C. F. Hooper

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.

Characterization of direct-drive-implosion core conditions on OMEGA with time-resolved Ar K-shell spectroscopy

Direct-drive-implosion core conditions have been characterized on the 60-beam OMEGA [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] laser system with time-resolved Ar K-shell spectroscopy. Plastic shells with an Ar-doped deuterium fill gas were driven with a 23 kJ, 1 ns square laser pulse smoothed with 1 THz smoothing by spectral dispersion (SSD) and polarization smoothing (PS) using birefringent wedges. The targets are predicted to have a convergence ratio of ∼15. The emissivity-averaged core electron temperature (Te) and density (ne) were inferred from the measured time-dependent Ar K-shell spectral line shapes. As the imploding shell decelerates the observed Te and ne increase to 2.0 (±0.2) keV and 2.5 (±0.5)×1024 cm−3 at peak neutron production, which is assumed to occur at the time of the peak emissivity-averaged Te. At peak compression the ne increases to 3.1 (±0.6)×1024 cm−3 and the Te decreases to 1.7 (±0.17) keV. The observed core conditions are close to those predicted by a one-dimensional h...

Measurements of core and pusher conditions in surrogate capsule implosions on the OMEGA laser system

Experiments have been carried out on the 60-beam, 30 kJ OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] as part of an integrated program to diagnose all phases of direct-drive capsule implosions. Laser-imprint levels and Rayleigh–Taylor growth rates associated with the spherical implosions have been inferred from planar-foil radiography experiments. In spherical targets, measurements of the combined effects of imprint and unstable growth at the ablation surface have been carried out using the burnthrough technique [J. Delettrez et al., Phys. Plasmas 1, 2342 (1994)]. Target behavior during the deceleration phase has been investigated using a series of surrogate cryogenic capsules in which the main fuel layer is represented by a Ti-doped CH shell and the hot spot is represented by an Ar-doped deuterium fill gas.

Density and temperature diagnostic based on the Ar He β line and associated Li‐like satellites

We have modeled the temperature and density dependence of the Li‐like satellites of the Ar He β line by performing NLTE kinetic modeling of level populations in conjunction with Stark broadening calculations. Composite line profiles are computed including resonance and satellite line transitions that have built‐in the temperature and density dependence characteristic of the level populations and Stark broadening of these transitions. These synthetic spectra can be used to analyze experimental data, providing a simultaneous diagnostic of temperature and density.

Spectroscopic line shape measurements at high densities

Abstract A comprehensive spectroscopic investigation of plasmas at extreme conditions produced by indirectly driven inertially confined implosions is described. In these experiments argon is doped into the gas filled core of implosion targets and the Ar K-shell emission is used to make time resolved measurements of electron density and electron temperature. The electron density is derived from the Stark broadened Ar XVII 1s 2 -1s3p line shape, the electron temperature is derived from the line intensity ratio of the Ar XVII ls 2 -ls3p transition and the lithium-like dielectronic satellites 2121′, 2131′ lying on the low energy side of the resonance line. We give examples of the experimental data and compare the extracted time histories of electron density and electron temperature with simple radiation hydrodynamic simulations, where broad agreement is found. Detailed line shape measurements of the Ar XVII 1s 2 -1s3p transition are presented and the absence of an intensity dip at line center in the experiment results is discussed. The validity of the quasi-static ion approximation for these plasma conditions is tested by varying the mass of the fill gas in the core. Results from deuterium, deuterated methane, and nitrogen filled implosions are presented and indicate ion dynamic effects are not responsible for the line center discrepancy. We discuss other possibilities including spatial gradients in the core affecting measurements of the intrinsic line shape.

