20
investigated.
Circuit-type 1
We first designed a circuit which
cover 65 dB. Though a 10 nC/bunch is themaximum accelerated charge, the BPM
design covers twice this (20 nC/bunch),futureaccount
intotakingimprovements. The mm1mum. charge inthe specification is 0.64 nC/bunch for the
positron beam, though 0.2 nC/bunch iscommissioning.
beam
positronfor
Therefore, the dynamic range for the
beam intensity is 45 dB, and a dynamic
range of ± 10 dB is added to it for beamdisplacement within ± 7 mm. A dynamic
range of 65 dB is divided into threeoverlapping ranges, each of which has
34 dB. The circuit comprises a band-passfilter, limiting amplifiers, a doublea sample/holdbalanced mixer andcircuit<l,2). The dynamic range of thecircuit is satisfactorily, but the linearity
does not meet our requirement. The
linearity will be compensated using acomputer.
Circuit-type 2
The circuit described above covers allof the beam modes; a common circuit canbe used through the linac. Other possiblemeans is to make two types of circuits
which can cover from the beginning of
the linac to the positron target (1 0nC/bunch-1.28 nC/bunch) and from thepositron target to the end of the linac(1.28 - 0.2nC/bunch), respectively. Twotypes of circuits having different gain
levels and the same dynamic range of 35
dB satisfy our requirements. In thisalternative, we aim for simplicity and low
cost of the circuits; also, a beamdisplacement within ± 5 mm is assumed. If
the circuit is simple and low cost, we canthen make the same number of the BPM
circuits as the BPM electrodes, and canmeasure one beam trace with one beampulse; otherwise, several BPM electrodes
will be connected to one common circuit

severalrequiring
switches,
throughpulses of beams to measure one beam
trace. A beam-position measurement with
one identical beam is very attractive andleads to a second circuit.
This circuit comprises a band-passfilter comprising an RC differentiating
circuit and RC integrating circuits. A
schematic diagram of the type-2 circuit isshown in fig. 2. The differentiating
circuit is followed by three stages ofintegrating circuits. The output from theband-pass part is amplified and stretched.
Figure 3 shows the input vs. output of thecircuit. Pulse width of input pulse was 1
ns and horizontal axis shows attenuation
passBandgenerator.
pulseofcharacteristics of the circuit are also
shown in fig. 4. Center frequencies for
andcircuit
differentiating
both
integrating circuits are 50 MHz. Althoughiscircuittheofstability
thesatisfactorily, linearity of this circuit isworse than our requirement.
4. Conclusion
We have developed two types of BPM
circuits. Type-2 has been investigatedvery recently, and has some room forimprovements. It was very difficult toobtain a linearity better than 1% for bothbewilllinearity
Thecircuits.
compensated for using a computer. Bothwill be tested in a real field from
September, 1995.
References
1) A. Lazos, T. Suwada, T. Urano and H.
Kobayashi; Proc. 1994 Linear Ace. Conf.
Tsukuba, Japan 1994, p. 872
2) H. Kobayashi, T. Urano, T. Suwada
Proc. 20th Linear Ace.
and A. Lazos;
Meeting in Japan Osaka, Japan 1995, p.
245-429-JAERI-Conf 95-021-_-_______________________________________ L~~'-~---------------------~____________________
1_,_. ___5_______---- _.!___2·l'J?9lG

lJ?--~'-----------_- _-~2-!L__JO 15llO[ -~~: --~0~~ -:~~~--- ------------------------- -- ---0 IU~~~~-1 0: I~'. -t}_}' '{.~~_:_~~ ··P}~_.j_!_(I,_QI

l·~LJ_J_O,o~lOC)

lO.,."(•)

LO-00Cl

0_Fig. 1. BPM electrode.

Fig. 2. Diagram of Circuit-2.

lOr-.--.--.-.--.--~~

0.35 .-----=----.--..---:::J;·~-->
g_-=

- 0.3
Q.
Q. 0.25?::, 0.2

1-6-s.

0.1

o.15
0.1

0 0.010.05
0. 00 1 .'-:-:---!:----:-'::---::1:--::L--L--.L___j-10 0 10 20 30 40 50 60
Input( dB)
Fig. 3. Input vs. output of Circuit-2.

00

150
10050
Frequency(MHz)Fig. 4. Band-pass characteritics of Circuit-2.

-430-