This part of our activity is theoretical and computational in nature. It concerns low-density plasmas — density of the order of 1/100th of that of the solid or below — in which the interaction between ions can be treated as a perturbation, or even totally neglected. Conversely, this approach applies to very hot plasmas — temperatures between one thousand and one million K — in which the ions may have several open subshells and the absorption spectra a very large number of lines. The purpose of this approach is to describe in detail atomic structure and transition rates. In the present context, the effects of the plasma environment are, if they are taken into account, treated in a perturbative way thanks to a phenomenological term added to the electron-ion interaction Hamiltonian.
A part of this work is done in a completely analytical way. This is particularly the case of the study of the effect of an ionic sphere potential called “uniform electron gas” on the hydrogen-like ions. It is also the case of the study of the configuration-average of the spin-orbit interaction, for which we have developed methods based on the explicit expression of this potential and on the second quantization.
However most of this activity is performed using atomic structure codes available in the literature. These include the HULLAC [BarShalom2001, Busquet2006] and FAC [Gu2008] parametric potential codes that are well suited to the description of highly ionized plasmas. The source code is in both cases available, which allows us if necessary to add new effects, such as the interaction via an ion-sphere potential. However, most of our work consists in writing post-processors that allow us to calculate the plasma spectra at the thermodynamic equilibrium, or to characterize the properties of plasmas outside thermodynamic equilibrium. We also make comparisons with the SCO-RCG hybrid code developed partly in our laboratory and partly at the CEA-DAM [Porcherot2011]. This numerical approach described above applies essentially to three types of studies.
1. Laser-plasma interaction experiments performed on kJ (LULI 2000) class facilities producing nanosecond pulses are interpreted using post-processors developed in our laboratory. These are based on the hypothesis of a plasma at local thermodynamic equilibrium (LTE) and weakly correlated.
2. We develop so-called “collisional-radiative” codes to study low-density and non-LTE plasmas, which are based on data produced by the HULLAC or FAC suites.
3. Effects related to the plasma environment are treated with an additional term in the electron-ion-interaction in the ion-sphere hypothesis. This term may be included in the FAC code. It describes the screening effect associated with free electrons. It allows us to calculate level shifts and changes in transition or collision rates.
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