JAERI-Conf 95-021

PA48
Beam Position Monitor for an Orbit Feedback System
Masaki.Tejima, Hitoshi Ishii and Yoshihiro Funakoshi
National Laboratory for High Energy Physics (KEK),
Oho 1-1, Tsukuba, lbaraki, 305, Japan

Abstract
We have developed a stripline type beam position
monitor system. This system was intended to be used in
the beam study on the orbit feedback system which will
be used in the KEKB. Characteristics of the monitor
system were investigated in both bench tests and beamtests. We succeeded to detect these positions with enoughaccuracy of a few micron using the system.

1. Introduction
An orbit feedback system is vital for maintaining
an optimum collision condition at a B factory where twobeams circulate in separate rings. For this purpose thebeam-beam deflection technique, pioneered at the SLC,
may be utilized to detect an offset of the two beams.
Methods of applying this technique to a ring collider aredescribed in a previous paper[ I]. Feasibility study of thetechnique has already conducted successfully by using
beams of the TRISTAN Main Ring. Details of this beam
test were shown in another paper[2].
In this paper we focus on characteristics of thebeam position monitors developed for the beam test.
Results of bench tests and beam tests on thecharacteristics of the monitors are described in detail.

I

impedance. Since we want to pickup the signal of two
beams separately, we chose this stripline monitor withfour electrodes. In addition, the stripline has the advantageof the higher signal level in comparison with button typeelectrode.
The design value of the characteristic impedance is50 Q. The length of the stripline electrodes was chosen
as 148 mm to maximize signal amplitude at the detection
frequency of 508MHz. Directivity of stripline electrodecan help us to distinguish a signal of one beam from thatof the other. To avoid degradation of accuracy due toinsufficient directivity of stripline electrodes, we haveworked out two countermeasures. First, the locations of
the monitors were carefuUy selected as shown in Fig.l sothat Fourier components of this signal are not disturbed .
very much by the counter-rotating beam at the detection
frequency. Second, we designed the structure of themonitors so that the characteristic impedance of thedipole mode matches to 50 n taking account of theelectromagnetic coupling with the other electrodes.
Details of the two countermeasures are described in aprevious paper[3].
The schematic view of the stripline monitor we
finally adopted is shown in Fig.2.
Stripline-~

1845

I
1576

.... ~{~; ..~·~···....................

c~p].
•:~I 1I" •w

1------2 920

Fig.l Beam position monitor locations.

2. Stripline Monitor
Stripline electrodes, which are also called
directional coupler pickup electrodes, are' essentiallytransmission lines with a well-defined characteristic

Button/SMA ,]Fig.2 Stripline monitor
Matching of the characteristic impedance ofelectrodes was measured by using a time domainreflectrometer (TDR), as shown in Fig.3. A mismatch of
the stripline electrode itself seems small. A large
mismatch exists at the part of line to connect thestripline and the center conductor pin of the feedthrough.
Although this mismatch can not be avoided, thedirectivity is hardly influenced by this at the detection
frequency. To obtain the beam position, we convert theelectrical position(H,V) to the geometrical position(X,Y)
following the first order approximation as;- 162-

X=kx·H, Y=ky·V

JAERI-Conf 95-021

where kx and ky are coefficients of conversion which are
determined from the geometry of the monitor chamber.
And the electrical positions are given by thenormalization procedure as follow;

Strlpllne/Button callbratlon(NL)

'E

%·2.6
~

H=(A-B-C+D)/(A+B+C+D), V=(A+B-C-D)/(A+B+C+D)~ ·2.8,g

where A,B,C and Dare the induced voltages of the fourelectrodes.

~·3~0.. ·3.2

·3.4 ......_,~_._.~...........~~..c.....~.....J...~~.L........~....J·0.085·0.09·0.095·0.1·0.105·0.11·0.115

Electrical position from Stripline

Fig.4 Calibration of Stripline monitor

Fig.3 TDR response of four stripline electrodes.
The waveforms are outputs of each electrodes
50mV/div., 500psec/div.
The kx and ky were determined by using actual
beams as is shown below. Two button electrodes arehoused in the monitor chamber at both upstream anddownstream sides of the stripline electrodes. We measured
beam positions at seven different beam orbits by using
both the stripline and the button electrodes. The beam
orbit was changed by making orbit bump around themonitors. According to the beam positions obtained fromthe button monitors, the calibration constants of thestripline monitors, kx,y were determined. This is because
we have mapping data for the button monitor from thebench test and we do not have those for the striplines.
Fig.4 shows plots for the calibration. We obtained thevalues of kx,y=33.1.

