A. Calisti

Fast electric microfield distribution calculations in extreme matter conditions

Abstract Spectral line shapes provide a powerful tool for characterizing strongly coupled plasmas that have become experimentally more accessible in recent years. Line shape calculations in turn require as input the electric microfield distribution at the emitting atom or ion. The APEX approximation for microfield distributions is computationally fast and suited for weakly as well as strongly coupled plasmas. The currently available APEX program, however, contains computationally difficulties that restrict its range of applicability. Consequently, the code has been improved removing many of its shortcomings. An important new feature is the incorporation of the HNC integral equation solution to the radial distribution functions necessary for the APEX approximation.

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.

Molecular dynamics simulation for modelling plasma spectroscopy

The ion-electron coupling properties for an ion impurity in an electron gas and for a two-component plasma are carried out on the basis of a regularized electron-ion potential removing the short-range Coulomb divergence. This work is largely motivated by the study of radiator dipole relaxation in plasmas which makes a real link between models and experiments. Current radiative property models for plasmas include single electron collisions neglecting charge-charge correlations within the classical quasi-particle approach commonly used in this field. The dipole relaxation simulation based on electron-ion molecular dynamics proposed here will provide a means to benchmark and improve model developments. Benefiting from a detailed study of a single ion embedded in an electron plasma, the challenging two-component ion-electron molecular dynamics simulations are proved accurate. They open new possibilities of obtaining reference lineshape data.

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.

Fourier-limited seeded soft x-ray laser pulse

We present what we believe to be the first measurement of the spectral properties of a soft x-ray laser seeded by a high-order harmonic beam. Using an interferometric method, the spectral profile of a seeded Ni-like krypton soft x-ray laser (32.8 nm) generated by optical field ionization has been experimentally determined, and the shortest possible pulse duration has been deduced. The source exhibits a Voigt spectral profile with an FWHM of 3.1+/-0.3 mA, leading to a Fourier-transform pulse duration of 4.7 ps. This value is comparable with the upper limit of the soft x-ray pulse duration determined by experimentally investigating the gain dynamics, from which we conclude that the source has reached the Fourier limit. The measured bandwidth is in good agreement with the predictions of a radiative transfer code, including gain line narrowing and saturation rebroadening.

Charge-charge coupling effects on dipole emitter relaxation within a classical electron-ion plasma description

Studies of charge-charge (ion-ion, ion-electron, and electron-electron) coupling properties for ion impurities in an electron gas are carried out on the basis of a regularized electron-ion potential without short-range Coulomb divergence. This work is motivated, in part, by questions arising from recent spectroscopic measurements revealing discrepancies with present-day theoretical descriptions. Many of the current radiative property models for plasmas include only single electron-emitter collisions and neglect some or all charge-charge interactions. A molecular-dynamics simulation of dipole relaxation is proposed here to allow proper account of many electron-emitter interactions and all charge-charge couplings. As illustrations, molecular-dynamics simulations are reported for the cases of a single ion embedded in an electron plasma and for a two-component ion-electron plasma. Charge-charge coupling effects are discussed for hydrogen-like Balmer alpha lines at weak coupling conditions.

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.

Ion dynamics in a linear radio-frequency trap with a single cooling laser

We analyse the possibility of cooling ions with a single laser beam, due to the coupling between the three components of their motion induced by the Coulomb interaction. For this purpose, we numerically study the dynamics of ion clouds of up to 140 particles, trapped in a linear quadrupole potential and cooled with a laser beam propagating in the radial plane. We use Molecular Dynamics simulations and model the laser cooling by a stochastic process. For each component of the motion, we systematically study the dependence of the temperature with the anisotropy of the trapping potential. Results obtained using the full radio-frequency (rf) potential are compared to those of the corresponding pseudo-potential. In the rf case, the rotation symmetry of the potential has to be broken to keep ions inside the trap. Then, as for the pseudo-potential case, we show that the efficiency of the Coulomb coupling to thermalize the components of motion depends on the geometrical configuration of the cloud. Coulomb coupling appears to be not efficient when the ions organise as a line or a pancake and the three components of motion reach the same temperature only if the cloud extends in three dimensions.

Strongly coupled laser produced plasmas: investigation of hollow ion formation and line shape analysis

Abstract X-ray line emission originating from hollow ions has been identified in dense laser-produced plasmas by means of two-dimensional X-ray optics and spectral simulations performed by the code MARIA. It is shown that for plasma coupling parameters Γ >1 excited states correlation effects of hollow ion configurations exceed the usual ground-state population channel by many orders of magnitude. The intensity of the emission of these excited-state hollow ions can be stronger than those of the usual satellite or resonance line transitions and lead to a remarkable distortion of the spectral emission. Detailed spectral simulations for the K-shell spectral interval are carried out for the 1s2l n l′-Rydberg configurations as well as the K 1 L 0 M 1 N 1 , K 1 L 0 M 1 N 0 O 1 , K 1 L 0 M 1 N 0 O 0 P 1 hollow ion configurations. Population kinetics, Stark broadening and spectral analysis are discussed along with experimental results of silicon laser-produced plasmas.