JAERI-Conf 95-021

PB04
BEAM POSITION DEPENDENCE OF A WALL-CURRENT MONITOR
K.Tamiya, illld A. As ami
Naruto University of Education, Takashima,Naruto-cho, Naruto-shi, 772 Japan
T.Suwada, T.Urano, and H.Kobayashi
KEK, National Laboratory for High Energy Physics, 1-1 Oho, Tsukuba-shi, 305 Japan

Abstract
It was pointed out recently that there eexists
an appreciable beam position deoendence in the wall current monitor widely used in electron accelerators.
Detailed study of this dependence is performed on a test
bench varying the pulse width and th~ frequency of theinput signal simulating the beam. The results ofexperiments show that when the pulse width becomesshorter more appreciable becomes the dependence, andit approaches to that of calculated from the metl10d ofimages. A unified analysis is under way.

Introduction
A wall-current monitor is one of the monitors which
are widely used in particle accelerators. It is used tomeasure the beam current and its waveform, and mostsuited for short pulse electron beams of a few nanosec.
However, it was pointed out recently that there is asignificant beam position dependence [I] ~ 13] . A detailed
study was then made to make clear the charactristics andperformance of this monitor. One of the results of thisstudy is that the output signal is not proportional to theinput signal in magnitude, but precisely proportionalin area. These results are reported elsewhere [4] .
This paper describes a study about the beam position
dependence of this detector; a detailed experiment is madeto clarify the property of this dependence.

through it, and the current flow is csused by the beam.
A schematic drawing of the monitor cross-section isshown in Fig.l, the monitor has four output ports tomake clear the current distribution around the register.
The layout of tl1e experimental apparatus is shownin Fig 2. The beam duct has an insulator in its middle,around which the monitor is installed. The beam issimulated by a current passing tlrrough a wire set in tl1e
center of tl1e beam duct. The current is supplied by a
voltage pulse from a pulse generator( HP 8131A ).
To measure the dependence of the monitor signal on
the beam position, the beam duct is varied with astepping motor instead of varying the wire position. Itis varied by a step of 1 mm up to 5 mm in thehorizontal plane as shown in Fig.l. Each signal fromthe four output ports is measured with an oscilloscope( TDA 6481A).

bl:'nm dur:twa II cu rrrn t monitor

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Experimental method
The operational principle of this monitor is wellkown; across the register set along the beam duct a
voltage difference is produced by a wall-current flow~-------- C II 2

beam ductcableZ

leadcable3

CIIJ(DIGITALOSCILLOSCOPE)
Cll~backFig.l Schematic drawing of the monitor cross-section

Fig.2 Schematic representation of the test bench

Two kinds of experiment are performed in studying
the beam position dependence of the monitor. One isthe experiment in which the width is varied of the input
signal, the other is that the frequency is varied for thesinusoidal waveform used as an input signal. Although
the beam does not have a sinusoidal waveform, thefrequency dependence of the cause to produce theposition dependence may probably be shown more
directly by using the sinusoidal input.
The attenuation of the signal in the cable and thevoltage gain of the oscilloscope depend on the pulse
width or the frequency of the signal, and both aremeasured and corrected for each measurement.

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JAERI-Conf

95-021

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Typical output waveforms are shown in Fig.3. The
pulse width is 1 nsec, and the beam position is +5 mrn.
The channel number of each signal from the top to
the bottom is 1 to 4, respectively, and corresponds tothe output port indicated in Fig.l. An appreciablereflection of the signal is observed, however, its
influence may be mostly avoided! in reading themagnitude by selecting an appropriate time point.
A result of the measurements is shown in Fig.4,in which the output signals from four channels arc
plotted as a function of the beam position. The pulse
width is 1nsec in all measurements. As is shown in
the figure, four curves give different behaviours which
are characteristic for the output positions relative tor: h 10 ch ~

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Fig.4 Output voltage vs. beam position for ch.l to 4

the beam position deviation from the center.
Figure 5 shows a summery of the measurements
for channel 1, where the output vs. the beam positionis shown and the parameter is the pulse width varied· from 750 ps to 50 ns.
When the pulse width is 50 nsec long, the output
secarcely depends on the beam position. However, when
the width becomes shorter, more apreciable becomes
the dependence, and when it comes near or less thana few nanoseconds, the outputs seem to approach
gradually to a solid curve shown in the Fig.5. This
curve is obtained by the method of images, and givenby [5]

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for 1 ns input

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Fig.5 Output voltage vs. beam position; the parameter· is tl1e pulse width-258-+2ns

JAERI-Conf 95-021

8' 3~</ZFig.6 Beam position in the beam duct
where the quantities used are those given in Fig.6. In
the calculation the value of r is required, and isassumed to be 35.25 mm which is about the averagevalue of the solid register.
The result for channel3 of the monitor output isvery similar to the channel 1 described above. As
shown in Fig.1, the channel 2 and 3 are much lesssensitive to the beam position displacement as is also
expected from Eq.(1).
Figure 7 represents the output waveforms for thechannel 1 to 4 when the sinusoidal input is supplied.
The frequency is 300 MHz and the beam position+ 5 mm.
The results of the measurements forchannel 1 is shown in Fig.8, where the frequency·of the input is varied. The parameter is the frequency
varied from 10 MHz to 300 MHz, and at 10 MHz theoutput does not seem to depend on the beam position.
However ,the frequency becomes higher ,more appreciably
depends the output on the beam position. This is very
similar to the previous experiment where the pulse
width is varied. The solid curve is the one calculatrd
by Eq.(l). At 300 MHz the experimental data show
more appreciable beam position dependence than thecalculated. Detailed examination of these data is underway. The reason to cause such beam position
dependence is suggested to be due to the rxistance of an

Fig.7 Waveforms of the monitor outputs from ch.1 to 4
for 300 MHz input
impedance ·along the beam duct in azimuthal direction[1] , [2] , and this impedance is shown most likely
to be an inductance [3] .. However, unified quantitative
analysis of this monitor is required on various
characteristics, and now in progress.
References[1] J.Komatsu, " Study of a wall-current monitor forelectron beams", private communication (1994)[2] J.Komatsu et al., "Dependence of the wall-current
monitor on the beam position", Proc. of 1994
Int. Unac Conf., Tsukuba (1994) 848-850[3] A.Asami et al., "Beam monitors for electron accelerators", Proc. 6th Workshop on Topics in Nuclear
Radiation Detection, Hiroshima Univ., 1995
[ 4] K.Tamiya et al., " Various characteristics of thewall-current monitor for electron beams ", Proc.
20th Linac Meeting in Japan, Osaka, (1995) 257259[5] R.T.Avery et al.," Non-intercepting monitor ofbeam current and position", IEEE Trans. Nucl.
Sci. NS-18 (3) (1971) 920-922

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Fig.8 Output signals vs. beam position; the parameter is the frequency-259-