Yong-Woon Choi

Low-Temperature Operation of a Bulk HTSC Staggered Array Undulator

A use of bulk high-temperature superconductors (HTSs) for an undulator is attractive since a high magnetic field can be generated at low-temperatures. While potential for generation of the high magnetic field is high, in-situ magnetization of the bulk HTSs for periodic field generation is challenging issue. Recently, we proposed a new type of undulator using bulk high-Tc superconductors (HTS) and a solenoid magnet. The undulator, named Bulk HTSC staggered array undulator (Bulk HTSC SAU), consists of a stacked array of bulk HTSs and copper insulators and a solenoid magnet. A proof of principle experiment at 77 K using liquid nitrogen has been carried out. The estimated performance at about 30 K was estimated using results of property measurements for the HTS used for the Bulk HTSC SAU. The expected undulator peak field reaches to 1.08 T for undulator period length of 9.9 mm for the undulator gap of 4.0 mm. This performance is about 2 times higher than that of existing technologies.

Analysis of SNIP Algorithm for Background Estimation in Spectra Measured with LaBr3: Ce Detectors

LaBr3:Ce scintillating detectors exhibit excellent properties for γ-ray spectroscopy such as high energy resolution and operation under room temperature as well as MHz counting rates. On the other hand, sever background radiations exist due to the internal contamination of radioactive materials that are very difficult to be avoided during the manufacture. To decrease the effect of these background levels, some analytical techniques, e.g. background subtraction, should be applied. In the present work, we investigate the efficiency of the sensitive nonlinear iterative clipping peak (SNIP) method for background estimation and subtraction. Optimization of the clipping window is discussed for range of energy up to 3 MeV. Enhancement of energy resolution up to 50% was obtained.

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.

Beam Stabilization by Using BPM in KU-FEL

A Mid-Infrared Free Electron Laser (MIR-FEL) facility has been constructed for developing energy materials in Institute of Advanced Energy (IAE), Kyoto University. Since high brightness electron beams are crucial for the FELs, stabilization of the electron beam (beam energy, beam current, bunch spacing and beam trajectory) is very important for a stable FEL operation. For these reasons, the electron beam position which includes the electron beam energy information should be monitored precisely to realize the highly stable electron beam. We have been developing a feedforward and a feedback system using 4-pickup electrode type Beam Position Monitor (BPM) in Kyoto University FEL (KU-FEL) to generate a stable FEL light. A basic design of the BPM readout system has been completed and we have installed BPMs with accuracy of 10 μm for non-destructive measurement in the KU-FEL linear accelerator. BPM signals were observed successfully and a phase stabilized electron beam has been generated by a feedforward system. A feedback system using BPM to stabilize the beam position is under development.