Kazuyoshi Koyama

Numerically optimized structures for dielectric asymmetric dual-grating laser accelerators

Optical scale dielectric structures are promising candidates to realize future compact, low cost particle accelerators, since they can sustain high acceleration gradients in the range of GeV/m. Here, we present numerical simulation results for a dielectric asymmetric dual-grating accelerator. It was found that the asymmetric dual-grating structures can efficiently modify the laser field to synchronize it with relativistic electrons, therefore increasing the average acceleration gradient by ∼10% in comparison to symmetric structures. The optimum pillar height which was determined by simulation agrees well with that estimated analytically. The effect of the initial kinetic energy of injected electrons on the acceleration gradient is also discussed. Finally, the required laser parameters were calculated analytically and a suitable laser is proposed as energy source.

Parameter study of a laser-driven dielectric accelerator for radiobiology research

A parameter study for a transmission grating type laser-driven dielectric accelerator (TG-LDA) was performed. The optimum pulse laser width was concluded to be 2 ps from the restrictions on the optical damage threshold intensity and the nonlinear optical effects such as the self-phase modulation and self-focus. An irradiation intensity of (2 GV m−1) was suitable for a silica TG-LDA with a pulse width range from 1 ps to 10 ps. The higher order harmonics of the axial electric field distribution was capable of accelerating electrons provided that the electron speed approximately satisfies the conditions of , or . The electrons at the initial energy of 20 kV are accelerated by an acceleration field strength of 20 MV m−1, and the electrons were accelerated by higher fields as the speed increased. For relativistic energy electrons,the acceleration gradient was .

Cracks measurement using fiber-phased array laser ultrasound generation

A phased array laser ultrasound generation system by using fiber optic delivery and a custom-designed focusing objective lens has been developed for crack inspection. The enhancement of crack tip diffraction by using phased array laser ultrasound is simulated with finite element method and validated by experiment. A non-contact and non-destructive measurement of inner-surface cracks by time-of-flight diffraction method using fiber-phased array laser ultrasound generation and electromagnetic acoustic transducer detection has been studied.

A study of internal defect testing with the laser-EMAT ultrasonic method

This paper studies the ultrasonic detection and evaluation of internal volume defects in metals using laser generation and electromagnetic acoustic transducer (EMAT) detection. A finite element model is developed to simulate the interaction of laser-generated ultrasonic waves with the defect in the material. Not only have the directly scattered shear waves been observed, but also the mode-converted creeping waves on the defect surface. A noncontact laser-EMAT ultrasonic testing experimental system was successfully applied to validate the observed phenomena in the simulation results. The defect can not only be detected and located by the directly scattered shear waves, but can also be quickly evaluated with a new method based on quantitative time-of-flight analysis of the directly scattered waves and the mode-converted waves on the defect surface.

Laser-driven dielectric electron accelerator for radiobiology researches

In order to estimate the health risk associated with a low dose radiation, the fundamental process of the radiation effects in a living cell must be understood. It is desired that an electron bunch or photon pulse precisely knock a cell nucleus and DNA. The required electron energy and electronic charge of the bunch are several tens keV to 1 MeV and 0.1 fC to 1 fC, respectively. The smaller beam size than micron is better for the precise observation. Since the laser-driven dielectric electron accelerator seems to suite for the compact micro-beam source, a phase-modulation-masked-type laser-driven dielectric accelerator was studied. Although the preliminary analysis made a conclusion that a grating period and an electron speed must satisfy the matching condition of LG/λ = v/c, a deformation of a wavefront in a pillar of the grating relaxed the matching condition and enabled the slow electron to be accelerated. The simulation results by using the free FDTD code, Meep, showed that the low energy electron of 20 keV felt the acceleration field strength of 20 MV/m and gradually felt higher field as the speed was increased. Finally the ultra relativistic electron felt the field strength of 600 MV/m. The Meep code also showed that a length of the accelerator to get energy of 1 MeV was 3.8 mm, the required laser power and energy were 11 GW and 350 mJ, respectively. Restrictions on the laser was eased by adopting sequential laser pulses. If the accelerator is illuminated by sequential N pulses, the pulse power, pulse width and the pulse energy are reduced to 1/N, 1/N and 1/N2, respectively. The required laser power per pulse is estimated to be 2.2 GW when ten pairs of sequential laser pulse is irradiated.

Numerical simulation of phased-array laser ultrasound and its application for defect inspection

Inthispaper, anumerical code isdeveloped for theanalysis of phased-array laserultrasound. Thesignal enhancement of ultrasonic waves induced by a multi-beam laser source in phased array is simulated by using finite element method (FEM). The use of phased-array laser ultrasound for crack inspection with shadow method is investigated by simulation method.

Advanced Accelerators for Medical Applications

We review advanced accelerators for medical applications with respect to the following key technologies: (i) higher RF electron linear accelerator (hereafter “linac”); (ii) optimization of alignment for the proton linac, cyclotron and synchrotron; (iii) superconducting magnet; (iv) laser technology. Advanced accelerators for medical applications are categorized into two groups. The first group consists of compact medical linacs with high RF, cyclotrons and synchrotrons downsized by optimization of alignment and superconducting magnets. The second group comprises laser-based acceleration systems aimed of medical applications in the future. Laser plasma electron/ion accelerating systems for cancer therapy and laser dielectric accelerating systems for radiation biology are mentioned. Since the second group has important potential for a compact system, the current status of the established energy and intensity and of the required stability are given.

Resonant enhancement of accelerating gradient with silicon dual-grating structure for dielectric laser acceleration of subrelativistic electrons

We show that the accelerating gradient of a dual-grating structure for dielectric laser acceleration of subrelativistic electrons can be enhanced by resonating with the zeroth diffraction order inside the channel. We analyze diffraction of light at a subwavelength grating (SWG) to illustrate the principle of the resonant enhancement. We present examples of dual-grating resonators for 50 keV electrons with different channel widths. The dependence of reflectivity and phase on the SWG dimensions provides flexibility in controlling the enhancement factor and filling time, thus enabling high-gradient acceleration driven by ultrashort low-power laser pulses.

Grating-based waveguides for dielectric laser acceleration

We propose a chip-scale hollow-core waveguide using high-contrast gratings as reflectors for dielectric laser acceleration. We show that confinement of a specified accelerating mode can be achieved by adjusting the thickness of a matching layer between the core and the highly reflective grating. Several examples of the grating-based waveguide and their characteristic parameters such as the group velocity, interaction impedance, and acceleration efficiency are presented. The planar structure of the waveguide makes fabrication and integration simple, which is required by an on-chip dielectric laser accelerator for high-energy, material, and medical applications.We propose a chip-scale hollow-core waveguide using high-contrast gratings as reflectors for dielectric laser acceleration. We show that confinement of a specified accelerating mode can be achieved by adjusting the thickness of a matching layer between the core and the highly reflective grating. Several examples of the grating-based waveguide and their characteristic parameters such as the group velocity, interaction impedance, and acceleration efficiency are presented. The planar structure of the waveguide makes fabrication and integration simple, which is required by an on-chip dielectric laser accelerator for high-energy, material, and medical applications.

Development of a Fibre-Phased Array Laser-EMAT Ultrasonic System for Defect Inspection

In this work, a phased array laser ultrasound system with using fibre optic delivery and a custom-designed focusing objective lens has been developed for enhancing the ultrasound generation. The fibre-phased array method is applied to improve the sensitivity and detecting ability of the laser-EMAT system for defect inspection.