High Energy Density Physics最新文献

The discrepancies between theoretical and experimental opacities reported by experiments performed at the Sandia National Laboratory Z-pinch relevant to the solar interior remain unexplained. The suggestion that two-photon ionization could help resolve the discrepancies was recently examined and found not to account for the higher than predicted measured opacities. That test, however, was limited in scope and is now extended to include excited configurations and different charge states of several elements. Comparisons of one- and two-photon ionization cross-sections show that the latter fail to resolve the aforementioned discrepancies.

The mode-1 x-ray drive asymmetry of indirect-drive Inertial Confinement Fusion(ICF) implosions at the National Ignition Facility(NIF) has been estimated using a simple static ViewFactor model. The model takes as input measured laser performance data in the foot and peak, the hohlraum configuration, and laser to hohlraum pointing. These estimates are compared with neutron time-of-flight measurements of directionality and magnitude of the resultant hotspot bulk velocity (~20–100 μm/ns) for 39 NIF shots using High Density Carbon (HDC) ablators and show strong correlation on a statistically significant number of shots. The most important factors identified so far are random quad-to-quad peak power laser imbalances, the presence of lossy diagnostic windows and gaps on the hohlraum waist, capsule sag and capsule thickness mode 1. Typical mode-1 asymmetry in drive is currently ~0.5% for many of these sources on their own and, when summed, can lead to a neutron hotspot velocity of up to 100 μm /ns and a reduction in yield of 35% for current NIF DT layered implosions. Our goal is to identify, quantify and mitigate all potential sources of mode-1 asymmetry (which also include target and laser alignment imperfections, foot power and Cross Beam Energy Transfer imbalances) to enable higher quality implosions on NIF.

Accurate modeling of the electronic structure of warm dense matter is a challenging problem whose solution would allow a better understanding of material properties like equation of state, opacity, and conductivity, with resulting applications from astrophysics to fusion energy research. Here we explore the real-space Green’s function method as a technique for solving the Kohn–Sham density functional theory equations under warm dense matter conditions. We find the method to be tractable and accurate throughout the density and temperature range of interest, in contrast to other approaches. Good agreement on equation of state is found when comparing to other methods, where they are thought to be accurate.

Particle annihilation means that nuclear particles annihilate each other (for example nucleons like a neutron and an anti-neutron) and generate showers of mesons (mainly kaons and pions) at high energy. The kaons decay via pions and muons to electrons, positrons, neutrinos and photons. The energy which can be extracted from the very fast particles is of the order of 50% of the total energy of the nucleon masses involved or 500 MeV per mass unit. Several reports have been published recently on the meson showers ejected by pulsed-laser impact on ultra-dense hydrogen H(0). Since the particle velocities often are relativistic at >100 MeVu⁻¹ it is clear that a much more efficient nuclear process is responsible than in a normal hydrogen isotope fusion process (which can give only 3 and 15 MeV per mass unit out). The first experiment showing heat production above break-even in a laser-induced nuclear process in H(0) was published in AIP Avances as early as 2015. Here, we use a standard method for relativistic particle detection to show that the particles ejected by the laser pulse from D(0) are charged (thus not photons), and in fact positive, and that the signals decay with the characteristic decay times of kaons and pions with uncertainty < 1%. Using the measured kinetic energies of the mesons gives exact energy conservation. We conclude that annihilation of nucleons in H(0) is observed. This may have profound effects on future energy production, since the efficiency of the fuel in annihilation is roughly a factor of 100 higher than in a nuclear fusion process. Ordinary hydrogen (protium and deuterium) can be used as fuel instead of radioactive tritium. This means that energy is generated at low cost and with very little harmful radiation both for terrestrial and space applications (Acta Astronautica 2020).

Predicting and matching radiation wave propagation with computational models has proven difficult. Information provided by experiments studying radiation flow has been limited when only radiation breakout is measured. We have developed the COAX (co-axial) diagnostic platform to provide spatial temperature profiles of a radiation wave through low density foams as a more detailed constraint for simulations. COAX uses a standard, laser-driven OMEGA-60 halfraum to drive radiation down a titanium-laden silicon oxide foam. Point-projection X-ray absorption spectroscopy perpendicular to the radiation flow measures the spatial profile of titanium ionization. The spectroscopic measurement utilizes a broadband capsule backlighter. Imaging and streak spectroscopy are used to characterize the size and spectrum of this source. Radiography provides an additional constraint by capturing the developing shock as the radiation flow becomes subsonic. The DANTE diagnostic is used to measure the halfraum temperature. We provide a spectroscopic analysis of COAX data to determine temperature, and we describe experimental sources of uncertainty. The temperature is obtained by comparison to multi-temperature synthetic spectra post-processed from radiation-hydrodynamics simulations. Quantitative comparison between data and synthetic spectra generated from temperature profiles at relevant simulation times enable determination of a peak temperature of 114 $\pm$ 8 eV at 265 $\pm$ 22.4 $\mu$m from the halfraum. This represents an improvement over the temperature uncertainties of previous radiation flow experiments. Further refinements to the spectroscopic analysis could achieve $\pm$ 4 eV. The combination between space-resolved spectroscopy and radiography enables us to determine the distance from the halfraum of both the radiation front and the shock front at the time of measurement. For the example shown in this paper the radiation front position is 600–630 $\mu$m at 3.43 $\pm$ 0.16 ns and the shock front position is 633 $\mu$m at 3.3 $\pm$ 0.24 ns.

