E. Dufour

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.

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.

Observation of ion-ion correlation effects on emissivity and opacity of hot ultra-dense low Z plasmas

The transmission function and the resulting emissivity and opacity of bound-bound transitions in hot ultra-dense, low Z (aluminum or fluorine) plasmas are investigated in this paper. It is shown that the treatment of both the area-normalized line profiles and the absorption oscillator strengths involved are crucial. Taking account, self-consistently, of all the interactions inside a radiating transient molecule, as proposed in the dicenter code IDEFIX, we computed the photo-excitation cross-sections, the emissivities, the opacities, and then compare the results with those from standard codes. We emphasize the strong dependence of the above quantities with the ionic correlations. A first experiment, devoted to measure opacities and emissivities of hot and ultra-dense aluminum plasmas, has been designed. The interpretation of the results shows the adaptability of the dicenter model for these extreme conditions

Access to Spectrally Resolved Ultra‐Dense Hot Low Z Emissivities and Opacities

We present here experimental studies of broadened bound‐bound emissivities and opacities of low Z plasmas that are the best candidates for exhibiting ion and electron correlations. First we report on an emission experiment where a new target design is used to access the highest densities. Such targets irradiated by an intense long laser pulse generate plasmas well adapted to model and extract opacities. The measurements are compared to theoretical results obtained from simulations involving new atomic/molecular physics models that take into account detailed line profiles. We end up with the description of an absorption experiment in progress, this experiment using the same targets.

Classical dynamics of electrons surrounding ions in hot and dense plasmas—related topics

Abstract The non-linear behavior of electrons around a positive impurity ion is studied using both classical many-body theory and classical molecular dynamics. The plasma conditions are such that quantum effects can be taken into account in an approximate way with the help of a regularized Coulomb potential without divergence at short distances. This preliminary study explores the possibility for a new means in order to describe electron broadening for ion emitters.

Low Z opacities at high densities

Abstract We present an experimental study devoted to measuring the opacity of bound–bound transitions in ultra-dense, hot, low Z plasmas, which are at the extreme limit for conditions of both emission spectroscopy and absorption spectroscopy. In this work, we develop an absorption spectroscopy experiment specially adapted to high-density diagnostics, using newly designed structured targets and an ultra-high resolution spectrograph. An aluminum plasma is chosen as the first candidate and the opacity of the He-like 1s 2 –1s2p (He β ) and 1s 2 –1s3p (He γ ) transitions are measured.

Charge correlation effects in electron broadening of ion emitters in hot and dense plasmas

Abstract Electron broadening for ion emitters is investigated with a molecular dynamics based spectral line shape simulation. A regularized Coulomb potential that removes the divergence at short distances is used for the ion–electron interaction. The method presented here allows one to account for all the correlations between charged particles, which is in distinction to the standard electron broadening of the impact approximation. Two cases are considered: first, a single ion impurity embedded into an electron gas is considered; and second, a two-component ion–electron plasma is studied. Simulations show non-negligible charge correlation effects on line shapes opening new possibilities to improve line shape models and interpretations of experiments.

Advanced Simulations for Signatures of Charge Exchange in Heterogeneous Plasma Emission

We present an advanced theory of x‐dips in spectral lines emitted from laser‐produced plasmas. We compare predictions of this theory with our previous experimental results where, in the process of a laser irradiation of targets made out of aluminum carbide, we observed two dips in the Lyγ aluminum line perturbed by fully stripped carbon. Our theory gives a reasonable agreement with our experimental results. The results are of importance for the diagnostics of fundamental processes as it opens up a way to experimentally produce not‐yet‐available fundamental data on charge exchange between multi‐charged ions, virtually inaccessible by other experimental methods. From the theoretical viewpoint, the x‐dips are the only one signature of charge exchange in profiles of spectral lines emitted by plasmas and they are the only one quasi‐molecular phenomenon that could be observed at relatively “low” densities of laser‐produced plasmas, all those aspects emphasize the interest for studying heterogeneous plasma emission.

Spectral Line Shape Simulation for Electron Stark‐Broadening of Ion Emitters in Plasmas

Electron broadening for ions in plasmas is investigated in the framework of a simplified semi‐classical model involving an ionic emitter imbedded in an electron gas. A regularized Coulomb potential that removes the divergence at short distances is postulated for the ion‐electron interaction. Line shape simulations based on Molecular Dynamics for the ion impurity and the electrons, accounting for all the correlations, are reported. Comparisons with line shapes obtained with a quasi‐particle model show expected correlation effects. Through an analysis of the results with the line shape code PPP, it is inferred that the correlation effect results mainly from the microfield dynamic properties.

Models For Electronic Electric Field Distribution Function At a Positive Ion

A single positive ion is imbedded in an electron gas with overall charge neutrality. A classical statistical mechanics is considered using an electron‐ion Coulomb potential regularized at distances within the de Broglie length. The electric field distribution at the ion is studied as a function of ion‐electron coupling using molecular dynamics simulation and theoretical model (APEX, Adjustable‐Parameter Exponential Approximation). Agreement between theory and simulation is quite good in general, although differences are observed for very strong ion‐electron coupling due to enhanced importance of close electron‐ion configurations.