We aimed at obtaining much better resolution thanthat in the TRISTAN. The configuration of position
monitor electronics is shown in Fig.5. Eight output
signals from a monitor chamber are sent to the localcontrol room(DlO) through their own coaxial cables. The
signals are selected by RF switches and processed by acommon front-end circuit. A programmable attenuator
adjusts the signal level to match to the circuit. The frontend circuit consists of a triple stage super -heterodyne
circuit, a synchronous detector and a 20 bits ADC.
These circuit components were made as CAMAC
modules such as an RF switching module and aprogrammable attenuator module. We are able to control
these modules from the central control room. A pick up
frequency was chosen as 521MHz which is the 5267th
harmonics of the revolution frequency (99.9KHz). Thisfrequency was determined experimentally so that we canmitigate the effect of insufficient directivity. We mention
this frequency choice in more detail in the next section.
The position resolution of this system is designed
to be a few micron and this value was actually achievedin the beam test. The time which is needed to get beam
positions of the two beams at one position monitor isabout 3 sec. The time is mainly determined by AID
conversion, the typical value of which is around 120msec
per conversion.

3. Electronics
We developed new electronics for the beam
position monitor. We adopted basically the same signal
processing method as that for the BPMs of the TRISTAN
Main Ring.

- 163-Fig.5 Configuration of position monitor electronics

JAERI-Conf 95-021

4. Measurement
We measured beam positions repeatedly by using
the monitor system to check performance of theelectronics and to observe an orbit change. Fig.6 shows ahistory of the beam positions at 8GeV for about threeboll!~· We con~ed that the monitor system has a highpos1t1on resolut1on of a few micron. We also measured
repeatedly beam positions of. the two beams at 29GeV as
shown in Fig.7. We observed some fluctuation of thebeam positions: Those were changing with a period of
about 150 sec and about 50 min. The FFf analysis of
those data reconfirms those periods as shown in Fig.8.
Position change at 8GeV(950516/20:50)
0.1["'""'~.,...~.....~~.......~,...,.....~..........~~

e o.os.§.

.§1!E -o.os-0.1-Q.15 0.._._..~._.0.5~~--'--'--~~1L.5~........J.~~2..1....5~.......JTime(hour)

To see an influence of the counter-rotating beam on
the beam position measurement, we made measurementsin the following two conditions. First, with the positron
beam we observed the signals coming from the upstream
port of each electrode .Secondly, with the two beams weobserved the signals from the same port. In themeasurements the beam current was 1.2mA for thepositron beam and 0.8mA for the electron. Fig.9 shows
results of the measurements. The vertical axis is thedifference of the signal levels in the two conditions. If thedirectivity of the stripline is perfect, the voltage shift
should be zero. In reality, however, the signal levels dochange due to the influence of the counter-rotating beam
as seen in Fig. 9. The horizontal axis is the relative
bucket difference of the two beams. Tli.e zero bucket shiftcorresponds to the situation where the two beams collide
at the nominal collision point. We intended that thevoltage shift should be zero with the zero bucket shift at
the desi~n detection freq.uency of 500MHz by choosing
the locauons of the momtors correctly. However, in theexperiment the shift of 15 RF buckets were needed tominimize the voltage shift We have not yet understood
the reason for this. In the experiment on the beam-beam
deflectio.n[2], the detection frequency was changed
temporanly to 521MHz to minimize the voltage shift.

Fig.6 History of beam position at 8 Gey.

Position change at 29GeV(950509/13:10)-0.85 r-~..,.-.~-.-,-~~,.....-~...,.--.~~~~

e -o.e~.Q-.i ·0.95

8.

J-1-1.05-1.1 ~~--:-1:-'~_.__t_~~L....~..L.~_._j_~..__._j
0
0.5
1.5
2
2.5

TIME(hour)

6.Acknowledgments

Fig.7 History of beam position at 29 GeV.

We wis~ to thank KEK Vacuum Group for helping
the construction of BPM chambers. We also wish tothank Dr.N.Hiramatsu, Dr.K.Satoh for useful somecomments and K.Mori, Y.Suetsugu and K.Hanaoka forassisting with carrying out experiments.

References.

.

Fig.8 FFf analysis of the position history.

[ 1] Y .Funakoshi, Proceedings of "The state of the art inaccelerators, detectors and physics" SLAC-400(1992)[2] ~.Funakoshi, M.Tejima and H.Ishii, Proceedings ofParticle Accelerator Conference and International
Conference on High-Energy Accelerators ·1995 Dallas .
~A'''[3] Y.Funakoshi, M.Tejima, H.lshii and K.Hanaoka,
Proceedings of the 9th Sympo. on Accelerator Scienceand Technology, 1993, Tsukuba, Japan-:- 164 -