Tatsuhiro Hori

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.

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.

Gamma-ray irradiation effects on the optical properties of bulk ZnO single crystals

Hydrothermal-grown bulk ZnO single crystals are investigated before and after gamma-ray irradiation. The irradiation does not alter the optical transparency in the visible region. The gamma rays only induce modified near-band-edge UV emission with blue-shifted peaks and shortened response times. From the initial values before irradiation, the peaks shift by 5 to 6 nm, and the response times shorten by 140 to 440 ps. We attribute these observations to the radiation-induced defects on the bulk crystals. Our results nevertheless lead to the realization of short-wavelength ZnO scintillators that can be utilized in high-energy-radiation environments.

Spatial Resolution Evaluation of ZnO Scintillator as an In-situ Imaging Device in EUV Region

We captured single shot images of ZnO emission patterns originating from excitation with the EUV laser at Japan Atomic Energy Agency (JAEA). The EUV laser was focused to a spot size of 1 μm on the ZnO crystal by a Fresnel Zone Plate (FZP). The FZP was shifted along the propagation direction of the EUV beam, essentially changing the spot size at the ZnO surface. The emission pattern was detected by a magnifier. The waist radii were determined from the fitting curve as 5.0 μm along the horizontal axis and 4.7 μm along the vertical axis. We also evaluated the spatial resolution of the magnifier, which consists of a Schwarzschild mirror, two lenses and a camera lens to be about 4 μm. These results suggest that the ZnO crystal has sub-micron spatial resolution when used as an in-situ imaging device for the diagnostics of EUV/X-ray sources.

Optical characterization of Nd 3+ :LiCaAlF 6 in the vacuum ultraviolet region at low temperature

Solid-state vacuum ultraviolet (VUV) lasers are currently being developed because these are more robust, easier to maintain, and more cost-efficient compared to conventional light sources such as excimer lasers, synchrotron radiation, and free electron lasers. Fluorides are excellent solid-state laser host materials for this purpose because of their wide bandgaps, making them transparent down to nearly 100 nm. Moreover, interconfigurational 4fⁿ⁻¹ to 4fⁿ transitions in rare-earth-doped fluorides result to broad fluorescence bandwidths. Previous work has shown that LiCaAlF<inf>6</inf> is an excellent laser host when doped with Ce³+ ions [1]. However, emission from Ce³⁺ activator ions cannot extend below 265 nm. In this work, we investigate the optical properties of Nd³⁺-doped LiCaAlFe (Nd:LiCAF). Nd³⁺-doping results to emissions in the VUV region.