C. Yamanaka

Characterization of extreme ultraviolet emission from laser-produced spherical tin plasma generated with multiple laser beams

Spherical solid tin targets were illuminated uniformly with twelve beams from the Gekko XII laser system to create spherical plasmas, and the extreme ultraviolet (EUV) emission spectra from the plasmas were measured. The highest conversion efficiency of 3% to 13.5nm EUV light in 2% bandwidth was attained for an irradiance of around 5×1010W∕cm2. The experimental results were reproduced fairly well using a theoretical model taking the power balance in the plasma into consideration.

Nd-doped phosphate glass laser systems for laser-fusion research

Nd:phosphate laser glasses are evaluated in this paper as the active medium for high-power and high-energy laser systems for laser-fusion research. Small-signal gains and parasitic limits of rod amplifiers and disk amplifiers are first shown. Then system performances at a short-pulse, high-power regime and at a long-pulse high-energy regime are investigated. At a short-pulse regime, very high output power of 3.4 terawatts (TW) has been obtained because of the large stimulated emission cross section and the small nonlinear refractive index. At a long-pulse regime, we found that the effective saturation fluence of the phosphate glass LHG-8 is 4.0 J/cm2and we can efficiently extract the energy stored in the laser glass. This lead to the conclusion that Nd: phosphate glasses are very suitable for short-pulse as well as for long-pulse amplifications. In this paper, system descriptions and hardware specifications of the glass laser systems at the Institute of Laser Engineering (ILE), Osaka, University, are also briefly given.

Improvement of The Bulk Laser Damage Threshold of Potassium Dihydrogen Phosphate Crystals by Ultraviolet Irradiation

Potassium dihydrogen phosphate (KDP) crystals were grown under the irradiation of ultraviolet light. The bulk laser damage threshold was improved to two to three times (15–20 J/cm2) compared to the case of crystals grown by conventional methods. Microbes such as germs and bacteria are frequently generated in the KDP solution with the usual growth method. The ultraviolet light reduces or eliminates organic materials such as microbes or their carcasses incorporated into the crystal, which are the cause of low damage threshold.

Radiation conversion and related ablation behavior of a gold‐foil target irradiated by 0.35, 0.53, 1.06, and 10.6 μm lasers

Ultra‐soft x‐ray emission from a gold‐foil target was studied by irradiating with four different wavelength lasers. The experimental data were analyzed under the blackbody approximation to evaluate the radiation temperature and the radiation conversion efficiency as functions of laser intensity and wavelength. The results show that the shorter‐wavelength laser leads to a remarkable conversion of absorbed energy into radiation. The ablation behavior due to radiation and related energy transport in a gold foil is also studied.

Soft x-ray emission from ω0, 2ω0, and 4ω0 laser-produced plasmas

The soft x‐ray spectra from Au plasmas in 0.1–1.6 keV range and x‐ray conversion efficiencies of Au (high Z) and Al (low Z) targets with three laser wavelengths of 0.26, 0.53, and 1.05 μm are measured. In addition, the intensity dependence of the x‐ray conversion efficiency at 0.26 μm is obtained for Au targets. For 0.26‐μm laser light, the conversion efficiency of Au targets can be as high as 80% at an intensity of 2.4×1013 W/cm2.

Strategy for finding new materials for ESR dosimeters

The right strategy for finding a new ESR dosimetric material sensitive to radiation is to follow the orthodox procedures used in the development of thermoluminescence dosimeters (TLD) and phosphorescence studies. Modern procedures used in materials sciences, such as computer calculation of molecular orbitals (MO), should be employed to estimate the ESR and optical properties of prospective materials. Radiation effects in lithium and magnesium sulfates and metal salts of organic acids, such as lithium and magnesium lactates, have been investigated in search for tissue-equivalent dosimeter with a large G value.

Experimental determination of fuel density‐radius product of inertial confinement fusion targets using secondary nuclear fusion reactions

The first demonstration of a fuel density‐radius product measurement using secondary nuclear fusion reactions is presented. This technique involves using neutrons and protons generated by DT {T(d,n)α} and D3He {3He(d,p)α} fusion reactions, respectively, in a pure deuterium fuel.

Laboratory simulation of space weathering: Changes of optical properties and TEM/ESR confirmation of nanophase metallic iron

Abstract Reflectance spectra of S-type asteroids are different from those of ordinary chondrites. This spectral mismatch is explained by space weathering processes, where high-velocity dust particle impacts change the optical properties of the uppermost regolith surface of asteroids. S-type asteroids exhibit more overall depletion and reddening of spectra, and more weakening of absorption bands relative to ordinary chondrites. Nanophase metallic iron particles, which are formed through vapor deposition from dust impact heating, are considered as the most essential cause of space weathering. In this study, we describe the spectral changes of olivine and pyroxene using nanosecond pulse laser irradiation and the presence of nanophase metallic iron particles in laser-irradiated materials by transmission electron microscopy (TEM) and electron spin resonance (ESR). The irradiated spectra of the samples show a reduction of the overall spectra (250–2600 nm) and a reddening with weakened absorption bands. Nanophase metallic iron particles were found not only in laser-irradiated olivine samples, but also in laser-irradiated pyroxene samples by TEM. Strong ESR signals, which derive from nanophase iron particles, are observed in the irradiated olivine samples. Moreover, ESR intensities increase with the space weathering degree simulated as laser irradiation time. One possible application of space weathering is the estimation of the relative age of asteroids using the relation between optical effects and quantities of produced nanophase iron particles.

Improvement of the bulk laser damage threshold of potassium dihydrogen phosphate crystals by reducing the organic impurities in growth solution

We obtained damage‐resistant crystals of potassium dihydrogen phosphate (KDP) by reducing organic impurities in the growth solution. To prevent organic impurity exuded from the wall of the growth vessel, Pyrex glass was used. The residual organic impurities, which mainly derived from KDP raw powder, were reduced by photochemical dissociation using a combination of ultraviolet light and an oxidant (hydrogen peroxide). The resultant total organic carbon of the solution was less than 0.1 ppm. The damage threshold of the crystals grown from this solution was always high (20–22 J/cm2), which is about three times higher than that of crystals grown by conventional methods.

Magnetically insulated and inertially confined fusion — MICF

By combining the benefits of magnetic and inertial fusion, a new fusion scheme is proposed. A plasma with a density of ?1021 cm?3 is confined by the inertia of a heavy, cannonball-type metallic shell; its heat is insulated by a self-generated magnetic field of ?100 T. The plasma and the magnetic field are produced by ablation due to direct impact of a laser (or particle) beam on solid fuel which constitutes the coating of the inner surface of the spherical metallic shell. Preliminary experimental and simulation results, using a 100 J CO2 laser on a target of a few millimetre parylene shell, gave n? ? 5 ? 1012 cm?3?S, with T ? 500 eV. A 1-D spherical hydrodynamic code, HISHO, with the radial heat conductivity reduced by an assumed magnetic field of 103 T, leads to ignition at an absorbed energy of the order of 20 MJ deposited during a confinement time of approximately 100 ns. These results provide supporting evidence for the feasibility of the scheme as a realistic reactor.