Sho Amano

A gamma ray generation facility was developed through laser Compton scattering on an electron beam storage ring, for the purpose of researching nuclear transmutation. Theoretical analysis on gamma ray yield and energy spectrum was performed, as well as EGS4 simulation code was employed to simulate the interaction of gamma ray and Ge detector. The acquired experimental results are in agreement with theory.

Nondiffracting laser beam-J(0) Bessel beam, is suggested to take the place of conventional Gaussian laser beam in a laser Compton scattering system, in order to increase the flux of scattered photons through maintaining an efficient interaction of electron beam and laser beam in a long distance. A novel cavity is proposed to produce and store the J(0) Bessel beam based on our present laser Compton scattering experimental setup for gamma-ray generation, and the propagation features and intensity distribution of the J(0) Bessel beam inside the cavity are analyzed. The flux of Compton scattering gamma-ray is estimated theoretically and the results reveal that a significant growth are accomplished with the use of J(0) Bessel laser beam contrasting with the use of Gaussian laser beam.

We are exploring to process nuclear waste through nuclear transmutation method with gamma ray irradiation. The gamma ray employed for transmutation is generated in the way of Compton backscattering. Preliminary experiment is being developed on the NewSUBARU storage ring providing an electron beam with energy of 1 GeV. and 17.6MeV gamma ray will be generated through a head-on collision of the electron beam and a 1.064 mum laser light. gamma ray flux about the present experiment is predicted theoretically. A planned scheme for promoting the flux is also introduced, as well as the maximum laser power available for the storage ring is discussed.

A new-type of laser-excited RF-gun using a needle photocathode is proposed. Design studies of the gun performances are executed with numerical calculations for electric field of the tip of the needle photocathode, laser heating and Joule heating of the tip, and beam parameters of a photoelectron beam in the RF-gun cavity. This calculation assumes a 2856 MHz RF-gun system that has the maximum field of 40 MV/m and a mode-locked pulsed-laser at a wavelength of 351 nm with a micropulse duration of 10 ps. Calculation results show that, for the optimized tip radius of 8.8 μm, a peak photocurrent reaches up to a space-charge limited current of ${\sim}90$ A with a pulse width of ${\sim}6$ ps and an energy spread of ${\sim}1$% at the cavity exit using an optimized laser with a peak intensity of 330 MW/cm2 and a laser injection phase of 20 deg. A preliminary experiment of a needle-RF-gun is also performed.

The propagation characteristics of a beam diffracted by a circular aperture are investigated. The beam-quality factor M-2 defined by an 86.5% power-content radius is given theoretically and experimentally as a function of the truncation ratio. It is found that the theoretical limit of M-2 is 2.37 times as great as that of an incident beam as the truncation ratio approaches 0. For a weakly diffracted beam a simple formula giving M-2 is derived. Although M-2 does not increase much with diffraction, the influence of diffraction should be taken into account in beam brightness. (C) 2002 Optical Society of America.

The influence of flash lamp characteristics on the performance of a high-power slab laser system is studied in high-repetition-rate operations. The pulse duration of the flash lamp was found to be lengthened at high repetition rates due to the decrease in the dynamic impedance of the lamp. It decreased the output efficiency of the PC-MOPA (phase conjugated-master oscillator power amplifier) system that we developed. The system nevertheless resulted in an average power of 235 W by Q-switched pulses at 320 pps. (C) 2002 Published by Elsevier Science B.V.

A femtosecond synchrotron radiation pulse train can be extracted from an electron storage ring by interaction between an ultrashort laser pulse and an electron beam in an undulator. Generation system of a femtosecond soft x-ray pulse by the slicing technique was studied with numerical calculations for its performance, as applicable for the NewSUBARU synchrotron radiation facility at LASTI. The femtosecond electron pulse, that is energy-modulated with a Ti:sapphire laser at a pulse energy of 100 μJ, a pulse width of 150 fs, and repetition frequency of 20 kHz, can be sufficiently separated in a bending magnet. A femtosecond soft x-ray pulse (the critical photon energy of 0.69 keV and a pulse width of 250 fs) is obtained with a collimator (diameter of 800 μmφ), and it has an average brightness 3 × 10⁶ photons/s/mm²/mrad²/0.1 %BW and an average photon flux 10⁵ photons/s/0.1 %BW.

