Kenta Naito

Conceptual Design Studies of a Laser Diode Pumped Solid State Laser System for the Laser Fusion Reactor Driver

We have developed a conceptual designing technique of a laser diode pumped solid state laser system having high peak power, high efficiency, good beam quality, and high repetition rate. Using this technique, we evaluated the feasibility of the systems with several solid state laser materials for the laser fusion reactor driver with 10 MJ blue output, 10% overall efficiency, and 10 Hz repetition rate. The systems with such as HAP4 glass, Nd:SiO2 glass, Nd:Y:CaF2 crystal, and Tm, Ho:YAG crystal are shown to be attractive candidates for the reactor driver while the systems with such as LHG8 glass, Nd:YAG crystal, and Tm:YAG crystal will not be selected as the candidates. For an economical laser fusion reactor driver, the solid state laser materials are required to have the following properties; \hbarω/σ~10 J/cm2 (\hbarω: the laser photon energy, σ: the stimulated emission cross section), a thermal shock parameter of ≥3 W/cm, a nonlinear index of refraction smaller than ~3×10-13 esu, and a fluorescence lifetime of ≥4 ms.

Parametric Studies on the Laser-Diode-Pumped, Thermal-Lensing-Compensated, Mode-Locked, Q-Switched Nd:YAG Laser

The parametric studies of a laser-diode (LD)-pumped, acousto-optic (AO)-mode-locked and Q-switched Nd:YAG laser were carried out. Adequate compensation for the thermal lensing in the longitudinally pumped laser rod was provided by analyzing the spot size in the astigmatically compensated cavity. The width of the mode-locked pulse with 0.5 nJ output energy per pulse was elongated to 910 ps by means of an intracavity etalon. Good stabilities of the output energy (±3%) and the peak power (±5%) were obtained, which are comparable to those of the flash-lamp-pumped mode-locked Q-switched lasers.

Intense-Electron-Beam Transport Through a Short Insulator Beam Guide

We previously proposed [Jpn. J. Appl. Phys. 34 (1995) L520] a new system for intense-electron-beam transport using an insulator guide which covers the whole beam-transport region. In this paper we present a numerical study of intense-electron-beam transport through an insulator guide whose length is short compared to beam-transport length. Positive ions are extracted in a self-regulated manner from the plasma, generated at the insulator surface, by the electron beam space charge. The ions expand over the transport region, and are extracted from the guide region by the electron beam space charge into the outer region which is not covered by the insulator. Consequently, the beam space charge is neutralized even in the outer region, and the beam propagates effectively in this system.

Intense-Electron-Beam Transportation through an Insulator Beam Guide

The purpose of our research is to elucidate the role of plasma located at the insulator surface in intense-electron-beam transportation. Positive ions are extracted from the plasma by the electron beam space charge. The ion extraction is self-regulated by the net space charge of the electron beam. The ions expand over the transport area, and the beam charge is neutralized effectively by the ions. Consequently, the electron beam propagates efficiently through the insulator beam guide. In our study, we discuss the transportation of the intense electron beam based on results of a computer simulation.

Effect of Plasma Generation at Insulator Surface on Transport of Intense Electron Beam through an Insulator Beam Guide.

In our previous paper [Jpn. J. Appl. Phys. 35 (1996) L120] we presented a new feature in an intense electron beam transport system with an insulator guide. This paper presents a numerical study of the effect of plasma generation at the insulator guide surface on the transport of an intense electron beam. The plasma at the insulator surface is generated by the local electric field which is created by the beam electrons. Ion extraction from the plasma is delayed by the plasma generation time. The simulation results indicate that 1) the front of the electron beam pulse is used to generate the plasma, and 2) the electron beam transport efficiency is not totally but only slightly degraded.

A Simple Method for Measurement of Wavelength Chirping of a High-Peak-Power Laser Diode Array during Pulsed Operation

The wavelength chirping of a two-dimensional high-power laser diode array was measured by a newly developed method utilizing an absorption band of Nd:Y3Al5O12 (Nd:YAG). The validity of this method was verified by the time-resolved spectroscopic measurement. It was shown that this method is useful for the quantitative evaluation of the wavelength chirping and other emitting properties of a two-dimensional laser diode array.

Study on transport control of high‐current electron beam

The purpose of our research is to clarify the role of plasma located at the insulator surface in high‐current‐electron‐beam transportation. Positive ions are extracted from the plasma by the electron beam space charge. The ion extraction is self‐regulated by the net space charge of the electron beam. The ions expand over the transport area, and the beam charge is neutralized effectively by the ions. Consequently, the electron beam propagates efficiently through the insulator beam guide. In our study, we discuss the transportation of the high‐current electron beam based on results of a computer simulation.

High power pulsed streamer corona reactor using a coaxial discharge tube for gas treatment applications

We report on our investigations of a pulsed high power streamer corona reader using a simple coaxial discharge tube for industrial gas treatment processes, such as ozone generation and flue gas cleanup. For industrial usages of these processes, higher discharge power density is favorable to achieve higher productivity. As a first step of research aiming at the higher discharge power density, we examined experimentally characteristics of the repetitive pulsed streamer corona discharge using dry air as feed gas. As a result, the dependency that the discharge current decreases with repetition rate and increases with gas flow rate has been found. The maximum discharge power density of about 100 W/l (/spl sim/25 kV, /spl sim/75 A, /spl sim/100 ns, 170 pps, 0.3l) was obtained in this experiment. The gas temperature rise at this condition was about 20/spl sim/25 %. Because it is not so high as to affect gas treatment badly, it will be possible to operate at still higher discharge power density. We also measured generated ozone in this experiment and the lowest ozone energy cost obtained was 9/spl sim/10 Wh/gO/sub 3/, which was lower than that of a commercial silent discharge type ozonizer.

Mechanical-Stress Induced Nd:YAG Active Quarter-Wave Plate

A quarter-wave retardation was obtained by mechanically induced stress in a Nd:YAG laser rod and a laser gain of 1.15 at 1064 nm was obtained by pumping with a quasi-CW 300 W laser-diode array at 808 nm. The laser rod was held in a brass heatsink in which the mechanical stress was induced horizontally by means of screws. The effective quarter-wave area was measured to be 1 mm (vertical)×2 mm (horizontal) in the center of the 4 mm-diameter Nd:YAG rod by means of a newly constructed polarimeter.