Fumio Sakai

Low-emittance electron-beam generation with laser pulse shaping in photocathode radio-frequency gun

A technique of laser-pulse shaping was developed for low-emittance electron-beam generation in a photocathode radio-frequency (rf) gun. The emittance growth due to space charge and rf effects in the rf gun was experimentally investigated with square and gaussian temporal pulse shapes. It was found that the square pulse shaping was a useful tool for both the reduction of nonlinear space-charge force and the correction of linear space charge. The normalized transverse rms emittance at 1 nC was obtained to be 1.20 πmm-mrad for the square pulse shape with pulse length of 9 ps full width at half maximum. The emittance was measured as a function of the electron bunch charge and the laser-pulse length.

In-line phase-contrast imaging of a biological specimen using a compact laser-Compton scattering-based x-ray source

Laser-Compton scattering (LCS) x-ray sources have recently attracted much attention for their potential use at local medical facilities because they can produce ultrashort pulsed, high-brilliance, and quasimonochromatic hard x rays with a small source size. The feasibility of in-line phase-contrast imaging for a “thick” biological specimens of rat lumbar vertebrae using the developed compact LCS-X in AIST was investigated for the promotion of clinical imaging. In the higher-quality images, anatomical details of the spinous processes of the vertebrae are more clearly observable than with conventional absorption radiography. The results demonstrate that phase-contrast radiography can be performed using LCS-X.

Quantum Efficiencies of Mg Photocathode under Illumination with 3rd and 4th Harmonics Nd: LiYF4 Laser Light in RF Gun

A Mg photocathode for use in a radio-frequency gun was manufactured by a technique of hot isostatic pressing with diamond polishing and tested under a peak electric field of 57 MV/m. A laser cleaning process for the Mg cathode was developed. The quantum efficiency (QE) of the Mg cathode before and after the laser cleaning was measured under illumination with 262 nm and 349 nm laser light. A high QE of up to 1.0×10-3 was achieved under the 262 nm laser-light illumination. The QE of the Mg cathode under the 349 nm laser-light illumination was measured to be 2.2×10-5. The dependence of the QE on the electric field was measured and compared with a theoretical analysis including the Schottky effect.

Short-Pulse X-Ray Generation via Thomson Scattering in 0° and 90° Interactions

Short-pulse X-ray generation was demonstrated in a scattering experiment between a 4 ps electron beam and a 10 ps Nd:YLF laser beam. The beams interacted at 0° and 90°. The X-ray signal was measured using a microchannel plate. Maximum X-ray energy was analytically estimated to be 3.55 keV in the 0° interaction and 1.77 keV in the 90° interaction with a pulse duration of 4 ps for both interactions. Measured X-ray intensities exhibited good linearity with the incident laser light pulse energy for both scattering configurations. The measured X-ray intensity was compared with the intensity analyzed from the beam sizes and energies of both beams. The intensities matched and were of the order of 104 photons in the 0° interaction.

Experimental studies of emittance growth and energy spread in a photocathode RF gun

Abstract In this paper we report on a low emittance electron source, based on a photocathode RF gun, a solenoid magnet and a subsequent linac. The dependencies of the beam transverse emittance and relative energy spread with respect to the laser injection phase of the radio-frequency (RF) gun, the RF phase of the linac and the bunch charge were investigated experimentally. It was found that a lower beam emittance is observed when the laser injection phase in the RF gun is low. The emittance increases almost linearly with the bunch charge under a constant solenoid magnetic field. The corrected relative energy spread of the beam is not strongly dependent on the bunch charge. Finally, an optimal normalized rms transverse emittance of 1.91±0.28 πmm mrad at a bunch charge of 0.6 nC was obtained when the RF gun was driven by a picosecond Nd:YAG laser. A corrected relative rms energy spread of 0.2–0.25% at a bunch charge of 0.3– 2 nC was obtained after the beam was accelerated to 14 MeV by the subsequent linac.

INJECTOR STUDY FOR COMPACT HARD X-RAY SOURCE VIA LASER COMPTON SCATTERING

Compact hard X-ray source via laser Compton sattering has been developed in SHI and AIST. Our system has the injector and the linac and the high power laser system. The injector has a photo-cathode rf gun with a solenoid magnet. To enhance the X-ray yeild, we are planning to increase electron beam charge up to 5 nC/bunch and to make multi-bunch beam. The beam tracking simulation in the injector have been performed by changing laser spot size, laser pulse width, rf phase and solenoid field to optimize the distance between the injector and the linac for 5 nC/bunch high charge beam. In addition, high charge multi-bunch beam simulation in rf-gun cavity have been carried out to investigate the influence by the beam loading and the wake field.

Hybrid time-resolved spectroscopic system for evaluating laser material using a table-top-sized, low-jitter, 3-MeV picosecond electron-beam source with a photocathode

Hybrid time-resolved spectroscopy of laser media comparing electron-beam excitation and optically excited cases is performed using a newly developed, table-top-sized, low-jitter, 3-MeV picosecond electron-beam source with a photocathode. The properties of an electron-beam-pumped Ce3+:LiCaAlF6 (Ce:LiCAF) ultraviolet laser medium significant differ from those of an optically pumped medium.

Non-destructive inspection system for special nuclear material using inertial electrostatic confinement fusion neutrons and Laser Compton Scattering Gamma-rays

A study of a non-destructive inspection system for hidden special nuclear materials in the cargo container at the sea port has been carried out under a promotion of Japan Science and Technology. This inspection system consists of an active neutron detection method for a fast screening purpose and a nuclear resonance fluorescence (NRF) method for isotope identification. The inertial electrostatic confinement fusion device has been developed for a neutron source and two neutron detection methods, the Feynman-alpha method and high energy neutron detection method, have been examined to realize the fast screening system. Generation of a quasi-monochromatic gamma-ray beam from the laser Compton Backscattering by using a compact microtron electron accelerator and an NRF experiment on uranium target using a new type of scintillation detector, LaBr3(Ce), has been studied to realize the isotope identification system.

Development of high-brightness hard x-ray source by Laser-Compton scattering

Laser-Compton scattering is a promising method for generating high-brightness, ultrashort, energy-tunable X-rays. We have developed a compact X-ray source using laser-Compton scattering. Hard X-rays, ranging from 15 keV to 34 keV, were generated with a low-emittance, 38 MeV, 0.8 nC electron accelerator and a femtosecond 4TW Ti:sapphire laser. The created X-rays were composed of 2×106 (5×105) photons/pulse for interaction angles between an electron bunch and a laser pulse of 165° (90°). A highly accurate timing synchronization scheme was employed, and the fluctuation of the generated X-rays was suppressed to 11% (rms) for the 90° scattering. The spatial (angular) distributions for the intensity and the energy of the generated X-ray were measured, and agreed well with theoretical calculations. Thus, X-ray imaging has been demonstrated using a phase-contrast technique with the interference of an X-ray beam.

Stable High-Brightness Electron Beam System with a Photocathode RF Gun for Short Pulse X-Ray Generation by Thomson Scattering

A high-brightness electron accelerator system with a photocathode RF gun and an all-solid stable laser for the photocathode was installed, and a commissioning test was performed to generate short-pulse X-ray beams by the Thomson scattering method. Electron energy was boosted by a linear accelerator (linac) up to 14 MeV. Energy dispersion of the electron beams was measured to be 0.7% (rms). The normalized emittance of the electron beam was 4 πmm-mrad with a 0.4 nC bunch charge. The electron beam size at the interaction point, where the electron beams and high peak power laser light interacted, was measured to be 100 µm (rms). Good stability in the spatial and temporal domains was also obtained.