Kunihiko Tsumori

High-Current Single Bunch Electron Linear Accelerator

An injector of an electron linear accelerator has been modified at ISIR of Osaka University in order to increase a single bunch charge from 14 nC to 60 nC. A 6th subharmonic prebuncher has been replaced with two 12th subharmonic prebunchers and a 6th subharmonic prebuncher which are newly constructed. A one-dimensional disk model has been used to calculate the bunching of the beam and to decide the optimum location of the subharmonic prebunchers. The subharmonic prebunchers are immersed in a solenoidal magnetic field so that the electron beam is confined during the travel through the drift region. The single bunch of 16 - 20 ps duration and up to 67 nC in charge, with the energy spread of 0.7 - 2.5 % over the range of 24 - 34 MeV, and a repetition rate from a single shot to 720 pps can be obtained. The energy spread depends on the charge and the minimum spread is 0.7 % at 33 nC. The single bunch of 25 - 45 nC in charge is used for the experiments in routine work.

Self-amplified spontaneous emission at wavelengths of 20 and 40 μm from single-bunch electron beams

Abstract The high-brightness single-bunch electron beams of a 38-MeV L-band linac have been used for free-electron laser amplifier experiments to investigate self-amplified spontaneous emission at wavelengths of 20 and 40 μm. At a charge of electrons in a bunch of 28 nC and a pulse length of about 30 ps the intensities of radiation measured have been 5 and 100 times those of the incoherent spontaneous emission estimated at wavelengths of 20 and 40 μm, respectively. The characteristics of the radiation are discussed for developing intense pulsed light-sources.

Fast decay of the visible band electron in e-irradiated crystalline ice at low temperature: The isotope effects and the role of a mobile proton in the decay

The trapped electron which absorbs in the visible region in crystalline ice e−vis has been studied by pulse radiolysis in the low temperature range 6 to 77 K using 2 μs pulses, and above 77 K using pulses of 40 ps to 6 ns width. The half‐life of e−vis in ice around 77 K is unusually short, 8 ns in H2O ice and 120 ns in D2O ice. The decay of e−vis in ice is found to fit Hummel’s empirical equation for the decay by geminate ion recombination in a spur. Several other pieces of evidence indicate that the decay occurs in a spur. Electron tunneling from e−vis to the OD radical does not occur in D2O ice. It is concluded that a proton (H3O+) or a deuteron (D3O+) produced in a spur by the irradiation migrates through ice to react with immobile e−vis in the same spur, and that the half‐life of e−vis is determined by the mobility of the proton or deuteron. The Arrhenius plot of the half‐life in the range 100 to 6 K is nonlinear and shows an activation energy of 20 meV at higher temperatures and much smaller values a...

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.

FEL amplifier and oscillator experiments with the ISIR linac

Abstract The accelerator system and the characteristics of the electron beams of the ISIR (Institute of Scientific and Industrial Research) 38 MeV L-band linear accelerator (linac) of Osaka University have been investigated for developing free electron lasers (FELs). The single-bunch and the multi-bunch beams generated by the linac have been evaluated for application to FEL amplifier and oscillator experiments, respectively. Spontaneous emission from these beams has been measured.

Beam Profile Measurement for Electron Accelerators

New methods have been developed for monitoring the beam profile of electron accelerators. For electron energies above and below 3 MeV, the monitor utilizes the secondary emission from the probe and the partial absorption of the primary beam in it, respectively, and the probe consists of cooper wire or thin aluminum plate. The performance of the monitor was investigated with electron beams of a 15 MeV linear accelerator and a 2 MeV resonance transformer. The advantages of these methods are their simpleness and utility for high power accelerators.

Status of the experiments for an infrared free-electron laser oscillator at ISIR

Abstract The multibunch electron beam of the 38-MeV L-band linac has been used for free-electron laser (FEL) oscillator experiments at ISIR in Osaka University. At a wavelength of 40 μm spontaneous emission has been measured and amplified radiation has been observed for a single passage of the beam in a wiggler. Experiments are being performed to observed oscillation at 40 μm. The will be improved by using a low-emittance gun.

Current Profile Monitor for Use in Scanning Electron Beam Irradiations

A current profile monitor for the scanning electron beam from the accelerator is described. It utilizes the fact that the electrons scattered through large angles by the exit window of the scanner show a profile similar to that of the main beam at the irradiation site. The scattered electrons are collected by small probes arranged in a linear array parallel to the scanning direction, and the beam profile in this direction is displayed by a storage oscilloscope or an X‐Y recorder. This device has been used for electrons of energies 4–16 MeV from a linear accelerator and proved to be useful for continual monitoring with negligible disturbance to the irradiation in progress.

Contribution of Obliquely Scattered Electrons to the Irradiation under the Scanner Window

Directional distribution of electrons has been investigated for the case where they impinge on the sample after passing through the scanner window. A simple theoretical treatment to estimate the fractions of oblique electron flux is given. An experiment has also been made by using electrons of energies 1.5–8 MeV from a Van de Graaff and a linear accelerator. The calculated and the observed results have shown a moderately good agreement, It is to be noted that the contribution of the oblique flux with angle of incidence greater than 5° is about 15–65% under a typical irradiation geometry.