The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

BEAM LOSS MONITOR SYSTEM FOR THE JAERI SUPERCONDUCTING RF LINAC-BASED FEL
E. J. Minehara, E. Tanaka, M. Sugimoto, M. Sawamura, R. Nagai, N. Kikuzawa and N. Nishimori
Free Electron Laser Laboratory,Department of Reactor Engineering, Tokai Research Establishment, Japan Atomic Energy Research Institute
2-4 Shirakata-shirane, Tokai, Naka, Ibaraki, 319-11 JAPAN
Abstract
In order to reduce substantially high radiationlevel of X- and gamma-rays at the JAERI FEL facility, we
have developed a beam cun:ent loss monitoring systembeing distributed along the beam line of the JAERISuperconducting rf Linac. Preliminary and experimental
results showed the reduction of about a few orders ofmagnitudes in the JAERI FEL experimental hall. Design
options of the photon detectors and monitoring ways arereported briefly.

a beam pulse duration, and the fall time an amplifier time
constant. In the case of the short pulse duration orgamma ray source, the rise time is the same with thecircuit rise time, and less than 1J.!S. The pulse is taken
about 2 meters after the JAERI superconducting rf-linac
based FEL driver along the beam line. The monitor was
fixed at the position of about one and half meter from thestainle:ss-steel beam duct, and heavily-shielded with several
50mm-thick Pb blocks on the floor. In the Fig.4, 2.5ms
and 30 J.!S duration components are shown in a typicalloss monitor pulse under beam acceleration. The long is
the same with an rf pulse length of the linac, the shortmacro pulse. The long comes from the X-rays being
originated from the field-emitted electron bombardment,
the short the electron beam loss.

In order to decrease substantially the hard X- andgamma-rays radiation level at the JAERI FEL facility, we
have developed a beam current loss monitoring systembeing distributed along the beam line of the JAERI
superconducting rf linac. Usually, we first try to
maximize a transmission of an electron beam through therf linac by adjusting beam optical elements, and bymonitoring the beam current. Once we succeed to transport
the electron beam with about a full transmission, and wecan not use a current monitor system to reduce a smaller
amount of the beam loss than a few%. In order to reducethe beam loss smaller than 0.01% or less, we have to use
a more sensitive monitor for the current Joss than thecurrent monitor.
500K
Preliminary and experimental results showed theOHM
reduction of about one and half orders of magnitudes in theJAERI FEL experimental hall. Plastic, Nai(TI) crystaland Glass scintillators have been used to reduce theradiation level. In order to monitor the radiation level
induced by an electron beam loss, we have developed anew semiconductor radiation monitor using a Si-photo
diode 1), Csl or BGO scintillators and a charge-sensitive

Loss Monitor Circuit Diagram AOUT PUT
1.5PICOFARAD

soOHM

PULSEhybrid amplifie~). In a heavy radiation circumference, weGENERATER
used to remove the scintillation crystal or glass to reducethe monitor sensitivity, and reduce the bias voltage.
Fig.l: A circuit diagram of the Si-photo diode Joss
Designmonitor.
options of the photon detectors and monitoring ways will
be discussed. In the Figs.land 2, a circuit diagram and aRefere111ces
photograph of the Si-photo diodeloss monitor is1) Hamamatsu Photonics Co. "Si PIN Photo diodeschematically illustrated. In the Fig.3, a typical pulse
S3590-05".
shape from the Joss monitor prototype of Si-photo diode 2) Hamamatsu Photonics Co. "a charge-sensitive hybrid
is shown to have a rise time of 30J.!s and a fall time ofamplifier H4083".
about one hundred J.!S. The rise time is the same with-444-

The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

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Fig.4: Long and short duration components in the
typical Joss monitor output under beam acceleration. Thelong one is 2.5ms long, the short 30~-Ls long. The longcomes from the X-rays being originated from the fieldemitted electron bombardment during the RF macro pulse,
A photograph of the Si-photo diode loss monitor. the short the electron beam loss.

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Fig.3: A typical pulse shape from a loss monitor
prototype of Si-photo diode is shown, and has a rise timeof 30~-Ls and a fall time of about a few hundreds I!S.

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