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Modeling of radiation transport in optically thick plasmas

par Elodie PICO - publié le

Thesis advisor : Joël Rosato
Email and address : joel.rosato Tél. +33-491288624

Subject description :
Radiative transfer is an issue that occurs in optically thick media, in applications relevant to gas and plasma spectroscopy (astrophysics ; thermonuclear fusion research ; etc.). The photons emitted by atoms or multi-charged ions can be absorbed before escaping the medium under consideration, resulting in an alteration of observable spectra with the presence of dips or absorption lines. Such spectra contain information on the medium, which can be extracted (e.g., from line widths and intensities) provided a suitable model accounting for opacity is used.
Recently, theoretical investigations have questioned the validity of the standard opacity
models in the case where the light is partially coherent. This occurs for radiation with a
narrow spectral band and concerns atomic lines in particular (coherence is associated with the space and time autocorrelation of the radiation electric field, whose strength is characterized with the inverse line width). It has been suggested that the standard opacity models are inaccurate if the coherence length is comparable to or larger than other relevant scales of the problem of interest (inverse gradients ; radiation absorption length ; etc.) [1].
This stems from the fact that the localization of the physical processes is ambiguous at a scale smaller than the coherence length, in a fashion similar to the ambiguity that occurs in the measurement of a particle’s position because of the Heisenberg uncertainty principle.
This ambiguity is not retained in the standard opacity models.
The aim of this work is to investigate the role of the light’s coherence in radiation-matter interaction problems. An ongoing research project in our group concerns the modeling of photon transport in optically thick divertors (magnetic fusion research), in the framework of the preparation of ITER [2]. A refinement of the current opacity models would provide a better characterization of the ionization-recombination balance in the divertor plasma. A preliminary stage in the work will concern an analysis of the standard radiative transfer theory from the first principles (QED) and the elaboration of an opacity model accounting for coherence, sufficiently simple so as to be implementable in a code. A special emphasis will be devoted to the description of the plasma’s microscopic electric field, which perturbs the atomic energy levels (Stark effect) and results in a broadening of the line under consideration. Perturbations due to a magnetic field (Zeeman effect) should also be considered. A part of the work will be devoted to programming, but comparisons to experiments involving partially coherent light (such as laser sources and / or gas discharge lamps) should be performed using the laboratory facilities.

Bibliography :
[1] J. Rosato, Phys. Rev. E 87, 043108 (2013)
[2] J. Rosato et al., Contrib. Plasma Phys. 50, 398 (2010)