L. Klein

Processing of multi-monochromatic x-ray images from indirect drive implosions at OMEGA

We report here on the processing of multi-monochromatic x-ray images recorded with the MMI instrument in a series of stable and low-convergence indirect-drive implosion experiments in which Ar-doped D2-filled plastic shells were imploded with the OMEGA laser system. MMI records numerous narrow-band x-ray images over a broad photon energy range. From these images, specific line- and continuum-based subimages can be extracted. A procedure for processing data from the array of images recorded by MMI was developed and implemented into a convenient interactive data language code. Data from narrow-band x-ray images are critical for diagnosing the spatial structure of ICF implosion cores.

Analysis of X-ray spectral data with genetic algorithms

An algorithmic method for the analysis of X-ray line spectra using genetic algorithms is presented. This technique permits the extraction of diagnostic information on the emitting medium from the spectral data. As an example of the method, plasma electron number density and temperature are extracted from the analysis of X-ray spectral data recorded in an Ar-doped inertial-confinement-fusion core. For the study of a sequence of gradually changing spectra, a combination of genetic algorithms and case-based reasoning that learns from experience is used to accelerate the analysis. The technique is general and can be applied to other plasma spectroscopy studies including analysis of spatially and temporally resolved line absorption or emission data.

ASPECTS OF PLASMA SPECTROSCOPY : RECENT ADVANCES

Abstract Two important recent developments in Stark broadening are presented in this work: The first describes the Frequency Separation Technique (FST), which is motivated by the standard electron-ion separation and has important applications in that, coupled to any of a variety of methods capable of treating the intermediate ion-dynamical regime, (but not capable of treating ion impact), presents a unified and practical solution to the ion-dynamical problem. That is, this technique allows the relaxation of the quasistatic approximation. The second describes recent improvements that allow the accurate calculation of electron impact widths.

Analysis of the spatial structure of inertial confinement fusion implosion cores at OMEGA

We report on an experimental and modeling study of the spatial structure in indirectly driven implosion cores. To this end, Ar-doped D2-filled plastic shells were irradiated with the OMEGA laser system. We focus on processing and analysis of data recorded with a new X-ray imager (MMI-2) that records numerous narrow-band images that include both line and continuum emission. The temperature and density gradients are determined by using a multi-objective analysis of X-ray narrow-band images and X-ray line spectra. This analysis self-consistently determines the temperature and density gradients that yield the best fits to the spatially-resolved emissivity and space-integrated spectral line shapes.

Experimental results on line shifts from dense plasmas

Abstract The dynamics of the implosion of a deuterium-filled microsphere has been investigated via the detailed analysis of the Ar XVII 1s 2 –1s3p 1 P line shape. Ar is doped into the deuterium core for diagnostic purposes. For the analysis calculations of Ar XVII 1–3 line shape including lithium-like dielectronic satellites were compared with time-resolved data. Three fitting parameters were used: (a) electron temperature, (b) electron density, and (c) relative shift of the wavelength axis between calculation and data. The temporal evolution of the core electron temperature and density were derived, and the shot-to-shot formation of the core plasma was shown to be reliable and reproducible. We report on the wavelength shift of the Ar XVII 1s 2 –1s3p 1 P line shape between electron densities of 10 23 – 10 24 cm −3 , results indicate a systematic red shift with increasing 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.

Ground work supporting the codes based upon the frequency fluctuation model

The development of the frequency fluctuation model (FFM) had two strong motivations. First, there was interest to model line shapes accounting for ion dynamics and second the inclusion of higher order radiative processes in plasmas was considered important for future development. The FFM relies on the hypothesis that the emitter-plasma system behaves approximately like a pseudo-molecule embedded into a thermal bath. As a result, the pseudo-system can be considered to have internal states connected to each others by collisions with the bath. This simple starting point has been translated into a powerful renormalization process, called FFM, resulting, a few years ago, in a fast line shape code called Pim Pam Poum (PPP) and more recently into a code for the computation of radiative redistribution. The authors present a few of the milestones in this evolution.

Multi-criteria search and optimization: an application to X-ray plasma spectroscopy

X-ray spectroscopy diagnostics have been widely used as a standard technique to determine the temperature and density of astrophysical and laboratory plasmas. Traditional techniques have relied on performing an interactive search with a graphical user interface to select theoretical model parameters that best fit the data. We use a Pareto optimal genetic algorithm to drive a search of model parameters that produce high-quality simultaneous fits of spectra and spatially-resolved emissivity profiles. Preliminary results indicate that our Pareto optimal genetic algorithm is able to quickly find physically meaningful solutions.

Satellite and opacity effects on resonance line shapes produced from short-pulse laser heated foils

Abstract We measure the He-like, time-resolved emission from two types of thin foils (1) consisting of 250 A of carbon together with 250 A of aluminum and (2) 500 A aluminum, illuminated with a 150 fs laser pulse at an intensity of 10 19 W / cm 2 . Dielectronic satellite contributions to the resonance transitions 1s 2 – 1 s 2 p ( 1 P ) , 1s 2 – 1 s 3 p ( 1 P ) , and 1s 2 – 1 s 4 p ( 1 P ) are modeled using the configuration-averaged code AVERROES and is found to be significant for all three resonance lines. The effects of opacity are inferred from the data and found to be significant only in the 1s 2 – 1 s 2 p ( 1 P ) .

X-RAY LASER PHOTOPUMPED RESONANCE FLUORESCENCE

Abstract A calculation of the resonance fluorescence spectrum of the 146.526 A 3d-2p transition of fluorine-like magnesium photopumped by the 146.515 A zirconium X-ray laser line is discussed in connection with an experimental measurement in a hot, dense laser produced plasma. The calculation of the fluorescence spectral profile is based on a recent extension of the Frequency Fluctuation Model (FFM) that enables the computation of radiative redistribution and other higher order radiative processes. The discussion of the photopumping experiment includes an analysis of the optimal plasma conditions and the associated characteristics of the redistributed radiation with specific reference to the experimental feasibility. The potential information to be acquired in the comparison of the computational and experimental results also will be considered.