The dependence of quantum efficiency on a high electric field was investigated using a tungsten needle photocathode irradiated by a 3rd-harmonics Nd:YAG laser whose photon energies were lower than the work function of tungsten. The tungsten needles were fabricated by an electrolytic etching technique. The tip radii could be controlled between 0.1 and 1 μm by changing the etching conditions. The obtained quantum efficiency of the needle tip is found to be proportional to the >10th power of the electric over 500 MV/m, and it reached up to 3% at about 800 MV/m. This observed field-enhancement of quantum efficiency is qualitatively explained with a field-emission process including the Schottky effect and photo-excitation.

The design considerations of a confocal multipath laser cavity to enhance Compton backscattering are presented. Ray trace simulation results predicted that 29-14-fold enhancement of the laser field fan be achieved at the confocal point by superposition of mode locked laser pulses. As a result, Campton backscattered X-rays, generated by interaction of on intense laser field with a relativistic electron beam, will be enhanced efficiently by a factor of more than 10.

A high average and high peak brightness Nd:YAG MOPA laser system composed of a laser-diode-pumped Nd:YAG master oscillator, flash-lamp-pumped slab power amplifiers and a phase-conjugated mirror was developed. The system demonstrates an average output power of 235 W at a repetition rate of 320 Hz and a peak power of 30 MW at a pulse duration of 24 ns with M-2 = 1.5. Both an average brightness of 7 x 10(9) W/cm(2).sr and a peak brightness of 1 x 10(15) W/cm(2) sr are achieved simultaneously. The system design rules that we confirmed suggest that by replacing lamp pumping in the amplifier with laser-diode pumping, an average output power of similar to1 kW can be obtained at similar to1 kHZ with a higher average brightness of similar to3 X 10(10) W/cm(2).sr and a higher peak brightness of similar to3 X 10(15) W/cm(2).sr.

Image relaying in a high-average-power slab laser system, especially one with thermal lenses, was studied.Ray-trace analysis using an ABCD matrix was conducted for a multi-cascade relaying system. The analysisclarified quantitatively the beam expansion, the focal shift and the power loss by thermal lenses that should betaken into account in the system design. An Nd: YAG slab laser system was developed, and its beam wasrelayed successfully by careful design, taking the above considerations into account. The laser system demonstratedan average power of 235 W with a high beam quality.

High repetition rate (320 pps), high average power (100 W) pulse Nd:YAG slab laser has been developed for x-ray driver. A strong X-ray emission around 10.8 nm, which wavelength is attractive for x-ray projection lithography, was generated from cryogenic Xe targets pumped by this laser pulses. CO2 and H2O were also tested for cryogenic targets and we found that the emission intensity from Xe around this wavelength region was at least 10 times higher than that from H2O, CO2. (C) 1999 Elsevier Science S.A. Ail rights reserved.

Coherent x-rays are generated not only by recent x-ray lasers but also by conventional x-ray sources. Parts of high bright x-ray from synchrotron radiation are one possible source of coherent x-rays for application use. Possibilities and characteristics of the synchrotron radiation as a coherent x-ray source are presented. As an example, we calculated a coherence of soft x-ray on the NewSUBARU, 1.5 GeV synchrotron radiation facility at Himeji Institute of Technology. The calculated results show that a longitudinal coherent length of 2 mu m and a coherent power of 4 mW for a long undulator (lambda=13 nm, N=200).

The aims of the New SUBARU project are to promote industrial applications in the VUV and soft X-ray region and to develop research and development towards new light sources. The main facility of the New SUBARU project is the 1.5 GeV electron storage ring which is under construction at the SPring-8 site in Harima Science Garden City, Japan. The storage ring is quasi-isochronous and has variable momentum dispersion for the deep study of beam dynamics in very short bunches.

The main facility of the New SUBARU project is the 1.5 GeV electron storage ring which is under construction at the SPring-8 site in Harima Science Garden City. The first beam commissioning from the SPring-8 LINAC is scheduled at the end of March, 1998. The aim of new SUBARU is to promote industrial activities in the local area (Hyogo prefecture) and to develop experimental investigations for new light sources.