Takeo Nakashima

Two-dimensional mode analysis of the Raman-type free-electron laser

The effects of an arbitrary beam thickness and a conducting wall in the Raman‐type free‐electron laser are investigated. To simplify the problem, a two‐dimensional model of a solid nonmagnetized relativistic electron beam enclosed with a parallel plate waveguide is considered. With the aid of the fluid theory for electron beams, the coupled mode equation relating the scattered wave (TE mode) and the electron plasma wave (TM mode) under the influence of the pump wave (TE mode) is derived. By using the solution to the coupled mode equation, together with the boundary conditions on the beam surface, the dispersion relation and the spatial growth rate for the coupled scattered and electron plasma waves are found. From detailed numerical analysis for the properties of the scattered wave, several interesting results are obtained. First, in order to get an appreciable magnitude of the growth rate, the beam width and the separation between the conducting walls of a parallel plate waveguide are required to be cons...

Resonant-ultrasound spectroscopy for studying annealing effect on elastic constant of thin film

In this paper, we study the elastic property of thin films using resonant-ultrasound spectroscopy (RUS). RUS determines the elastic constants of a solid from its resonance frequencies of free vibration. There were two problems to be solved for applying RUS to thin films: accurate measurement of the resonance frequencies and mode identification of each resonance frequency. We solve these problems using the tripod needle transducers and the laser-Doppler interferometry (LDI). In this paper, we describe the RUS/LDI measurement setup we developed, and show the relationship between the elastic constant and annealing temperature for Cu thin films. Then, we discuss the effects of recrystallization and recovery on the elastic constant referring the X-ray diffraction measurement.

Fast Recovery of Elasticity of Cu Thin Films at Room Temperature Studied by Resonant-Ultrasound Spectroscopy

In this study, we investigate the behavior of the room-temperature recovery of elastic constants of Cu thin films by monitoring the mechanical resonance frequencies of the film/substrate specimen throughout the deposition process using resonant-ultrasound spectroscopy. The thin films were deposited on a (001) Si substrate located on a needle tripod transducer set in a vacuum chamber. The free-vibration resonance frequencies of the Cu/Si specimen were measured before, during, and after magnetron-sputtering deposition, yielding the evolution of the elastic constants of the Cu thin films. Recovery of the elastic constants of Cu thin films is completed in a much shorter time than that for bulk Cu; after the deposition, the elastic constants C11 and C66 increased by 1.0–2.4% and 2.2–5.3%, respectively, within 20 min. The recovery behaviors are well explained by the dislocation-damping theory. The recovery rate is larger than that of bulk Cu by a factor of 100, indicating much lower activation energies for point defect migration and dislocation sliding in the film.

Optimum beam thickness for the Raman‐type free‐electron laser using a two‐dimensional hollow beam

The effect of an arbitrary beam thickness is investigated for a two‐dimensional Raman‐type free‐electron laser composed of a hollow relativistic electron beam and a parallel plate waveguide containing it. On the basis of the fluid theory for electron beams, analytical expressions are given for the dispersion relation and the spatial growth rate of a scattered wave (even TE mode) and an electron plasma wave (even TM mode) coupled under the influence of a pump wave (even TE mode). Then, from the numerical analysis for an important case where both the pump and scattered waves are the lowest‐order even TE mode, the following interesting results are obtained. First, the hollow beam can propagate two distinct modes of the electron plasma wave with even symmetry characterized by different field distributions. One mode (mode 1) corresponds to the normal mode of the solid beam while the other mode is inherent to the hollow beam. In addition, the growth rate for the mode 1 is considerably larger than that for the m...

Fast Recovery of Elastic Stiffness in Ag Thin Film Studied by Resonant-Ultrasound Spectroscopy

We studied the recovery of the elastic constant of sputtered Ag films on monocrystal Si substrates by monitoring the resonance frequency of film/substrate specimens throughout the deposition process using resonant-ultrasound spectroscopy. An Ag film is deposited on a (001) Si substrate, which is located on the tripod transducer set in the sputtering chamber. The free-vibration resonance frequency of the Ag/Si specimen is measured before, during, and after the magnetron sputtering deposition, inducing the evolution of the elastic constants of the deposited film. Recovery of the elastic constants of the Ag film is completed within 40 min. The recovery behavior of the elastic constants is similar to that of residual stress; the recovery rate is comparable to that for residual stress.

Fast recovery of elastic constant in thin films studied by resonant-ultrasound spectroscopy

This paper reports incredibly large and rapid evolution of elastic constants in deposited copper and silver films observed by the resonant-ultrasound spectroscopy. The evolution begins just after stopping the deposition with the temperature dependent recovery rate. To explain the mechanism, we propose a model, where the elastic constants at grain boundary regions increase by 67% at least. Diffusion of atoms along the grain boundary region is a possible reason, and we confirm that the activation energy is much smaller than that for grain-boundary diffusion in bulk materials. These results are explained by drastic structure change at grain boundaries, being similar to phase transition from liquid into solid phase.