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Title: Development of a Pseudo Non-LTE model for Z-pinch simulations
Author: Niasse, Nicolas-Pierre Louis
ISNI:       0000 0004 2715 2370
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2011
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Predicting the energetic and spectral characteristics of Z-pinch sources is a very delicate task. Z-pinch plasmas conditions are spread across a wide range of parameter space and often far from Local Thermodynamic Equilibrium (LTE). In addition, the increasing optical depth of the plasma at stagnation can have a strong influence on its own dynamics, suggesting that simultaneous solution of both the magnetohydrodynamic (MHD) and radiative response is required. Unfortunately, the estimation of the frequency and time dependent radiative properties of Z-pinches is computationally challenging and the recent improvements made to the parallel architecture of the 3D resistive Eulerian MHD code GORGON have only reinforced the strong emphasis already placed on optimizing the physics solvers used in Z-pinch simulations. To address these issues, we have developed a simple and fast pseudo NLTE code based on a Screened Hydrogenic Model (SHM) that can be run in-line with GORGON or as a post processing tool with synthetic spectra capabilities. Making use of a computationally inexpensive modification of the Saha equation, this highly optimized code has demonstrated a good ability to represent Non-LTE plasma conditions. In order to handle the amount of data generated by the spectral treatment of the billions of numerical cells constituting the simulation grids, an original data structure derived from a self-balancing binary search tree has been developed, enabling the use of Non-LTE DCA calculations in a large scale three dimensional environment for the first time. The implementation of this model is described in detail and comparisons with a commercial package are offered. Results from Z-pinch simulations performed with the new code are discussed and possible future improvements are presented.
Supervisor: Chittenden, Jeremy Sponsor: CEA Gramat ; MAGPIE
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral