M. Yamanaka

Study of the stability of optical parametric amplification

Numerical simulation and analysis of the output stability of optical parametric amplification process versus pump intensity variations are presented. It is shown that there exists an optimal working point where excellent stability can be achieved by properly balancing relevant system parameters within certain ranges of pump variations.

Radiochemistry and secondary reactions for the diagnostics of laser‐driven fusion plasmas

Radiochemical measurements have been developed for the diagnostics of laser‐driven implosion plasmas. The excellent calibration for neutron‐yield measurement has been done using β‐γ coincidence technique. The multiactivable tracer method has been examined for measuring the pusher areal density by means of a high‐purity germanium detector. The first experimental success of the secondary nuclear fusion reaction method is also demonstrated for the direct measurement of the fuel ρR.

Fuel areal density measurement of laser-imploded targets by use of elastically scattered protons

A new method for the measurement of the density‐radius product (ρR) of laser‐imploded plasmas is presented. Deuterium‐tritium‐hydrogen fuel mixture is used and the spectrum of protons elastically scattered by 14.1 MeV neutrons is measured with nuclear emulsion. The reliability of this method was certified by comparing the inferred ρR value with that from the secondary reaction method in equivalent experimental conditions.

Pellet Implosion and Interaction Studies by Gekko XII Green Laser

The world record for neutron yield (1.25x1012 /shot) has been achieved using the GEKKO XII green laser system. The implosion process is carefully controlled on the basis of experimental data and improved computer simulations. High aspect ratio targets are uniformly imploded with twelve green beams. The multiplexing of shock waves generated with the accelerating pusher by a quasi-tailored pulse was observed at the center of a pellet. The developments of the new diagnostics are presented. Neutron activation is a very important tool for the measurement of implosion parameters.

Direct areal density measurement by activation technique for plastic hollow shell implosion experiments

The fuel areal density ρR measurement of imploded plastic hollow shell targets by use of a neutron activation technique is reported. Silicon was doped as an activatable tracer into a hollow shell of tritium‐contained deuterated polymer(CDTSi). This target was imploded by 0.53 μm Nd:glass laser light and the induced radioactivity of 28 Al was measured with a β‐γ coincidence counting system. The collection efficiency was calibrated by the use of radioactive tracer 24 Na and was verified by comparing the estimated ρR with that simultaneously measured by a knock‐on method.

Thermonuclear burn time and duration in laser-driven high-aspect-ratio targets

Thermonuclear burn time and burn duration in ablatively driven high‐aspect‐ratio targets have been measured. While the observed burn times were in agreement with results of a one‐dimensional hydrodynamic simulation, the observed burn durations were excessively long.

High Density Compression of Hollow-Shell Target by Gekko XII and Laser Fusion Research at Ile, Osaka University

High density compression of over 600 times solid density has been achieved by the laser implosion of a hollow shell pellet of deuterated and tritiated plastics (CDT). This is the highest density of material which has ever been realized in a laboratory and corresponds to the density four times higher than the center of the sun.

Efficient low-energy near-infrared KTiOPO(4) optical parametric converter.

We report a widely tunable near-infrared holosteric KTiOPO(4) optical parametric converter featuring a low pump energy threshold and high conversion efficiency. The converter uses two 10-mm-long crystals in a simple optical parametric oscillator amplif ier configuration and is tunable from 750 to 1040 nm for the signal. The pump energy threshold at 920 nm is 0.44 mJ for the 532-nm 8-ns 50-Hz input. The system produces as much as 0.45 mJ of energy at 920 nm with an external pump-to-signal energy conversion efficiency of 27% for pump inputs from just two times the threshold value. The internal pump-to-signal and idler energy conversion efficiency is estimated to be 58%. The low pump energy requirement of this system makes it compatible with small laser-diode-pumped solid-state lasers.

Gated neutron streak camera with a uranium cathode

A gated neutron streak camera with a uranium oxide cathode has been developed for laser fusion experiments. The energy spectrum of secondary electrons was improved by coating the uranium oxide cathode with thin cesium iodine. The tube design was carried out by computer calculation of electron trajectories. The newly developed streak tube showed a temporal resolution of about 70 ps. This streak camera can be applicable to a neutron yield greater than 1012 with a reasonable signal‐to‐noise ratio by gating a microchannel plate electron multiplier.

Neutron penumbral imaging at Gekko XII (abstract)

14 MeV DT neutrons emitted from a laser‐imploded target have been imaged by using a penumbral aperture camera. This system consists of a 6‐cm‐thick tungsten toroidal aperture, an array of plastic scintillators, an optical fiber coupled two‐stage image intensifier, and a cooled CCD camera. A burn region has been observed with a spatial resolution of ∼35 μm at a neutron yield of 5×1011. The spatial resolution was determined by a pixel resolution, the accuracy of the aperture fabrication, the alignment accuracy of the aperture, the signal‐to‐noise ratio of an encoded image, and the statistical fluctuation in detecting neutrons.