12. Others

概要

INTRODUCTION: As peak power of light sources in the terahertz (THz) region increase notably, novel appli- cations using THz lights are pioneered to solve various issues. THz lights are no longer just probe lights for in- spections. It has been reported that a THz light beam causes dissolution of a fibrous peptide [1], and it is ex- pected that the THz light beam is applied to a new treat- ment technology. Developments of intense THz light sources will become increasingly important.
　A key technique for increasing power of light pulses is to superimpose the electromagnetic field of the light pulses. We have confirmed that coherent diffraction radi- ation nonlinearly amplifies at resonant frequencies by using periodical diffractors [2]. In order to further devel- op this technique, we have experimented with confining and superimposing coherent radiation generated by a pulse train of electron bunches in a ring-type resonator. Although a resonant light extracted from the ring-type resonator though a coupling hole of a mirror was ob- served, a background level of coherent diffraction radia- tion was high. Therefor, in order to decrease the back- ground, we performed experiments with a thin substrate inserted in the resonator as a coupling device.

EXPERIMENTS: The experiments were performed using an electron beam with the energy of 42 MeV and the macropulse duration of 47 ns in an L-band linac at Kyoto University Institute for Integrated Radiation and Nuclear Science. The repetition frequency of the macro- pulse of the electron beam was 30 Hz. Schematic layout of the ring-type resonator used in the experiments is shown in Fig. 1. The electron beam generated coherent transition radiation (CTR) at two thin polyethylene films vapor-deposited with aluminum when it passed through them. The thickness of the polyethylene film was 6 m. The current of the electron beam passed through the pol- yethylene film was approximately 60 A. These polyeth- ylene films were also used as mirrors constituting the ring resonator. The two CTR beams were confined in the ring resonator composed of four mirrors, which included two parabolic mirrors with the focal length of 508 mm. The length of the resonator was 922 mm, which was four times the interval of the electron bunches. In order to extract the resonate CTR beam from the optical cavity, a ZEONEX substrate with a thickness of 3 mm was insert- ed into the resonator at an angle of 45 degrees with re- spect to the optical axis. The ZEONEX was almost transparent in a frequency range below 1 THz, and its refractive index was 1.53. Although the CER was re- flected on both sides of the substrate, the two reflected beams did not interfere due to the thickness of the sub- strate.

RESULTS: The CTR beam transported to the exper- imental room was measured by a D-band diode detector (Militech Inc., DXP-06). As shown in Fig. 2, several peaks were observed in the macropulse of the measured CTR power but did not have periodicity. Because micro- pulse structure of the CTR beam disappeared in the macropulse, it was considered that the ring-type resonator deviated from the resonance condition. By adjusting po- sition of a stage on which the parabolic mirror was in- stalled, the resonance conditions could have been im- proved. Moreover, it was difficult to detect detailed structure in the macropulse due to the low CTR power. To increase efficiency of CTR extraction from the reso- nator, we consider using a material with a high refractive index as the coupling device. We also plan to use thin coupling device so that the reflected CTR beams on the both sides of the coupling device can be coherent.

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