Yasuhiro Okada

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.

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.

Development of a positron trap for the laser cooling experiment of positronium

A cloud of ortho-positronium produced at the room temperature can be cooled down to 0.6 K utilizing a long pulse (180 ns) laser with 243 nm wavelength. We developed a time bunching system of a slow positron beam to synchronize production of positronium atoms to the laser pulse. After a theoretical examination using a Monte Carlo simulation, an experiment was made for confirmation.

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.

Development of a picosecond Nd:YLF amplifier for laser-Compton x-ray generator

A multipass amplifier for a picosecond Nd:YLF laser was designed and developed for the light source of a laser-Compton X-ray generator. The developed amplifier was constructed of a water-cooled Nd:YLF rod (20mm long, 4mm in diameter) and a pair of quasi-CW laser diodes (peak power 1kW) with optical systems for rod pumping. The amplifier was operated as a laser with a generated output energy of 218mJ and slope efficiency of 31% at 10Hz. We then operated the laser as an amplifier with 0.4mJ, at 10ps, 1047nm seed light. The amplified energy was 4mJ in the single-pass configuration and 3OmJ in the double-pass configuration. The experiment result was compared with the theoretical result derived from the Frantz-Nodvik equation. The results agreed well in the lower energy region but diverged in the higher energy region.

Measurement of depth profile of hydrogen isotope atom contained in solid material using resonant laser ablation

The depth profile of hydrogen isotope atoms was measured by using mass spectrometry combined with resonant laser ablation. A graphite sample was implanted with deuterium by a cyclotron and was employed for the measurements. The graphite sample was ablated by a tunable laser which wavelength was corresponding to the resonant wavelength of 1S - 2S for 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 (TOF) mass spectrometer. The deuterium ions were detected very clearly with resonant ablation contrast to non-resonant measurement. The depth profile was determined from the intensity of the mass spectrum and the etching depth. The depth profile was compared to the simulated profiles. Each profile coincided within the error of the numerical simulation.