The 9th Symposium on Accelerator Science and Technology, Tsukuba, Japan 1993

A BEAM POSITION MONITOR USING AN AMORPHOUS
MAGNETIC CORE
Toshiaki Kobayashi, Toru Ueda, Yoichi Yoshida, Takahiro Kozawa,
Mitsuru Uesaka and Kenzo Miya
Nuclear Engineering Research Laboratory, Faculty of Engineering, University of Tokyo
22-2, Tokai-mura, Naka-l(un, lbaraki 319-11

Seiichi Tagawa
The Institute of Scientific and Industrial Research, University of Osaka
8-1, Milwgaoka,lbaraki, Osaka 567

Hitoshi Kobayashi
National Laboratory for High Energy Physics
1-1, Oho, Tsukuba, Ibaraki 305

ABSTRACT
A beam position monitor for anelectron accelerator has been developedby using an amorphous magnetic core.
The position is detected by the differenceof leakage inductances of four pickup
coils wound on the amorphous magnetic
core. The accuracy of the beam position
monitor is less than 1mm for the various
electron pulses from nanosecond to
microsecond.

Fig. 1 shows the structure of thebeam position monitor. The monitor iscomposed of an amorphous magnetic
core, the four pickup coils (Sturn),
housing, SMA type connectors and four
50 ohm terminations. This monitor ismounted on the outside of a ceramic
beam duct in the atmosphere. The
pickup coil is 0.3mm diameter wire andits end part is shorted.
AMORPHOUS CORE

INTRODUCTION
For the fine operation of thelinear accelerator, it is important tomeasure the current, the pulse shape andposition of the beam. A beam current
monitor using an amorphous magnetic
core (ACM) for measuring short pulse
beam was been developed in 1987. The
ACM shows fast response and good SIN

SMA CONNECTOR- - t - - - t O - --+---

ratio due to the high permeability of theamorphous magnetic core [1- 4].
The pulse shape from the ACM
does not depend on the position of thebeam, because, the leakage inductance ofthe troidal pickup coil wound on theamorphous core is very small. On theother hand, a pickup coil wound on thesmall region of the core, the leakage
inductance can not be neglected. Theleakage inductance depends on theposition of the beam. By using thiseffect, a new beam position monitor has
been developed.

AMORPHOUS
CORE

PICKUP COIL

Fig. 1 Schematic cross section of beam position monitor

CONSTRUCTION-348-

PRICIPLE
This beam position monitor
picks up the magnetic field generated byelectron beam. Fig. 2 shows theequivalent circuit. The induced voltage,V, at pickup coil for the input voltage ofthe step function is described as follows,

were simulated by an electric analysis
code (MICROCAP-3). The results areshown in Fig. 5. The preliminary
experimental results almost agreed withsimulative results.

V = (4RE I L) t e-2Rt/L •where R is the load resistance, L is theleakage inductance, C is the stray
capacitance, E is the input voltage and t isthe time.
The leakage inductance depends
on the beam position. The rise time of theinduced voltage is especially changed.
Therefore, the beam position can beobtained by analyzing the time-profile ofthe rise shape.
R1

OSCILLOSCOPEFig. 3 Block diagram for test bench of
calibrating beam position monitor

L2RL

Fig. 2 Equivalent circuit of the beam
position monitor
CALIBRATION OF THE MONITOR
AND SIMULATIONFor calibration of the beam
position monitor, a test bench using apulse generator, a single wire, theposition monitor, matching register (50ohm) and ·x-Y stage were constructed.
The test bench is shown in Fig.3. Thepulse. current through the single wire wasmeasured by using a fast current probe(tektronix CT-2, inner diameter I mm)
and the beam position monitor.
Fig. 4 shows the position
dependent response of the monitor.
Upper traces (4a) were detected byadjusting the wire at center position ofthe core. The pulse shapes from the four
pickup coils were quite agreed. Lower
traces (4b) were detected by moving thewire to the position 20mm away from thecenter. The difference of the pulse shapes
from the four pickup coils depended on
the position of the wire. The pulse
shapes of the beam position monitor

Fig. 4 Pulse shape of the beam position monitor
using test bench•-..

Trerelen..............
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s.

3.
~

3.

s z.

§

..
l.l ; -o.

:--//// rr'I II/i 1/
VI v
ljj/
--- --·--A ~-o.:JC-··

I lnH-- r-- -·--Fig. 5 Simulation of the beam position monitor-349-

PERFOR MANCE OF THE POSITIO NMONITOR
The accuracy of the beam position
measurement was measured by using theelectron beam from a 28Me V linear
accelerator [5-6]. The beam pulse width
and the peak current were SOns and60mA, respectiv ely. The position
monitor was assemble d on the vacuum
duct with the accuracy within lmm.
Fig. 6 shows the pulse shapes
from the four pickup coils. Upper traces( 6a) were obtained when the beam
passed the center of the core. The 'pulse
shapes of the four pickup coils wereagreed approxim ately. Also, lower
traces (6b) were obtained when the beam
passed the position 20mm away from thecenter. The difference of the pulse shapes
from each pickup coils depends on theposition of the beam monitor.
As a next step, we are going tostudy the compute r tomography by using
this monitor.

ACKNOWLEDGMENT
The author would like to thankDr. H. Shibata for his suggestion and M.
Yuura (Pulse Electric Engineer ing Co.)in several discussions.
REFERENCES[I] T. Kobayashi, IONICS (1988) I.
[2]T.Kobayashi,OYO-BUTURI,57(1988) 1382.
[3] T. Kobayashi, T. Ueda, Y. Yoshida,
K. Miya, ~- Tagawa, H. Kobayas hi,
Proc. 17th hnear accelerat or meeting inJapan (1992) 210.
[4] T. Kobayashi, T. Ueda, Y. Yoshida,K. Miya, S. Tagawa, H. Kobayas hi,
Nucl. Instr. and Meth., in press.
[5] H. Kobayashi, T. Ueda, T.
Kobayashi, S. Tagawa and Y. Tabata,Nucl. Instr. and Meth. 179(1981) 223.
[6] H.Kobay ashi,T.U eda, T.Kobayashi,S.Tagawa and Y.Tabata, UTNL-R0252,Nucl. Engng. Res. Lab., Faculty
Engng., Univ. Tokyo, (1990) 38.

Fig. 6 Pulse shape of the beam position monitor
using a 28MeV linear accelerator
CONCLU SIONA beam position monitor for anelectron accelerat or has been developedby using an amorpho us magnetic core.
The accuracy of the beam position
monitor is less than lmm for the various
electron pulses of which pulse width
were from nanosecond to microsecond.

-350-