High Energy Density Physics最新文献

Phase transitions of materials can be investigated by shock compression. It is necessary to obtain the equations governing the shock waves formation. We introduce the similarity variable $\xi =r/S\left(t\right)$ for the radial distance $r$ and the radius of the shock front $S\left(t\right)$ to handle the continuity equation ${\partial }_t\rho +{\partial }_r\left(\rho v\right)+2\rho v/r=0$ for the density $\rho (r,t)$ and for the radial velocity $v(r,t)$ considering a spherically symmetric flow. To illustrate our correct approach via a test case we make detailed as well as crucial remarks on Joy et al.’s (2021) paper.

Photoionization fronts are ubiquitous in astrophysics, but difficult to produce in a laboratory experiment. Recently, it was reported that photoionization fronts may be produced in nitrogen gas at pressure of ten atmospheres irradiated by a radiation source with temperature T_r ∼ 100 eV. We present two computational approaches to describe photoionization propagation in nitrogen gas induced by soft x-rays: 1. A time dependent in-line model that solves the ionization and the energy balance and the radiation transfer in a self-consistent way, and 2. A multi-group flux limited diffusion model based on two temperatures for the electrons and radiation, ionization and atomic data tables. Calculations were done for two spectrally different radiation sources, a laser irradiated gold foil and a blackbody Planckian source. It is shown that the atomic modeling and the spectral content of the source clearly affect the evolution of the nitrogen gas.

The THERMOS toolkit has been improved by adding the module for calculating the properties of non-stationary plasma. Low-density photoionized neon plasma obtained in experiments was simulated. Calculated ion populations, rates of elementary processes, and radiation spectra were compared with results obtained by other scientific groups.

A formula for supershell partition functions, which play a major role in the Super Transition Array approach to radiative-opacity calculations, is derived as a functional of the distribution of energies within the supershell. It consists in an alternative expansion for an arbitrary number of electrons or holes which also allows for quick approximate evaluations with truncated number of terms in the expansion.

The soft X-ray driven subsonic ablation of five different materials with atomic numbers ranging from 3.5 to 22 is investigated for radiation drive temperatures of up to 400 eV. Simulations were performed using the one-dimensional radiation hydrodynamics simulation code HYADES. For each material, ablation pressure scaling-laws are determined as a function of drive radiation-temperature and time, assuming that the irradiation lasts for a period of a few nanoseconds and that the drive temperature remains constant during this period. For all the materials, the maximum drive-temperature for subsonic operation is identified. As expected, lower-Z materials demonstrate a stronger scaling of ablation pressure with radiation temperature and a more gradual fall off with time. However, the lowest-Z materials transition to trans- and super-sonic ablation at temperatures of only a few hundred eV.

X-ray diffraction measurements have been performed on tin ramp compressed at the Orion laser facility to pressures in the range 50 – 100 GPa. The relationship between density and stress that we infer along the adiabat of the tin sample is consistent with previous hydrostatic and quasi-isentropic data and the existence of the BCC phase of tin in this pressure region. Within the limitations of the experimental technique, we detect no anisotropy of strain in the compressed tin sample.

The use of Non-Local Thermal Equilibrium (NLTE) material properties within radiation-hydrodynamic simulations remains a major challenge. The plasma radiative and EOS properties in NLTE can depend on the detailed local radiation field and electron energy distribution. Fully characterizing each of these quantities may require 10’s to 100’s of parameters, making tabulation effectively impossible and requiring expensive calculations for each set of conditions within the simulation. In this work we present a new method that characterizes the local radiation field with a limited number of parameters. This permits pre-calculation and tabulation of the material properties, allowing codes to perform NLTE radiation-hydrodynamic simulations which would otherwise be computationally prohibitive.

We report an increase in MeV energy bremsstrahlung x-ray production using compound parabolic concentrators (CPC) compared to flat solid targets during relativistic laser-plasma experiments on a 140 J, 150 fs laser system using an $f$/40 focusing optic. CPC enhanced targets show a $>3\times$ increase in high energy x-ray production over planar foil targets. This enhancement in x-ray energy spectra shows a direct improvement in the radiography of an image quality indicator (IQI) object with a 20 g/cm${}^2$ areal density.

High backlighter brightness is important to maximize the number of detected photons in radiography experiments and to minimize the background while backlighting high-energy-density plasmas with strong self-emission. Several different configurations were tested to improve the brightness of the Si He_α x-ray line emission at a photon energy of 1865 eV from high-energy (>1 kJ), short-pulse (∼20 ps), laser-driven backlighter targets. The emission from low-density SiO₂ foam targets, the effects of a laser prepulse, and Si targets with a CH “shield” that form a small cavity were compared to solid-density, flat Si targets. The CH “shield” targets showed the best performance with a>5×improvement in time-integrated emission and an x-ray pulse duration of ∼25 ps with no measurable spectral shift of the Si He_α emission line. A conversion efficiency from laser light into Si He_α photons on the order of 1 × 10^–5 was inferred from the data.