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Hot plasmas
Permanents: Jacques Bauche, Claire Bauche-Arnoult, Jean-François Wyart
In the laboratory, theoretical methods have been developed for several years for calculating the dynamical equilibrium of hot plasma ions away from Local Thermodynamical Equilibrium, and applying the results to the simulation of emission and absorption spectra. These plasmas are very complex media: the effects of many atomic processes ought to be computed for many levels in many ions. Some global methods give a realistic description of the ions, and the simulation of the corresponding emission and absorption spectra. Progresses are presently being made in two directions.
1) Plasma absorption spectra.
By means of the RTA model (Resolved Transition Arrays), it is possible to obtain a line-by-line simulation of a transition array, i.e., of the totality of the radiative lines linking two electronic configurations. Actually, in the existing models, e.g., UTA (Unresolved Transition Arrays), the spectrum is represented by a superposition of Gaussian-shaped features. Such smooth and continuous profiles are not adequate for the computation of the absorption spectrum, which is very sensitive to the occurrence of the gaps between the lines, even when they coalesce in the lower part of their profiles. Formulas have been obtained for a Monte Carlo sorting of the lines in a joint distribution of their wavenumbers and intensities. In this way, it is ensured that the lowest-order moments of the wavenumber distribution, weighted by the intensities, are equal to their exact values, i.e., to the values computed by detailed methods [1].
2) Spectra of non-LTE plasmas.
The plasmas generated by short and intense laser pulses are not in Local Thermodynamical Equilibrium (LTE). The Planck, Saha and Boltzmann laws cannot be applied. One ought to compute the dynamical equilibrium of the populations of the ionic levels, which results from the effects of the atomic processes (spontaneous emission and radiative absorption, collisional excitation and de-excitation, photoionization and radiative recombination, collisional ionization and 3-body recombination, autoionization and resonant capture. We have demonstrated that the level populations tend to follow a Boltzmann-like exponential law, for a temperature specific to the configuration to which they belong. For computing these temperatures, one can solve a collisional-radiative system of linear equations, which is built from the rates of the processes. This system contains one equation per configuration. But so many configurations must be introduced, that it is interesting to build a system for superconfigurations (SCs) instead of configurations. We define each SC [2] as the totality of the configurations having the same set of occupation numbers of the electronic shells [3].
For an application, we have simulated the spectrum of a Xenon plasma, by means of 109 SCs in 8 ions, containing 18328 configurations, or about 66 millions of levels, for the electronic density and temperature ne = 1.2 1020 cm-3 and kTe = 450 eV [4]. The agreement with the experimental spectrum is very good, as it can be seen in the Figure.
Figure: Interpretation of the emission spectrum of a Xenon laser plasma recorded at LULI [3].
Further reading:
[1] J. Bauche and C. Bauche-Arnoult, in Laser Interaction with Atoms, Solids and Plasmas, ed. R. M. More (Plenum Press, 1994).
[2] C. Bauche-Arnoult and J. Bauche, JQSRT 71, 189 (2001).
[3] J. Bauche, C. Bauche-Arnoult and K.B. Fournier, Phys. Rev. E 69, 026403 (2004).
[4] C. Chenais-Popovics et al., Phys. Rev. E 65, 046418 (2002).
Publications
Laser Interaction with Atoms, Solids and Plasmas,
J. Bauche and C . Bauche-Arnoult,
ed. R. M. More (Plenum Press, 1994).
[2] J. Bauche, C. Bauche-Arnoult and O. Peyrusse,
JQSRT 99, 55 (2006).
[3] J. Bauche, C. Bauche-Arnoult and K.B. Fournier,
Phys. Rev. E 69, 026403 (2004).
[4] C. Chenais-Popovics et al.,
Phys. Rev. E 65, 046418 (2002).
