Session GO6: X Rays, Nonlocal Transport, and IFE

A New Approach to Quantitative NIF GXD Image Analysis

A Gated X-ray Detector (GXD) is used by the National Ignition Facility (NIF) as a shot diagnostic instrument to record the ablator implosion process. Quantitative information must be retrieved from a series of time lapsed images to guide the NIF target design and laser tuning. Varying shapes and the presence of localized or contiguous hot spots make it very difficult to quantitatively compare shots from different experiments. Based on the observation that any deviation from a perfect spherical shape is undesirable, we have developed a symmetry based algorithm to decompose a GXD image into two parts: a main blob image that retains symmetry around the hohlraum axis and a residual image that describes the hot spots. This approach allows the retrieval of laser tuning information from the main blob symmetry without assuming user-defined thresholds, the computation of x-ray emission profiles through Abel inversion, and the study of ablator premix from the hot spots.   

Spectroscopic Observations of Ablator Mass Mixed into the Hot Spot of NIF Implosions

Megajoule-class hohlraums at the National Ignition Facility (NIF) were used to implode gas-filled (helium/deuterium) plastic shell inertial confinement fusion targets with a buried Ge-doped shell layer offset from the inner gas--shell interface. Hydrodynamic instabilities and jets seeded by isolated shell-surface mass modulations and the gas-fill tube are predicted to mix ablator mass with the hot spot.\footnote{B. A. Hammel et al., \textit{High Energy Density Physics} \textbf{6}, 171-178 (2010).} The measured Ge K-shell line emission (10 to 13 keV) is direct evidence that the Ge-doped ablator material mixed into the hot spot. Estimates of the ablator mass mixed into the hot spot are inferred from the measured brightness of the Ge K-shell emission lines. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

Spectrally resolved imaging of x-ray self emission from NIF symmetry surrogate capsules

Control of drive asymmetry is crucial to achieve conditions of ignition at the National Ignition Facility. For the measurement of drive asymmetry, we have been using surrogate capsules made of plastic (SymCap). Thickness of the surrogate capsules are designed so that the shape of x-ray self emission from the imploded core reflects asymmetry of the peak drive. However the core shape is affected also by hydrodynamic instabilities initiated by initial surface imperfections of the capsules. An interpenetration mixing of the gas and the shell material also affects the x-ray emission profile. A spectrally resolved x-ray imaging is one of a promising way to separate those complex phenomena. We designed pairs of matched x-ray filters (Ross filter) and applied them to time integrated pinhole imaging (with imaging plates). Concept of the filter setup and the experimental results will be presented.   

A Computational Study of X-ray Emissions from High-Z X-ray Sources on the National Ignition Facility Laser

We have begun to use 350-500 kJ of 1/3-micron laser light from the National Ignition Facility (NIF) laser to create millimeter-scale, bright multi-keV x-ray sources. In the first set of shots we achieved 15{\%} -18{\%} x-ray conversion efficiency into Xe M-shell ($\sim $1.5-2.5 keV), Ar K-shell ($\sim $3 keV) and Xe L-shell ($\sim $4-5.5 keV) emission (Fournier \textit{et al}., Phys. Plasmas July 2010), in good agreement with the emission modeled using a 2D radiation-hydrodynamics code incorporating a modern Detailed Configuration Accounting atomic model in non-LTE (Colvin \textit{et al}., Phys. Plasmas, July 2010). In this presentation we first briefly review details of the computational model and comparisons of the simulations with the Ar/Xe NIF data. We then discuss a computational study showing sensitivity of the x-ray emission to various beam illumination details (beam configuration, pointing, peak power, pulse shape, etc.) and target parameters (size, initial density, etc.), and finally make some predictions of how the x-ray conversion efficiency expected from NIF shots scales with atomic number of the emitting plasma.   

Expansion of a foil tracer and wall shocks as diagnostics for supersonic radiation flow in a foam tube

Modeling radiation transport is complicated by the flow of radiation across boundaries between different materials. Methods such as gray diffusion, multigroup diffusion, and Implicit Monte Carlo (IMC) lead to differences in the calculated heating of materials. We have conducted laboratory experiments at the Omega Laser Facility for the purpose of radiation transport modeling and diagnostic development. A laser-heated Au hohlraum is used as a thermal source to drive supersonic radiation through a foam-filled Be tube containing a Ti foil tracer. X-ray radiographs show the expanding Ti foil tracer and the shocked Be wall which provide information about the radiation flow through the foam and the absorption of energy in the Ti foil and Be wall. These results are compared with simulations to evaluate radiation transport methods.   

The creation of electric fields within plasma shock fronts

The propagation of shocks in high temperature plasmas is, in general, a well understood phenomena in high energy density science. Although the creation of a self-consistent electric field that develops at the shock front was predicted decades ago, it has not been investigated in detail, since in most cases of interest this effect was deemed small and therefore is always ignored in hydrodynamic calculations of shocks. However, certain parameter regimes exist in which the strength of this electric field is sufficiently large that it can affect the shock width, owing to excessive diffusion of electrons across the shock interface. For example, in strong shocks a precursor electric field ahead of the shock is observed which can then give rise to a precursor electric shock. We present a number of examples from a variety of parameter regimes, and then compare theoretical predictions with results obtained from collisional particle-based plasma simulations (LSP) that include this self-consistent electric field.   

Electron Heat Transport Models and Flux Limiters in the CRASH Code

The Center For Radiative Shock Hydrodynamics (CRASH) at the University of Michigan is an effort to create a radiation-hydrodynamics (RH) simulation code and quantify its predictive ability using experimental results. In the RH regime, much of the kinetic energy of the ions is expended by heating the electrons. The coefficient of electron heat conductivity can be calculated by one of several methods. When a discontinuity is encountered, many methods are prone to numerical errors. To correct for this an electron flux limiter can be used. However, flux limiters often blur real details. Simulation output in CRASH using different electron heat transport models with various flux limiters can be compared to experimental results to find the most accurate combination and its overall effect on the simulation. This research was supported by the DOE NNSA under the Predictive Science Academic Alliance Program by grant DEFC52-08NA28616.   

