R. C. Mancini

Hot Dense Capsule-Implosion Cores Produced by Z -Pinch Dynamic Hohlraum Radiation

Hot dense capsule implosions driven by Z-pinch x rays have been measured using a approximately 220 eV dynamic Hohlraum to implode 1.7-2.1 mm diameter gas-filled CH capsules. The capsules absorbed up to approximately 20 kJ of x rays. Argon tracer atom spectra were used to measure the T(e) approximately 1 keV electron temperature and the n(e) approximately 1-4 x 10(23) cm(-3) electron density. Spectra from multiple directions provide core symmetry estimates. Computer simulations agree well with the peak emission values of T(e), n(e), and symmetry, indicating reasonable understanding of the Hohlraum and implosion physics.

Experimental investigation of opacity models for stellar interior, inertial fusion, and high energy density plasmas

Theoretical opacities are required for calculating energy transport in plasmas. In particular, understanding stellar interiors, inertial fusion, and Z pinches depends on the opacities of mid-atomic-number elements over a wide range of temperatures. The 150–300 eV temperature range is particularly interesting. The opacity models are complex and experimental validation is crucial. For example, solar models presently disagree with helioseismology and one possible explanation is inadequate theoretical opacities. Testing these opacities requires well-characterized plasmas at temperatures high enough to produce the ion charge states that exist in the sun. Typical opacity experiments heat a sample using x rays and measure the spectrally resolved transmission with a backlight. The difficulty grows as the temperature increases because the heating x-ray source must supply more energy and the backlight must be bright enough to overwhelm the plasma self-emission. These problems can be overcome with the new generation...

Calculational aspects of the Stark line broadening of multielectron ions in plasmas

Abstract Recently a theoretical formalism and computer code were developed suitable for computing the Stark broadening of X-ray line transitions in highly ionized multielectron ions immersed in hot, dense plasmas. This problem can be formulated in terms of time-dependent autocorrelation functions. In this paper we present and discuss the formulation, computational techniques and algorithms used in the implementation of the theory into a computer code. We also present some examples of our results.

Hot-spot mix in ignition-scale implosions on the NIF

Ignition of an inertial confinement fusion (ICF) target depends on the formation of a central hot spot with sufficient temperature and areal density. Radiative and conductive losses from the hot spot can be enhanced by hydrodynamic instabilities. The concentric spherical layers of current National Ignition Facility (NIF) ignition targets consist of a plastic ablator surrounding a thin shell of cryogenic thermonuclear fuel (i.e., hydrogen isotopes), with fuel vapor filling the interior volume [S. W. Haan et al., Phys. Plasmas 18, 051001 (2011)]. The Rev. 5 ablator is doped with Ge to minimize preheat of the ablator closest to the DT ice caused by Au M-band emission from the hohlraum x-ray drive [D. S. Clark et al., Phys. Plasmas 17, 052703 (2010)]. Richtmyer–Meshkov and Rayleigh–Taylor hydrodynamic instabilities seeded by high-mode (50<l<200) ablator-surface perturbations can cause Ge-doped ablator to mix into the interior of the shell at the end of the acceleration phase [B. A. Hammel et al., Phys. Plasma...

Escape factors for Stark-broadened line profiles

Escape factors for upper and lower limits to the source function appropriate for spherical geometry have been evaluated using a set of Stark-broadened line profiles, computed with different approximations, for the Lyman- alpha line of argon XVIII. The method used to compute the escape factors, which is based on a general formalism can be applied to any kind of line profile and is suitable for any geometry. Comparisons are made with previous calculations that used Holtsmarkian line profiles and significant differences are noted.

