Demonstration of a spherical plasma mirror for the counter-propagating kilojoule-class petawatt LFEX laser system.

A counter-propagating laser-beam platform using a spherical plasma mirror was developed for the kilojoule-class petawatt LFEX laser. The temporal and spatial overlaps of the incoming and redirected beams were measured with an optical interferometer and an x-ray pinhole camera. The plasma mirror performance was evaluated by measuring fast electrons, ions, and neutrons generated in the counter-propagating laser interaction with a Cu-doped deuterated film on both sides. The reflectivity and peak intensity were estimated as ∼50% and ∼5 × 1018 W/cm2, respectively. The platform could enable studies of counter-streaming charged particles in high-energy-density plasmas for fundamental and inertial confinement fusion research.

Refractive index measurements of solid deuterium-tritium.

Physical properties of tritium (T) and deuterium (D) have been of great interest as a fuel for nuclear fusion. However, several kinds of the physical properties in a cryogenic environment have not been reported. Optical properties in liquid and solid phases are indispensable for the quality control of the DT fuel. We study the dependence of the refractive index of solid DT on temperature. A dedicated cryogenic system has been developed and forms a transparent solid DT in a prism cell. Refractive index measurements based on Snell's law were conducted. The refractive indexes of solid DT are from 1.1618 ± 0.0002 to 1.1628 ± 0.0002 in the temperature range of 19.40 K to 17.89 K.

Laser astrophysics experiment on the amplification of magnetic fields by shock-induced interfacial instabilities.

Laser experiments are becoming established as tools for astronomical research that complement observations and theoretical modeling. Localized strong magnetic fields have been observed at a shock front of supernova explosions. Experimental confirmation and identification of the physical mechanism for this observation are of great importance in understanding the evolution of the interstellar medium. However, it has been challenging to treat the interaction between hydrodynamic instabilities and an ambient magnetic field in the laboratory. Here, we developed an experimental platform to examine magnetized Richtmyer-Meshkov instability (RMI). The measured growth velocity was consistent with the linear theory, and the magnetic-field amplification was correlated with RMI growth. Our experiment validated the turbulent amplification of magnetic fields associated with the shock-induced interfacial instability in astrophysical conditions. Experimental elucidation of fundamental processes in magnetized plasmas is generally essential in various situations such as fusion plasmas and planetary sciences.

Surface structure on diamond foils generated by spatially nonuniform laser irradiation.

Here we report on the effects of material strength factors on the generation of surface structure due to nonuniform laser irradiation. The influence of material strength on the generation of perturbation on a diamond surface subjected to nonuniform laser irradiation was experimentally investigated. Our previous investigations suggested that stiffer and denser materials reduce surface perturbation due to spatially nonuniform laser irradiation, which was reproduced well by calculations with multi-dimensional hydrodynamic simulation code. In this work, we found that local fractures due to yield strength failure are generated by high degrees of irradiation non-uniformity. A characteristic crack-like surface structure was observed, which was not reproduced by the 2D simulation code calculations at all. The 2D simulations showed that the pressure at the diamond surface locally exceeds the Hugoniot elastic limit due to nonuniform irradiation, implying the potential for development of surface perturbations. We also measured the areal-density distribution of perturbations for single-crystal diamond and diamond with a thin high atomic number (high-Z) coating on its surface. The experimental results imply that the combination of a stiff material and thin high-Z coating can suppress the solid-strength effects caused by large irradiation non-uniformity. The knowledge given here is applicable to inertial confinement fusion target design, laser material processing, and universal problems involving solids and high-energy-density plasmas.

Iodide-Mediated or Iodide-Catalyzed Demethylation and Friedel-Crafts C-H Borylative Cyclization Leading to Thiophene-Fused 1,2-Oxaborine Derivatives.

The first synthesis of dithieno-1,2-oxaborine derivatives was achieved via iodide-mediated or iodide-catalyzed demethylation of 3-methoxy-2,2'-bithiophene and subsequent C-H borylation. A wide variety of thiophene-fused oxaborines could be synthesized by the procedure.

Synthesis and Properties of Dithieno-Fused 1,4-Azaborine Derivatives.