This work is the further development of the general model, Multi-Average Ion Collisional-Radiative Model (MAICRM), to calculate the plasma spectral properties of hot dense plasmas. In this model, an average ion is used to characterize the average orbital occupations and the total populations of the configurations at a single charge state. The orbital occupations and population of the average ion are obtained by solving two sets of rate equations sequentially and iteratively. The calculated spectra of Xe and Au plasmas under different plasma conditions are in good agreement with the DCA/SCA calculations while the computational cost is much lower.

Preheat in laser-driven experiments can have negative impacts on inertial confinement fusion (ICF) and hydrodynamic experiments. While many groups employ the use of dopants to reduce or block preheat, direct quantification has not previously been explored. We developed a planar platform and a series of ablator targets to measure the electron and x-ray spectra generated by laser-plasma interactions with a direct drive using OMEGA-60. By comparing both thin ablators (75m) and thick ablators (270 m) that were either pure CH, 3% Si doped or 3% I doped, we were able to measure differences in electron and x-ray spectra. In addition, we observed the preheat growth of tracer layers and observed reductions in the growth with different materials. We find that iodine or a thin gold layer is the best at tamping the direct-drive preheat at OMEGA, but that the growth is still significant.

This manuscript presents verification cases that are developed to study the electrothermal instability (ETI). Specific verification cases are included to ensure that the unit physics components necessary to model the ETI are accurate, providing a path for fluid-based codes to effectively simulate ETI in the linear and nonlinear growth regimes. Two software frameworks with different algorithmic approaches are compared for accuracy in their ability to simulate diffusion of a magnetic field, linear growth of the ETI, and a fully nonlinear ETI evolution. The nonlinear ETI simulations show early time agreement, with some differences emerging, as noted in the wavenumber spectrum, late into the nonlinear development of ETI. A sensitivity study explores the role of equation-of-state (EOS), vacuum density, and vacuum resistivity. EOS and vacuum resistivity are found to be the most critical factors in the modeling of nonlinear ETI development.

From experiments, we analyzed the spatiotemporal characteristics of a laser pulse in the Fourier plane in the 4-f arrangement. Based on these characteristics, we demonstrated two-stage frequency domain optical parametric amplification (FDOPA). The 275-fs signal seed pulse of the 1054-nm central wavelength was amplified in Type I β-BaB₂O₄ crystals pumped by a 180-ps pulse of 532-nm wavelength 532 nm. Using two-stage FDOPA at pump energy of 27 mJ (1.5 GW/cm²), a total net gain of 67 was obtained, yielding an output energy of 174 μJ while maintaining pulse width. We report the new concept of a two-stage FDOPA with beam magnification inside the 4-f arrangement that avoids damage to the output diffraction grating.

In fast ignition laser fusion, a high-intensity picosecond laser heats a compressed dense core to achieve ignition. It is theoretically expected that the energy coupling efficiency from the heating laser to the compressed core becomes higher as the wavelength of the heating laser is shorter. This prediction is ready to be experimentally demonstrated using second harmonic generation at Institute of Laser Engineering (ILE), Osaka University. Fundamental and converted second harmonic waves irradiate a target simultaneously in the experiment because crystals for wavelength conversion is installed after a final optical system. In addition, the polarization of the second harmonic wave becomes perpendicular to the original fundamental wave after the wavelength conversion. These features make laser-plasma-interactions complicated. Therefore, three-dimensional particle-in-cell simulations have been conducted to investigate effects of conversion to short wavelength and mixed-beam irradiation with different wavelengths. Simulation results show that the temperature of the generated fast electrons decreases for second harmonic conversion compared to the case of pure fundamental wave and the laser propagates deeper as the wavelength conversion efficiency becomes high. In the case of mixed-beam irradiation, it is found that the second harmonic wave drills the plasma and guides the fundamental wave to the deep region, where pure fundamental wave cannot reach.