PROGRESS OF BEAM POSITION MONITOR IN KEK PROTON SYNCHROTRQN
D. Arakawa, S. Hiramatsu, H. Ishii, J. Kishiro and K. SatohNational Laboratory for High Energy Physics
Oho-machi, Tsukuba-gum, Ibaraki-ken, 305-32, JapanThe observation of the beam position in the accelerator is indispensable
to steer the beam in a small vacuum pipe during the acceleration period. It
offers the significant information to RF feedback system etc. The correctionof the closed orbit distortion can not be accomplished without the informationof the beam position.
The detectors to observe the beam position are classified into two. One
is electrostatic pick-up and the other is electromagnetic one. We developed
the former[l] and studied the electric characteristics during last year. In
this study following defects were observed. There were anormalous noisy
components in the signal. The noises in the signal were induced by RF signal
and its harmonicses and were arised from the sudden change of the beam shapeand position at the transition period. Moreover, the noises were induced in
the signal transmission cable. The method to obtain the beam position data bysubtructing between two analogue signals was too sensitive to the frequency
characteristics of the long transmission cable.
It can be expected to reduce the coupling between the sources of the noiseand pick-up by making the impedance of the pick-up lower using the electromagnetic pick-up. The reduction of the noise induced in the transmission cable
can be performed by transmitting only the lower frequency fundamental componentof the beam signal.
We developed a new type wall current pick-up as the beam position detector[2]. It has a inner pipe segmented into four parts as illustrated in Fig. 1.
The segmentation of the pipe enables us to observe the horizontal and vetricalbeam position, sumiltaneously. And it makes the inductance high so that thedecay time constant increases to about 400ns. The wall current induced on eachsegment flows out to outer vacuum pipe through a rod, whose resistance is about
0.05~, and is picked up by the small transformer wound by Cu wire of 30 turns.
The covar rod is metalized on the ceramic which seales the vacuum[3]. In thisway the maintemances of the detectors are carried without the interruption tovacuum system.
The block diagram of the electric circuits is shown in Fig. 2. An amplifire, placed in the accelerator room, has a low pass filter (LPF) with highfrequency cut-off of llMHz and a rectifire. The LPF eliminates the highfrequency noises and enables us to obtain a fundamental frequency component of
the beam signal. The rectifire is used to pick up the signal caused from thebeam position deviation from the closed orbit which is superimposed on the beamsignal.
In the scheduled accelerator shut down in last summer, we installed 56
detectors around the PS. From November we studied the perfoemances of thedetectors. The position error arised from the noises were lesser than o.Smm.
The results of the measurements of the closed orbit dist:ortion are shown inFig. 3. The agreement of the measured distortion at the local orbit bump to
the calculated one is very good. By using the electromagnetic pick-up andtransmitting the low frequency component of the signal, overall accuracy of thesystem is improved to about O.Smm.

~169~

The author wish to thank Drs. A. Ando and N. Kumagai for their colaboratio nin operating the PS magnets and would like to thank Dr K. Muto for his skillful
RF tuning.
References
1) H. Ishii, K. Muto and s. Shibata
KEK-Accele rator-78-4 Nov. 1978
s. Shibata, K. Muto, H. Ishii and Z. Igarashi
Particle Ace. Conf. IEEE Trans. on Nucl. Sci.NS24(19 77)1736
2) K. SatohRev. Sci. In strum. 50(1979)450
3) H. Ishimaru
Rev. Sci. Instrum. 49(1978)545,a-~

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Fig. 3

Measured closed orbit distortion-170-