Dynamic hohlraum radiation hydrodynamics

Z-pinch dynamic hohlraums are a promising indirect-drive inertial confinement fusion approach. Comparison of multiple experimental methods with integrated Z-pinch∕hohlraum∕capsule computer simulations builds understanding of the hohlraum interior conditions. Time-resolved x-ray images determine the motion of the radiating shock that heats the hohlraum as it propagates toward the hohlraum axis. The images also measure the radius of radiation-driven capsules as they implode. Dynamic hohlraum LASNEX [G. Zimmerman and W. Kruer, Comments Plasma Phys. Control. Fusion 2, 85 (1975)] simulations are found to overpredict the shock velocity by ∼20–40%, but simulated capsule implosion trajectories agree reasonably well with the data. Measurements of the capsule implosion core conditions using time- and space-resolved Ar tracer x-ray spectroscopy and the fusion neutron yield provide additional tests of the integrated hohlraum-implosion system understanding. The neutron yield in the highest performing CH capsule implos...

X-ray spectroscopy of high-energy density inertial confinement fusion plasmas

Analysis is presented of K‐ and L‐shell spectra obtained from Ar and Xe dopants seeded into the fuel region of plastic capsules indirectly imploded using the Nova laser. Stark broadening measurements of the n=3‐1 lines in H‐ and He‐like Ar (Ar Ly‐β and He‐β, respectively) are used to infer fuel electron density, while spatially averaged fuel electron temperature is deduced from the ratio of the intensities of these lines. Systematic variations in Ar spectral features are observed as a function of drive conditions. A spectral postprocessing code has been developed to simulate experimental spectra by taking into account spatial gradients and line transfer effects, and shows good agreement with experimental data. It is shown that correct modeling of the x‐ray emission requires a proper treatment of the coupled radiative transfer and kinetics problem. Continuum lowering effects are shown not to affect diagnostic line ratios, within the confines of a simple model. A recently developed diagnostic based on fitti...

Multispectral X-Ray Imaging With A Pinhole Array And A Flat Bragg Mirror

We describe a multiple monochromatic x-ray imager designed for implosion experiments. This instrument uses an array of pinholes in front of a flat multilayered Bragg mirror to provide many individual quasi-monochromatic x-ray pinhole images spread over a wide spectral range. We discuss design constraints and optimizations, and we discuss the specific details of the instrument we have used to obtain temperature and density maps of implosion plasmas.

K-shell spectra from hot dense aluminum layers buried in carbon and heated by ultrashort laser pulses

Ultrashort laser pulses allow for the generation of hot plasmas near solid state densities. For this purpose a Ti:Sapphire laser was used, which delivers after frequency doubling, pulses of high contrast with an energy of about 60mJ and a duration of 150fs at 395nm. The typical intensity on the target was a few 1017W/cm2. To achieve a high degree of uniformity we used targets consisting of a 25nm thin Al tracer layer buried at different depths up to 400nm in solid carbon. Time-integrated Al K-shell spectra are presented. Characteristic features of the spectra are significant high-order satellite line emission, strong line broadening and a center-of-mass line shift to the red, which was observed in transitions from principal quantum number n=2 or 3 to 1. Accurate measurement of the shift was made possible by using the cold Si Kα line as an absolute wavelength calibration. In addition to time-integrated measurements, we used an ultrafast X-ray streak camera to obtain time and spectrally resolved spectra. Typical durations of the Lyα and Heα lines are in the range 2–4 ps. The experimental results are compared with a time-dependent model, which combines hydrodynamic simulations, time-dependent atomic kinetics, detailed spectral line shapes including line shifts, and radiation transport.

High-order satellites and plasma gradients effects on the Ar Heβ line opacity and intensity distribution

Abstract We study the effects of high-order n =4 Li-like satellites and plasma density and temperature gradients on the Ar He β line opacity and intensity distribution, using a spectroscopic-quality atomic kinetics and radiation transport model. The model self-consistently accounts for NLTE level population kinetics, Stark broadened line shapes, and radiation transport effects. High-order satellites with a spectator electron in n =4 make a significant contribution to both opacity and line intensity distribution of the Ar He β composite line for plasma conditions typical of ICF imploded cores. We also discuss the impact of plasma density and temperature gradients on the formation of this spectral feature and show that the combined effect of gradients and opacity can lead to a characteristic round shape in the center of the He β line.