Masafumi Yorozu

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.

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.

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.

Laser microprobe and resonant laser ablation for depth profile measurements of hydrogen isotope atoms contained in graphite

We measured the depth profile of hydrogen atoms in graphite by laser microprobing combined with resonant laser ablation. Deuterium-implanted graphite was employed for the measurements. The sample was ablated by a tunable laser with a wavelength corresponding to the resonant wavelength of 1S-2S of deuterium with two-photon excitation. The ablated deuterium was ionized by a 2 + 1 resonant ionization process. The ions were analyzed by a time-of-flight mass spectrometer. The deuterium ions were detected clearly with the resonant ablation. The detection limit was estimated to be less than 10(16) atoms/cm(3) in our experiments. We determined the depth profile by considering the etching profile and the etching rate. The depth profile agreed well with Monte Carlo simulations to within a precision of 23 mum for the center position and 4-mum precision for distributions for three different implantation depths.

Two-Dimensional Measurements of X-ray Spectra Generated by Laser-Compton Scattering

Measurements of X-ray spectra generated by laser-Compton scattering are described. Basically, laser-Compton X-rays present distinctive characteristics, such as good-directivity, tunability, and a quasi-monochromatic X-ray spectrum. However, the actual X-ray spectrum is degraded by the effects of the broad spectrum of the laser pulse, and by the intrinsic spatial divergence of the electron bunch defined by the beam emittance. It is important to measure the spectral properties of laser-Compton X-rays for applications involving protein crystallography or mammography. The energy distribution in the forward direction can be observed using a front-illuminated deep-depletion X-ray charge-coupled device (CCD) detector. The spectral width of the measured X-rays within a detection angle of 4 mrad is estimated to be 5.0% (rms) for a CCD resolution of 2.6% (rms). These results can be accurately explained using a simple relationship between the electrons and the laser beam parameters.

Status of laser-Compton x-ray generation project for FESTA

FESTA has been developing short pulse X-ray generation technologies using Laser-Compton scattering. Two years ago, X-ray generation with a 90-degree collision configuration was achieved using a photocathode as the electron source and a 100fs Ti:sapphire laser as the photon source. Electron and laser light pulses were synchronized so that the fluctuation of X-ray intensity was 25% (rms). In the next stage, the x-ray generation system is being modified for use in practical applications. The electron energy has been raised to 40 MeV to increase to X-ray photon energy. Laser power will be increased to 0.5 J/pulse with a 100Hz repetition rate. The laser gain medium has been changed to Yb:S-FAP which is pumped by laser diodes. The synchronization system will be further modified to increase its stability. These technologies will be used to generate 15 -to 30 keV X-ray pulses with a stable peak-to-peak intensity for use in several applications.

Performance of compact pulse radiolysis system using a photocathode RF gun

A compact pulse radiolysis system using a photocathode RF gun was installed at Sumitomo Heavy Industries. Some performance tests were conducted concerning the electron beam and the laser pulse. The energy and the per-pulse charge of the electron beam were measured to be 1.75 MeV max. and 1 nC. The fluctuation of the charge was restricted within 2%. The pulse widths of the electron pulse and the analyzing laser pulse were 20 ps and 15 ps, respectively. The timing jitter between electron pulse and the laser pulse was ±5.7 ps. Based on these measurements, the all-over time resolution can deduced to be about 25 ps.

Laser synchrotron femtosecond x-ray source for novel imaging

A description is given in this paper on the present progress of a compact laser synchrotron femtosecond x-ray source based on the inverse Compton scattering of high energy femtosecond laser pulses by high energy electrons. The present research program is reviewed by the target energy and number of the femtosecond x-ray photons for phase 1 (1996 - 2000) and phase II (2001 - 2004). Possible examples are considered for ultrafast imaging and pump- probe experiments by using this x-ray source.

All-solid-state picosecond laser system for photocathode RF gun

A compact diode-pumped picosecond laser was developed for the illumination of the photocathode of RF guns. Nd:YLF and Nd:YAG were selected as the active medium and the laser stability was the main interest during the development. Achieved performances were 0.5 ps timing jitter in the oscillator, 1% W pulse energy fluctuation and 5 /spl mu/rad beam pointing stability.