A relaxation theory of plasma-broadened He II lines

The relaxation theory of plasma line broadening developed by Smith and Hooper, and extended by Smith, is applied to a charged radiator. The 304 A Lyman alpha line of He II is chosen as an example. The radiator is assumed perturbed only by electronic and ionic electric microfields. The ions are treated in the static approximation. Two-component ion microfield distribution functions that allow for the presence of both He+ and He++ perturbers are employed. The fact that the radiator is charged complicates the treatment of electron perturbation; two different approaches are included. The first neglects the effect of the charge of the radiator on the perturbing electron and assumes that the perturbing electrons can be represented by momentum wave functions. This method corresponds to Smiths treatment of the electron perturbers in neutral hydrogen. The second includes the effect of the charge of the radiator on the perturbing electrons and represents the perturbing electrons by Coulomb wave functions; this calculation requires evaluation of free-free gaunt factors. The theory, with both the momentum and Coulomb perturber wave functions, predicts a blue asymmetry in the near wings of the line. These two approaches can be compared with the classical path approach of Griem and Shen.

Diagnosis of High-Temperature Implosions Using Low- and High-Opacity Krypton Lines

High-temperature laser target implosions can be achieved by using relatively thin-shell targets, and they can be diagnosed by doping the fuel with krypton and measuring K-shell and L-shell lines. Electron temperatures of up to 5 keV at modest compressed densities (∼1–5 g/cm3) are predicted for such experiments, with ion temperatures peaking above 10 keV at the center. It is found that the profiles of low-opacity (optically thin) lines in the expected density range are dominated by the Doppler broadening and can provide a measurement of the ion temperature if spectrometers of spectral resolution Δλ/λ ≥ 1000 are used. For high-opacity lines, obtained with a higher krypton fill pressure, the measurement of the escape factor can yield the ρRof the compressed fuel. At higher densities, Stark broadening of low-opacity lines becomes important and can provide a density measurement, whereas lines of higher opacity can be used to estimate the extent of mixing.

Spectroscopic analysis of Ar‐doped laser‐driven implosions

In a series of experiments performed at the Laboratory for Laser Energetics plastic microballoons filled with DD and doped with small amounts of Ar were imploded using the Omega laser system. Time‐resolved K‐shell Ar spectra were simultaneously recorded using two spectrographs (SPEAXS and flat‐crystal). We focus on the analysis of the He‐β line and its associated Li‐like satellites. The density and temperature sensitivity of this composite spectral feature has been studied previously [R. C. Mancini et al., Rev. Sci. Instrum. 63, 5119 (1992)]. Here, we use it as a diagnostic. Modeling results take into account the built‐in density and temperature dependence characteristic of the level populations and broadening properties of these transitions; in addition, we also consider the effects of ion dynamics and opacity. To check the consistency of our analysis we include in the model the He‐γ and Ly‐β lines.

Analysis of K- and L-shell spectra from indirectly driven implosions

Abstract We report on the analysis 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 of the Ar Ly-β and He-β lines is used to infer fuel electron density, while spatially averaged fuel electron temperature is deduced from the ratio of Ar Ly-β to He-β. A postprocessing code has been developed to simulate experimental spectra by taking into account spatial gradients and line transfer effects. It is shown that correct modeling of the x-ray emission requires a proper treatment of the coupled radiative transfer and kinetics problem. A recently developed diagnostic based on the lineshape of Ar He-β and its associated dielectronic satellites is shown to provide a simultaneous measure of electron temperature and electron density. Comparisons of TOTAL and MERL line broadening calculations for this case are shown. L-shell Xe spectroscopy is under continuing development as an electron temperature and electron density diagnostic. Density and temperature sensitive ratios of spectral features each consisting of many lines have been identified. Observed Xe spectra from imploded cores show the same qualitative behavior with temperature as predicted by model calculations of Xe emission spectra.

Modeling of absorption spectra in dense argon plasmas

Abstract We have modeled the optical depth of hot, dense Ar plasmas due to line absorption in n = 1 to n = 2 inner-shell transitions in L -shell Ar ions by performing Stark broadened absorption line profile and ionization balance calculations. Our results have built-in the temperature and density dependence characteristic of the level populations as well as the density sensitivity inherent in the Stark broadening of these transitions. Results are presented that illustrate the temperature and density dependence of the optical depth. As an application, we use these results to analyse absorption spectra recorded during the collapse of laser driven implosions of Ar-filled plastic microballoons.