Shoji Suemine

Development of subpicosecond pulse radiolysis system

The highest time resolution of the pulse radiolysis had remained about 30 ps since the late 1960s. To make clear the primary processes in the radiation chemistry and physics within 30 ps, we developed a stroboscopic pulse radiolysis system for the absorption spectroscopy with the time resolution of 2.0 ps (10}90% rise time). The time resolution of 2.0 ps was estimated from the time-dependent behavior of the hydrated electrons. The system consists of a subpicosecond electron linac as an irradiation source, a femtosecond laser as an analyzing light and a jitter compensation system. ( 2000 Elsevier Science B.V. All rights reserved.

Single-bunch and multibunch electron beam generation for FEL experiments with the 38 MeV L-band linear accelerator at ISIR

Abstract The generation of multibunch electron beams used for FEL oscillation experiments with the 38 MeV L-band linear accelerator at The Institute of Scientific and Industrial Research has been studied. Originally, the components of the accelerator system have been optimized for generating a high-brightness single-bunch beam. The operational conditions of the system have been investigated for a multibunch beam. A multibunch beam with a macropulse length of 4 μs has been accelerated at energies of 17 to 19 MeV. The latter part of the pulsed beam with a length of 1.8 μs has an energy spread of 1.8%. The charge per bunch is 2 nC. The beam of this part has been successfully used for oscillation experiments at a wavelength of 32 to 40 μm.

Free-electron laser oscillation with a multibunch electron beam of the 38 MeV L-band linear accelerator at ISIR

Abstract Free-electron laser oscillation has been achieved with a multibunch electron beam generated with the 38 MeV L-band linear accelerator at The Institute of Scientific and Industrial Research in Osaka University. The wavelengths of the observed FEL are 32 to 40 μm for energies of the electron beams of 17 to 19 MeV. The macropulse length of the beam is 1.8 μs. The net FEL gain measured at a wavelength of 40 μm is 25%. The tuning range of the length of the optical cavity and the FEL spectra have also been measured. The total output FEL energy of 12 mJ has been obtained in the recent experiments at a wavelength of 40μm. The intracavity peak power of the FEL is estimated to be about 150 MW.

Amplification of spontaneous emission with two high-brightness electron bunches of the ISIR linac

Abstract Recently, self-amplified spontaneous emission (SASE) has been observed at wavelengths of 20 and 40 μm with high-brightness single-bunch electron beams of the 38 MeV L-band linear accelerator (linac) at The Institute of Scientific and Industrial Research in Osaka University. A study has been made to generate two electron bunches for amplifying the SASE with another electron bunch under an oscillator configuration at a wavelength of 40 μm. A grid pulser of the electron gun has been developed for generating two electron bunches at the interval corresponding to the round-trip time of an optical cavity. The total charge of the electrons and the energy spread of the beam accelerated at an energy of 17.1 MeV are 40 nC and 2% (FWHM), respectively. After further optimization of the conditions of the microwave supplied to each component of the accelerator system, experiments of two-bunch amplification will be performed.

Characteristic measurements of higher harmonics generated in the SASE-FEL process

The nonlinear harmonic generation of self-amplified spontaneous emission (SASE) has been studied in the far-infrared region. Angular distributions of the second harmonic and the third harmonic as well as the fundamental of SASE have been measured. The third harmonic radiation is emitted on the electron beam axis, while the second harmonic radiation is emitted slightly off-axis in the horizontal direction. It seems that there is competition in development of radiation between the fundamental and the other higher harmonics.

Upgrade of the ISIR-FEL at Osaka University and oscillation experiments in the sub-millimeter wavelength region

Abstract The far-infrared free electron laser (FEL) at the Institute of Scientific and Industrial Research (ISIR), Osaka University has been upgraded to extend the wavelength region further to the long wavelength side. The present modification includes replacement of the optical resonator and installation of an evacuated optical transport line from the FEL to the measurement room as well as installation of a far-infrared monochromator for measuring the wavelength of FEL light. We recently started experiments using the new FEL system. The loss of light in the optical resonator was measured and it is experimentally confirmed that the diffraction loss is reduced considerably after the upgrade of the optical resonator.

Bunch-Shape Monitor for a Picosecond Single-Bunch Beam of a 35 MeV Electron Linear Accelerator

A non-interactive-type bunch-shape and beam intensity monitor for a 35 MeV electron linear accelerator (linac) has been developed. The monitor consists of an electric SMA-type connector and an Al pipe of 50 mm inner diameter. Test measurements of the present monitor have been made under the conditions of the accelerated charges of lower than 27 nC/pulse and the pulse width ranging from 6 to 30 ps (Full Width at Half Maximum). The results show that the present monitor is applicable to bunch-shape measurement of the picosecond single-bunch beam. The monitor output is also found to be proportional to the beam intensity of more than 0.05 nC/pulse.

Response of a wireline pickup for subnanosecond electron beam pulses

The response of a wireline-type beam monitor was measured for electron beams with energies of 28 MeV and pulsewidths of 20 ps from an electron linear accelerator. The output waveform from the wireline pickup was calculated according to the general formulation for responses of beam monitors based on electromagnetic interaction. In the calculations, a transfer function H(/spl omega/) and an effective length l/sub eff/ are introduced to express all the effects not included in the analysis. The function H(/spl omega/) is assumed to be Gaussian, and the FWHM of H(/spl omega/) and l/sub eff/ is experimentally determined. The effects of the termination, wireline length, height, and angle between the wireline and the beam direction are discussed. The test supported the analysis and, for the standard pickup, a charge sensitivity of 20 V/nC and an FWHM of H(/spl omega/) of 55 ps are obtained.

Measurement of the time profile of a single-bunch electron beam with wireline pickup by considering the wake-field effect

Abstract The effect of a wake-field generated by an electron beam passing through a linac beam exit window was studied experimentally, to establish the response of a beam monitor with a wireline pickup. To suppress the effect of the wake-field, a copper disk with a beam entrance-hole was placed in front of the pickup. A discussion of the frequency domain response for this monitor was also made, to corroborate the data of suppression of the effect. A considerable improvement in the time domain response was observed for a single-bunch electron beam from the linac at ISIR of Osaka University. A waveform with 50 ps rise time and 150 ps FWHM was obtained.

High-power solid-state switch using static induction thyristors for the klystron modurator

We have developed a solid-state switch with static induction thyristors (SI-thyristors) for the klystron modulator of the L-band electron linac at the Institute of Scientific and Industrial Research, Osaka University. The switch is designed to meet the specifications that the maximum voltage is 25 kV and the maximum current is 5 kA at a repetition rate of 10 pps, so that it consists of six parallel and ten series circuits of 60 SI thyrsitors. The turn-on time of the switch is measured to be 270 ns. It is retrofitted in the modulator to generate 1.3 GHz RF pulses of the maximum peak power of 30 MW for a duration of 4 jis or 25 MW for 8 μs. The linac is successfully operated with the switch under the specified conditions, which demonstrates that it can be used as a high-power switch for the modulator. The pulse-to-pulse variation of the klystron voltage is measured to be 0.015% though it is close to the noise level of our measurement system. Variations of the RF power and the phase - as well as the variation of the klystron voltage measured with the solid state switch are significantly smaller than those obtained with a thyratron, and hence the stability of the main RF system is remarkably improved. The solid-state switch has been used in normal operation of the linac for one year or more than 2,000 hours without any serious troubles. Thus we have confirmed its robustness and long-term reliability.