The first synthesis of dithieno[3,2- b:2',3'- e][1,4]azaborinine (DTAB) derivatives has been achieved by Buchwald-Hartwig coupling and subsequent Friedel-Crafts-type C-H borylation. A facile method for further π-extension of DTAB was also developed via stannylation and subsequent Kosugi-Migita-Stille cross-coupling reaction. The fundamental properties of DTAB derivatives were also investigated.

Heating efficiency evaluation with mimicking plasma conditions of integrated fast-ignition experiment.

A series of experiments were carried out to evaluate the energy-coupling efficiency from heating laser to a fuel core in the fast-ignition scheme of laser-driven inertial confinement fusion. Although the efficiency is determined by a wide variety of complex physics, from intense laser plasma interactions to the properties of high-energy density plasmas and the transport of relativistic electron beams (REB), here we simplify the physics by breaking down the efficiency into three measurable parameters: (i) energy conversion ratio from laser to REB, (ii) probability of collision between the REB and the fusion fuel core, and (iii) fraction of energy deposited in the fuel core from the REB. These three parameters were measured with the newly developed experimental platform designed for mimicking the plasma conditions of a realistic integrated fast-ignition experiment. The experimental results indicate that the high-energy tail of REB must be suppressed to heat the fuel core efficiently.

Kilotesla magnetic field due to a capacitor-coil target driven by high power laser.

Laboratory generation of strong magnetic fields opens new frontiers in plasma and beam physics, astro- and solar-physics, materials science, and atomic and molecular physics. Although kilotesla magnetic fields have already been produced by magnetic flux compression using an imploding metal tube or plasma shell, accessibility at multiple points and better controlled shapes of the field are desirable. Here we have generated kilotesla magnetic fields using a capacitor-coil target, in which two nickel disks are connected by a U-turn coil. A magnetic flux density of 1.5 kT was measured using the Faraday effect 650 µm away from the coil, when the capacitor was driven by two beams from the GEKKO-XII laser (at 1 kJ (total), 1.3 ns, 0.53 or 1 µm, and 5 × 10(16) W/cm(2)).

Measurement of heating laser injection time to imploded core plasma by using x-ray framing camera.

A simultaneous measurement of imploded core plasma and injection time of heating laser is conducted by using an x-ray framing camera (XFC). The experiments are performed using Gekko XII laser system for implosion of the deuterated polystyrene (CD) plastic shell target and Peta Watt (PW) laser system for heating. The time of PW laser injection is observed as the bright zone in the XFC image. The measured x-ray intensity profiles fit the Gaussian profiles well. The calculations of microchannel plate by using dynode model explain these broadened temporal profiles qualitatively. The peak position of fitted x-ray intensity profile is almost in agreement with the time when the high energy x ray is observed by x-ray streak camera. Moreover, the peak position is delayed corresponding to the delayed setting of PW laser injection time. From these results, it is concluded that we can estimate the heating laser injection time with resolution of the order of 10 ps by using XFC.

Opacity effect on extreme ultraviolet radiation from laser-produced tin plasmas.

Opacity effects on extreme ultraviolet (EUV) emission from laser-produced tin (Sn) plasma have been experimentally investigated. An absorption spectrum of a uniform Sn plasma generated by thermal x rays has been measured in the EUV range (9-19 nm wavelength) for the first time. Experimental results indicate that control of the optical depth of the laser-produced Sn plasma is essential for obtaining high conversion to 13.5 nm-wavelength EUV radiation; 1.8% of the conversion efficiency was attained with the use of 2.2 ns laser pulses.

Suppression of the Rayleigh-Taylor instability due to self-radiation in a multiablation target.

A scheme to suppress the Rayleigh-Taylor instability has been investigated for a direct-drive inertial fusion target. In a high-Z doped-plastic target, two ablation surfaces are formed separately-one driven by thermal radiation and the other driven by electron conduction. The growth of the Rayleigh-Taylor instability is significantly suppressed on the radiation-driven ablation surface inside the target due to the large ablation velocity and long density scale length. A significant reduction of the growth rate was observed in simulations and experiments using a brominated plastic target. A new direct-drive pellet was designed using this scheme.