Detailed measurements of a diffusive supersonic wave in a radiatively heated foam

Measurement of Time-Dependent Drive Flux on the Capsule for Indirectly Driven Inertial Confinement Fusion Experiments

A new method for measuring the time-dependent drive flux at the hohlraum center is proposed as a better alternative to conventional wall-based techniques. The drive flux here is obtained by simultaneous measurement of the reemitted flux and shock velocity from a three-layered "cakelike" sample. With these two independent observables, the influence induced by the uncertainty of the material parameters of the sample can be effectively decreased. The influence from the closure of the laser entrance hole, which was the main challenge in conventional wall-based techniques, was avoided through localized reemitted flux measurement, facilitating drive flux measurement throughout the entire time history. These studies pave a new way for probing the time-dependent drive flux, for both cylindrical hohlraums and novel hohlraums with six laser entrance holes.

Number of populated electronic configurations in a hot dense plasma

In hot dense plasmas of intermediate or high-Z elements in the state of local thermodynamic equilibrium, the number of electronic configurations contributing to key macroscopic quantities such as the spectral opacity and equation of state can be enormous. In this work we present systematic methods for the analysis of the number of relativistic electronic configurations in a plasma. While the combinatoric number of configurations can be huge even for mid-Z elements, the number of configurations which have non-negligible population is much lower and depends strongly and nontrivially on temperature and density. We discuss two useful methods for the estimation of the number of populated configurations: (i) using an exact calculation of the total combinatoric number of configurations within superconfigurations in a converged super-transition-array (STA) calculation, and (ii) by using an estimate for the multidimensional width of the probability distribution for electronic population over bound shells, which is binomial if electron exchange and correlation effects are neglected. These methods are analyzed, and the mechanism which leads to the huge number of populated configurations is discussed in detail. Comprehensive average-atom finite-temperature density functional theory (DFT) calculations are performed in a wide range of temperature and density for several low-, mid-, and high-Z plasmas. The effects of temperature and density on the number of populated configurations are discussed and explained.

A split imaging spectrometer for temporally and spatially resolved titanium absorption spectroscopy

We present a temporally and a spatially resolved spectrometer for titanium x-ray absorption spectroscopy along 2 axial symmetric lines-of-sight. Each line-of-sight of the instrument uses an elliptical crystal to acquire both the 2p and 3p Ti absorption lines on a single, time gated channel of the instrument. The 2 axial symmetric lines-of-sight allow the 2p and 3p absorption features to be measured through the same point in space using both channels of the instrument. The spatially dependent material temperature can be inferred by observing the 2p and the 3p Ti absorption features. The data are recorded on a two strip framing camera with each strip collecting data from a single line-of-sight. The design is compatible for use at both the OMEGA laser and the National Ignition Facility. The spectrometer is intended to measure the material temperature behind a Marshak wave in a radiatively driven SiO2 foam with a Ti foam tracer. In this configuration, a broad band CsI backlighter will be used for a source and the Ti absorption spectrum measured.

Observation of high soft x-ray drive in large-scale hohlraums at the National Ignition Facility

The first soft x-ray radiation flux measurements from hohlraums using both a 96 and a 192 beam configuration at the National Ignition Facility have shown high x-ray conversion efficiencies of ∼85%-90%. These experiments employed gold vacuum hohlraums, 6.4 mm long and 3.55 mm in diameter, heated with laser energies between 150-635 kJ. The hohlraums reached radiation temperatures of up to 340 eV. These hohlraums for the first time reached coronal plasma conditions sufficient for two-electron processes and coronal heat conduction to be important for determining the radiation drive.

The first measurements of soft x-ray flux from ignition scale Hohlraums at the National Ignition Facility using DANTE (invited)

The first 96 and 192 beam vacuum Hohlraum target experiments have been fielded at the National Ignition Facility demonstrating radiation temperatures up to 340 eV and fluxes of 20 TW/sr as viewed by DANTE representing an ∼20 times flux increase over NOVA/Omega scale Hohlraums. The vacuum Hohlraums were irradiated with 2 ns square laser pulses with energies between 150 and 635 kJ. They produced nearly Planckian spectra with about 30±10% more flux than predicted by the preshot radiation hydrodynamic simulations. To validate these results, careful verification of all component calibrations, cable deconvolution, and software analysis routines has been conducted. In addition, a half Hohlraum experiment was conducted using a single 2 ns long axial quad with an irradiance of ∼2×10(15) W/cm(2) for comparison with NIF Early Light experiments completed in 2004. We have also completed a conversion efficiency test using a 128-beam nearly uniformly illuminated gold sphere with intensities kept low (at 1×10(14) W/cm(2) over 5 ns) to avoid sensitivity to modeling uncertainties for nonlocal heat conduction and nonlinear absorption mechanisms, to compare with similar intensity, 3 ns OMEGA sphere results. The 2004 and 2009 NIF half-Hohlraums agreed to 10% in flux, but more importantly, the 2006 OMEGA Au Sphere, the 2009 NIF Au sphere, and the calculated Au conversion efficiency agree to ±5% in flux, which is estimated to be the absolute calibration accuracy of the DANTEs. Hence we conclude that the 30±10% higher than expected radiation fluxes from the 96 and 192 beam vacuum Hohlraums are attributable to differences in physics of the larger Hohlraums.

