Kensuke Okumura

Experimental demonstration of mode-selective phonon excitation of 6H-SiC by a mid-infrared laser with anti-Stokes Raman scattering spectroscopy

Mode-selective phonon excitation by a mid-infrared laser (MIR-FEL) is demonstrated via anti-Stokes Raman scattering measurements of 6H-silicon carbide (SiC). Irradiation of SiC with MIR-FEL and a Nd-YAG laser at 14 K produced a peak where the Raman shift corresponds to a photon energy of 119 meV (10.4 μm). This phenomenon is induced by mode-selective phonon excitation through the irradiation of MIR-FEL, whose photon energy corresponds to the photon-absorption of a particular phonon mode.

Beam test of the coaxial cavity for the thermionic triode type RF gun

The KU-FEL (Kyoto University Free Electron Laser) facility uses an S-band 4,5 cell thermionic RF gun as an electron source. The gun is strongly suffering from back-bombarding effect. In order to mitigate the energy of back streaming electrons the used RF gun shall be modified to triode type. For this reason a RF driven pre-bunching small coaxial cavity should be attached to the main 4,5 cell gun. The coaxial cavity was designed and fabricated. We report here results of a hot test of the coaxial cavity.

Optimization of the New Designed FEL Beam Transport Line

A mid-infrared free electron laser (MIR-FEL) (target wavelength: 5 ~ 20 μm) facility named KU-FEL (Kyoto University Free Electron Laser) was constructed to aid various energy science researches at the Institute of Advanced Energy, Kyoto University. In December 2011, KU-FEL was upgraded by replacing its undulator and optical cavity mirrors. By this upgrade, the tunable range of KU-FEL was improved to 5.5 ~ 15 μm. According to replacing the cavity mirrors, size and divergence of FEL beam at the emitting point was changed. Therefore, we designed and constructed a new MIR-FEL transport line. By using calculation code Zemax (http://www.zemax.com), the condition for keeping the FEL beam radius less than 25 mm during the transportation length of 24 m was determined. In addition, the FEL intensity profile was measured after passing through the constructed transport line. The beam waist at the out-coupling hole was calculated from the measured FEL intensity profile. By using the beam waist calculated from the measurement, we confirmed the validity of the calculated optimum condition.

Development of a field measurement system for the Bulk HTSC SAU

To realize a short-period strong-field undulator, we proposed a high temperature superconducting bulk staggered array undulator (Bulk HTSC SAU) and proceeded proof of principle experiments and numerical studies. We have succeeded to generate periodic transverse magnetic fields whose strength was controlled by an external solenoid field. At the same time, we revealed a problem; at both ends of undulator, field distribution is substantially distorted. We proposed several approaches of field correction. To verify the effectiveness of these field correction methods, it is necessary to measure the magnetic field distribution precisely, not only inside of the undulator but also both ends. For this purpose, we developed a rotary measurement system to measure the magnetic field distribution at the end of the undulator. Multiple Hall sensors are placed on a circuit board at equal intervals from the centre of the board. By rotating and moving the board, the probe can measure axial field in 3D space on the undulator ends. In this paper, we deliver specifics of the system.

Design Study for Direction Variable Compton Scattering Gamma Ray

A monochromatic gamma ray beam is attractive for isotope-specific material/medical imaging or non-destructive inspection. A laser Compton scattering (LCS) gamma ray source which is based on the backward Compton scattering of laser light on high-energy electrons can generate energy variable quasi-monochromatic gamma ray. Due to the principle of the LCS gamma ray, the direction of the gamma beam is limited to the direction of the high-energy electrons. Then the target object is placed on the beam axis, and is usually moved if spatial scanning is required. In this work, we proposed an electron beam transport system consisting of four bending magnets which can stick the collision point and control the electron beam direction, and a laser system consisting of a spheroidal mirror and a parabolic mirror which can also stick the collision point. Then the collision point can be placed on one focus of the spheroid. Thus gamma ray direction and collision angle between the electron beam and the laser beam can be easily controlled. As the results, travelling direction of the LCS gamma ray can be controlled under the limitation of the beam transport system, energy of the gamma ray can be controlled by controlling incident angle of the colliding beams, and energy spread can be controlled by changing the divergence of the laser beam.