Advances in Nonlocal Transport Models for Laser Fusion

We have developed a Krook model for nonlocal electron energy transport [1-5]. It gives an analytic solution for the nonthermal electron energy flux, and is relatively simple and inexpensive to incorporate in a fluid simulation. It shows that in some sense, preheat is subtracted from the main electron energy flux, thereby giving rise to flux limitation. In our cited work we have compared our theory with Fokker Planck simulations of simple configurations. The Krook model clearly gives much better agreement than does any other model for electron energy transport. Additional work included a treatment of the center in a spherical plasma, and of the vacuum plasma interface. We find that the Krook model predicts some effects on laser fusion due to preheat and modification of the temporal pressure profile, but the effects at this point do not appear to be major. Further investigations looked into whether a diffusion model for preheat is valid, and we proposed and investigated an improved numerical approach.\\[4pt] [1] W.Manheimer (WM), D.Colombant (DC) and V.Goncharov, Phys. Plasmas (PP), 15, 083103, 2008\\[0pt] [2] DC {\&} WM, PP, 15, 083104, 2008\\[0pt] [3] DC {\&} WM, PP, 16, 0627051, 2009\\[0pt] [4] DC {\&} WM, J. Comp. Phys (2010)\\[0pt] [5] DC {\&} WM, submitted to PP   

Fast advection of magnetic fields by hot electrons

Experiments where a laser generated proton beam is used to probe the megagauss strength self-generated magnetic fields from a nanosecond laser interaction with an aluminum target are presented. At intensities of $10^{15} \; \rm{Wcm}^{-2}$ and under conditions of significant fast electron production and strong heat fluxes, the electron mean-free-path is long compared with the temperature gradient scale-length and hence non-local transport is important for the dynamics of the magnetic field in the plasma. The hot electron flux transports self-generated magnetic fields away from the focal region through the Nernst effect [1] at significantly higher velocities than the fluid velocity. Two-dimensional implicit Vlasov-Fokker-Planck modeling shows that the Nernst effect allows advection and self-generation transports magnetic fields at significantly faster than the ion fluid velocity, $v_N/c_s\approx10$.\\[4pt] [1] A.~Nishiguchi \textit{et al.}, Phys.\ Rev.\ Lett., \textbf{53}, 262 (1984).   

Thermonuclear ignition criterion in ICF

The Lawson criterion, which determines the onset of thermonuclear ignition in inertial confinement fusion (ICF), is re-derived in terms of physical measurable quantities: the hot spot ion temperature T and the areal density ($\rho R$) of the deuterium-tritium (DT) gas. From this criterion, an ignition curve is generated in the $\rho R -T$ plane. In addition, a minimal required implosion energy for laser-drive and a minimal DT gas mass for a sustainable ignition with respect to the condition are derived.   

Experimental Study of Plasma Cooling and Laser Beam Interaction in Gas Filled ICF Engines

ICF power plants, such as the LIFE scheme under development at LLNL, may employ a high-Z, target-chamber gas-fill to moderate the first-wall heat-pulse due to x-rays and energetic ions released during target detonation. This gas-fill is heated and ionized by this energy release. It must cool and recombine before the next shot (at nominally 70-ms intervals) to a temperature where the next target and laser pulse can propagate to chamber center with minimal degradation. While we expect rapid cooling to 2eV by radiation, our modeling of cooling below 2 eV has a high degree of uncertainty. We have developed a plasma source to study the cooling rates and laser propagation in high-Z gaseous plasmas. The source is a theta discharge configuration driven by a low-inductance, 5-kJ, 100-ns pulsed power system. This configuration delivers high peak power levels, has an electrode-less discharge, and has unobstructed axial access for diagnostics and beam propagation studies. Our diagnostics include Thompson scattering, time resolved spectroscopy, and plasma probes. We will report on the system design, operation, and initial results.   

Modeling of the LIFE minichamber Xe theta pinch experiment

The LIFE minichamber experiment is being designed to investigate cooling of the Xe buffer gas protecting the LIFE chamber wall. A magnetically driven theta pinch configuration will be used to inductively heat a few-cm long cylinder of Xe at ion density 2e16/cc to several eV. Thomson scattering will be used to determine the electron temperature and ionization state of the strongly radiating, cooling Xe. The experiment is being modeled using the magnetohydrodynamic code HYDRA with an external circuit mode and inductive feedback from the plasma to the external circuit. Designing the experiment is challenging due to the current paucity of opacity and conductivity data for Xe in the buffer gas regime of temperature and density. Results of the modeling will be presented.   

Simulation on recent hohlraum physics experiments on SGIII prototype

Recent hohlraum physics experiments on SGIII prototype using eight laser beams from angle of incidence cone, four per side, measured radiation flux and radiation spectrum by an array of x-ray diodes (XRDs) and peak radiation temperature by shock wave method. We present the simulation results from our 2D hydrodynamic code LARED-H with post-processed, which agree with the observations. From the simulation results from our one-dimensional multigroups radiation transfer code RDMG, it showed that the scaling relation of the peak temperatures of the x-ray sources with the shock velocities depends on the temporal profile and the length of the x-ray sources, and more, the shock velocities produced in Al sample strongly depends on M-band fraction. A shock scaling of the peak temperature with M-band fraction was given for Al sample. Finally, we discuss the laser x-ray coupling efficiency on SGIII prototyp.