Radiative heating of plastic-tamped aluminum foil by x rays from a foam-buffered hohlraum

The time dependence of the x-ray absorption of aluminum samples heated with intense radiation sources from a foam-buffered gold hohlraum has been studied in this work. Hydrodynamic simulations were used to illustrate the plasma conditions in the plastic-tamped aluminum foils contained in this type of hohlraum. Experiments were conducted to measure the K -shell x-ray absorption spectra of the aluminum sample. With densities taken from the hydrodynamic simulations, electron temperatures were then inferred by fitting the measured absorption spectra with detailed-term-accounting calculations. The inferred temperatures have a maximum of about 93eV and were found to agree within 25% with the simulated results at times after 1ns , indicating that the use of foam shields, together with a compact cavity, has created a clean and high-temperature radiation source preferable to opacity measurements.

X-ray scattering measurements of radiative heating and cooling dynamics

Spectrally and time-resolved x-ray scattering is used to extract the temperature and charge state evolution in a near solid density carbon foam driven by a supersonic soft x-ray heat wave. The measurements show a rapid heating of the foam material (approximately 200 eV/ns) followed by a similarly fast decline in the electron temperature as the foam cools. The results are compared to an analytic power balance model and to results from radiation-hydrodynamics simulations. Finally, the combination of charge state and temperature extracted from this known density isochorically heated plasma is used to distinguish between dense plasma ionization balance models.

Measurement and analysis of x-ray absorption in Al and MgF2 plasmas heated by Z-pinch radiation

High-power Z pinches on Sandia National Laboratories' Z facility can be used in a variety of experiments to radiatively heat samples placed some distance away from the Z-pinch plasma. In such experiments, the heating radiation spectrum is influenced by both the Z-pinch emission and the re-emission of radiation from the high-Z surfaces that make up the Z-pinch diode. To test the understanding of the amplitude and spectral distribution of the heating radiation, thin foils containing both Al and MgF2 were heated by a 100-130 TW Z pinch. The heating of these samples was studied through the ionization distribution in each material as measured by x-ray absorption spectra. The resulting plasma conditions are inferred from a least-squares comparison between the measured spectra and calculations of the Al and Mg 1s-->2p absorption over a large range of temperatures and densities. These plasma conditions are then compared to radiation-hydrodynamics simulations of the sample dynamics and are found to agree within 1sigma to the best-fit conditions. This agreement indicates that both the driving radiation spectrum and the heating of the Al and MgF2 samples is understood within the accuracy of the spectroscopic method.

Radiation-driven hydrodynamics of high- hohlraums on the national ignition facility

The first hohlraum experiments on the National Ignition Facility (NIF) using the initial four laser beams tested radiation temperature limits imposed by plasma filling. For a variety of hohlraum sizes and pulse lengths, the measured x-ray flux shows signatures of filling that coincide with hard x-ray emission from plasma streaming out of the hohlraum. These observations agree with hydrodynamic simulations and with an analytical model that includes hydrodynamic and coronal radiative losses. The modeling predicts radiation temperature limits with full NIF (1.8 MJ), greater, and of longer duration than required for ignition hohlraums.

Observation of a hydrodynamically driven, radiative-precursor shock

Observations of a radiative-precursor shock that evolves from a purely hydrodynamic system are presented. The radiative precursor is observed in low-density SiO2 aerogel foam using x-ray absorption spectroscopy. A plastic slab, shocked and accelerated by high-intensity laser irradiation, drives the shock which then produces the radiative precursor. The length and temperature profile of the radiative precursor are examined as the intensity of the laser is varied.

X-ray absorption spectroscopy measurements of thin foil heating by Z-pinch radiation

Absorption spectroscopy measurements of the time-dependent heating of thin foils exposed to intense z-pinch radiation sources are presented. These measurements and their analysis provide valuable benchmarks for, and insights into, the radiative heating of matter by x-ray sources. Z-pinch radiation sources with peak powers of up to 160 TW radiatively heated thin plastic-tamped aluminum foils to temperatures approximately 60 eV. The foils were located in open slots at the boundary of z-pinch hohlraums surrounding the pinch. Time-resolved Kalpha satellite absorption spectroscopy was used to measure the evolution of the Al ionization distribution, using a geometry in which the pinch served as the backlighter. The time-dependent pinch radius and x-ray power were monitored using framing camera, x-ray diode array, and bolometer measurements. A three-dimensional view factor code, within which one-dimensional (1D) radiation-hydrodynamics calculations were performed for each surface element in the view factor grid, was used to compute the incident and reemitted radiation flux distribution throughout the hohlraum and across the foil surface. Simulated absorption spectra were then generated by postprocessing radiation-hydrodynamics results for the foil heating using a 1D collisional-radiative code. Our simulated results were found to be in good general agreement with experimental x-ray spectra, indicating that the spectral measurements are consistent with independent measurements of the pinch power. We also discuss the sensitivity of our results to the spectrum of the radiation field incident on the foil, and the role of nonlocal thermodynamic equilibrium atomic kinetics in affecting the spectra.