Status of Coherent Radiation Beamline at KURRI-LINAC
T. Takahashi
Research Reactor Institute, Kyoto University, Japan
In the electron linear accelerator at ResearchReactor Institute in Kyoto University (KURRILINAC), properties of several types of coherent
radiation (synchrotron radiation [1], transition
radiation [2], Cherenkov radiation [3], diffraction
radiation, and Smith-Purcell radiation [4], prebunched FEL [5]) in the THz-wave and millimeterwave regions have been experimentally investigatedsince 1991. The beamline for the millimeter-wave
spectroscopy has been constructed [6], in whichcoherent transition radiation (CTR) has been used asa light source and the spectroscopic research of N2O
gas [6] and electron spin resonance using a pulsed
magnet [7] have been demonstrated.
The KURRI-LINAC consists of an injector,
pre-buncher, two accelerator tubes. The specifications
are represented in Table. 1.
Table 1. Specifications of KURRI-LINAC
Operation Mode
Short Pulse
Long Pulse
RF frequency
1300 MHz
Energy46 MeV
30 MeV
Pulse Width2～100 ns
0.1～4 µs
Repetition Rate
1～300 Hz
1～200 Hz
Peak Current
8A500 mA
Beam Power
Max. 10 kWThis linac has been operated for collaboration
research programs with other universities since 1964
and research activities have covered the nuclear datawith pulsed neutron source, electron irradiation, andcoherent radiation. The time of operation in 2006 wasabout 2,700 hr. The distributed user-time to theresearch of coherent radiation is about ten weeks peryear. The schematic diagram of the beamline isshown in Fig. 1.
Ti window

Al foilelectronbeam

from
L-band Linac

The forward coherent transition radiation emitted
from a Ti window and the backward one from an
Al-foil are used as a light source. The beamline isequipped with a grating-type monochromator and a
Martin-Puplett type interferometer in series. Whenthe interferometer is used as a spectrometer thegrating in the monochromator is replaced with a flat
mirror. Three types of detectors are prepared, i.e. aliquid-helium-cooled Si bolometer (Infrared Lab.), a
hot-electron InSb bolometer (Infrared Lab.), and amillimeter-wave diode-type detector (DXP-10,Millitec) according to the sensitivity and the response
time. The degree of instability of the observed
intensity is within 2%.
Coherent radiation is especially useful as a picosecond pulsed light source for the time-resolved
spectroscopy and the pulseradiolysis study. Since theaccelerating frequency of this linac is 1.3 GHz(L-band), the interval between pulses in the CTR
pulse train is 770 ps (23 cm). The spectrum from thesuccessive bunches is constituted of the higher
harmonics of 1.3 GHz [6]. As a result, the CTR canbe treated as a light source with a continuous
spectrum only when the spectral resolution is lower
than 1/23 = 0.0434 cm-1 and the delay in thetime-resolved measurement is restricted within 770 ps.
In order to lift these restrictions, the single-bunch
beam has been generated by installing a high-speed
avalanche-type pulser in the electron injector. The
degree of impurity of single bunch was estimated tobe 1.5% by means of the cross-correlationinterferogram.
The following researches in the millimeter-wave
region are in progress at this beamline:a) Optical conductivity of superionic conductors(collaboration research),b) Optical properties of water (collaborationresearch),c) Optical properties of polymeric materials underirradiation (collaboration research),d) Development of pulseradiolysis system.

Accelerator Room
Shield WallExperimental Room
MonochromatorInterferometer

Fig. 1. The schematic diagram of the coherent
radiation beamline.

[1] Y. Shibata, et al., Phys. Rev. A 44 (1991) R3449.
[2] Y. Shibata, et al., Phys. Rev. A 45 (1992) R8340.
[3] T. Takahashi, et al., Phys. Rev. E 50 (1994) 4041.
[4] Y. Shibata, et al., Phys. Rev. E 57 (1998) 1061.
[5] Y. Shibata, et al., NIM 528 (2004) 162.
[6] T. Takahshi, et al., Rev. Sci. Instrum. 69 (1998)3770.
[7] Y. H . Matsuda, et al., Physica B 346-347(2004) 519-523.