Akiyuki Shiroshita

High-Mach number collisionless shock and photo-ionized non-LTE plasma for laboratory astrophysics with intense lasers

We propose that most of the collisionless shocks in the Universe, for example, supernova remnant shocks, are produced because of the magnetic field generated by Weibel instability and its nonlinear process. In order to verify and validate the computational result confirming this theory, we are carrying out model experiments with intense lasers. We are going to make a collisionless counter-streaming plasma with intense laser ablation based on the scaling law to laser plasma with the particle-in-cell simulation resulting in Weibel-mediated shock formation. Preliminary experimental data are shown. The photo-ionization and resultant non-LTE plasma physics are also very important subjects in astrophysics related to mainly compact objects, for example, black hole, neutron star and white dwarf. Planckian radiation with its temperature 80–100 eV has been produced in gold cavity with irradiation of intense lasers inside the cavity. The sample materials are irradiated by the radiation inside the cavity and absorption and self-emission spectra are observed and analyzed theoretically. It is demonstrated how the effect of non-LTE is essential to reproduce the experimental spectra with the use of a precision computational code.

Shock Hugoniot and temperature data for polystyrene obtained with quartz standard

Equation-of-state data, not only pressure and density but also temperature, for polystyrene (CH) are obtained up to 510 GPa. The region investigated in this work corresponds to an intermediate region, bridging a large gap between available gas-gun data below 60 GPa and laser shock data above 500 GPa. The Hugoniot parameters and shock temperature were simultaneously determined by using optical velocimeters and pyrometers as the diagnostic tools and the α-quartz as a new standard material. The CH Hugoniot obtained tends to become stiffer than a semiempirical chemical theoretical model predictions at ultrahigh pressures but is consistent with other models and available experimental data.

Laser-shock compression and Hugoniot measurements of liquid hydrogen to 55 GPa

KYOKUGEN, Center for Quantum Science and Technology under Extreme Conditions,Osaka University, Toyonaka, Osaka 560-8531, Japan(Dated: January 7, 2011)The principal Hugoniot for liquid hydrogen was obtained up to 55 GPa under laser-driven shockloading. Pressure and density of compressed hydrogen were determined by impedance-matching toa quartz standard. The shock temperature was independently measured from the brightness of theshock front. Hugoniot data of hydrogen provide a good benchmark to modern theories of condensedmatter. The initial number density of liquid hydrogen is lower than that for liquid deuterium, andthis results in shock compressed hydrogen having a higher compression and higher temperature thandeuterium at the same shock pressure.

Time-resolved spectroscopic observations of shockinduced silicate ionization

We conducted time-resolved spectroscopic observations of shock-heated quartz and forsterite using a high-power laser. The results revealed that ionization occur easily under shockinduced warm dense conditions. We compare the obtained temperatures on the Hugoniot with a few theories. The comparison suggests that the contribution of shock-induced ionization to the isochoric specific heat during shock compression is ~1 kb/atom for quartz and forsterite. Shock-induced ionization and subsequent electron recombination leads to a larger amount of vapor and may lead to dynamical and chemical evolution of silicate vapor clouds different from our current understandings.

IMPACT EXPERIMENTS WITH PROJECTILES AT VELOCITIES HIGHER THAN 10 KM/S

Impact velocity of meteorites on planetary and satellite surfaces at the final stage of planetary accretion becomes more than 10 km/s. The impacts with velocities higher than 10 km/s generate very large craters and a large amount of silicate vapor, melt, and fast ejecta, and would make great effects on the planetary surface environments. However, the details of the effects by such impacts on the environments have not been understood well yet. The reasons are probably that macroscopic (>∼0.1 mm) projectiles are not easily accelerated to more than 10 km/s in laboratories. This makes it difficult to investigate experimentally the impact phenomenon with impact velocities higher than 10 km/s. In this paper, we demonstrate that higher impact velocities than 10 km/s can be achieved using projectiles with a diameter of 0.1–0.3 mm: we accelerate glass and aluminum projectiles using a high‐power laser, GEKKO XII—HIPER. The projectiles are collided into LiF targets. We observe some lines of Li gas using a time‐resol...

Optical pyrometer system for collisionless shock experiments in high-power laser-produced plasmas

A temporally and spatially resolved optical pyrometer system has been fielded on Gekko XII experiments. The system is based on the self-emission measurements with a gated optical imager (GOI) and a streaked optical pyrometer (SOP). Both detectors measure the intensity of the self-emission from laser-produced plasmas at the wavelength of 450 nm with a bandpass filter with a width of ~10 nm in FWHM. The measurements were calibrated with different methods, and both results agreed with each other within 30% as previously reported [T. Morita et al., Astrophys. Space Sci. 336, 283 (2011)]. As a tool for measuring the properties of low-density plasmas, the system is applicable for the measurements of the electron temperature and density in collisionless shock experiments [Y. Kuramitsu et al., Phys. Rev. Lett. 106, 175002 (2011)].

Hugoniot and temperature measurements of liquid hydrogen by laser-shock compression

Hydrogen at high pressure in the fluid state is of great interest for target design of inertial confinement fusion and understanding the interior structure of gas giant planets. In this work, we successfully obtained the Hugoniot data for liquid hydrogen up to 55 GPa under laser-driven shock loading using impedance matching to a quartz standard. The shocked temperature was determined simultaneously by the brightness temperature. The compression and temperature along the principal Hugoniot are in good agreement with theoretical models. High reflectivity of hydrogen was observed at 40 GPa, which suggests the fluid becomes conducting.

WIDE ANGLE X‐RAY DIFFRACTION FOR SHOCKED PERICLASE

We performed wide angle x‐ray diffraction for Laser shocked MgO single crystal. Wide angle x‐ray diffraction technique was applied to measure the transient atomic motion near the Hugoniot elastic limit to determine the three dimensional atomic motion under shock loading. The perfect atomic uni‐axial motion was observed at the compression of 4.6%. This results was equivalent to the cubic‐tetragonal distortion within 5 ns.

Observation of Complex Optical Processes in ZnSe under Extreme Optical Excitation from a Kilojoule-Class Nd:Glass Laser

In this paper, we report on three-photon fluorescence and the possible observation of other complex optical processes in a ZnSe crystal for a kilojoule-class, 100 ps pulse, Nd:glass laser excitation. The emission properties of the ZnSe crystal and its excitation energy dependence were investigated under low and high-energy excitations. A 15 to 10 nm full width at half maximum (FWHM) spectral collapse, a 5 nm blue shift, and a shortened lifetime of fluorescence under high-energy excitation could indicate an amplified spontaneous emission. These findings present the feasibility of investigating the dynamics and spectral properties of three-photon fluorescence and other complex nonlinear optical processes in ZnSe at an extremely high excitation energy.