Toichi Okada

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.

Molecular design of an epoxy for cryogenic temperatures

The mechanical and thermal properties of several epoxy resins were measured to obtain guidelines for the molecular design of an epoxy resin for cryogenic temperatures. Two types of epoxy resin with different numbers of epoxy groups were mixed (with different mixing ratios) and cured. Fracture toughness, flexural strength and thermal contraction of the hybrid epoxy resins were measured down to cryogenic temperatures. The results suggest that epoxies with larger molecular weights between crosslinkings relaxed stress at the crack tip, even at cryogenic temperatures. Intermolecular forces and stress relaxation at the crack tip were found to be important for high fracture toughness.

Thermal Strain of Pipes Composed with High Strength Polyethylene Fiber Reinforced Plastics at Cryogenic Temperatures

High strength polyethylene fiber (Dyneema® fiber; hereinafter abbreviated to DF) has a large negative thermal expansion coefficient. Several kinds of pipes were prepared by means of filament winding or sheet winding method. The thermal strain or residual stress of those pipes were measured at liquid nitrogen temperature. The thermal strain was also calculated and was compared with the measured values. The circumferential thermal strain of the inner surface was found to be much different from that of outer surface. The circumferential strain changed with the ratio of inner diameter to thickness of pipes. The mean thermal strain of inner and outer surface was found to agree well with that of calculated value. It was confirmed that the negative thermal expansion can be realized even in the pipes. The design methodology of the pipes with negative thermal expansion was discussed.

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.

Tensile strain/transverse compressive stress dependence of critical current in Ag-sheathed Bi(2212) 7-core superconducting wires☆

Abstract The mechanical properties and tensile strain/transverse compressive stress dependence of the critical current I c in Ag-sheathed Bi(2212) 7-core superconducting wires of different constructions were evaluated at a temperature of 4.2 K and a magnetic field of 14 T. The increase in the volume fraction of the superconductor shifted the stress-strain curves upwards. The tensile strain for I c degradation onset was in the range 0.14–0.27%. This strain depends upon the residual compressive strain and the thickness of the superconductor layer. On the other hand, the transverse compressive stress for I c degradation onset was in the range 9–20 MPa. Wires with a thin Ag-sheath layer showed a remarkably low stress for I c degradation onset. There was no welldefined correlation between the onset stress for degradation of I c caused by the tensile stress and that caused by the transverse compressive stress.

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.

Evaluation of Epoxy Resin by Positron Annihilation for Cryogenic Use

The physical properties of commercially available epoxy resins for cryogenic use have been measured and were compared with the free volume evaluated by positron annihilation. The macroscopic physical properties of epoxy resin were changed by adding the plasticizer or changing the molecular weight between crosslinks. The life time of ortho-positronium were used to evaluate the free volume. The crosslinking density were changed systematically by changing the amount of plasticizer. The lifetime of ortho-positronium were used to evaluate the free volume in the epoxy resins. The different free volume dependence of macroscopic physical properties, such as fracture toughness, were found between the two systems even if the glass transition temperature was almost same. The fracture toughness of the two systems also showed the different crosslinking density dependence at liquid helium temperature. This means that the positron annihilation spectroscopy could be applicable to the molecular design of the glassy polymer epoxy resins for cryogenic use.

Low temperature irradiation effects on mechanical properties of epoxy used in superconducting magnets

Abstract Superconducting magnets for fusion devices are greatly influenced by the radiation environment and the principal radiation-induced troubles arise from changes in the mechanical fracture behaviour of organic materials used as insulating and/or potting materials. Small compression test specimens of epoxy resin were tested after gamma and pile irradiation at low temperatures. Three kinds of mechanical properties, ie a — breaking stress, b — breaking strain and c — elastic moduli were investigated. A statistical analysis of the data from a large number of small specimens revealedt the following: 1. the different effects of gamma and neutron irradiation on the fracture behaviour were observed. This fact suggests that a simulation study by gamma irradiation is not valid for neutron irradiation. 2. in the case of reactor irradiation overall softening was induced accompanied by local hardening and embrittlement. A qualtitative explanation of the above results is presented and their relationship to the performance of the magnets is given.

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.

Research and development of thermal shield support under radiation environment in large helical device

Abstract The thermal shield support used under radiation environments for Large Helical Device (LHD) has been developed considering the thermal properties and the radiation resistance. The ceramic materials were chosen as the candidate. After the screening was made considering the thermal conductivity, mechanical properties and thermal expansion coefficient at room temperature, the zirconia was chosen as the support. The thermal conductivity down to 10K was measured and the thermal penetration through the zirconia support was calculated comparing that with the stainless steel support. It was found that approximately 1 5 of heat penetration to 80K and 1 4 of that to 4.2K shield will be realized using zirconia support compared with using the stainless steel support. The machinability of the zirconia was also examined and found that the bolt shaped shield support was confirmed fabricable. It was concluded that the zirconia cryogenic support is the most possible candidate for thermal shield under radiation environments in LHD.