Modelling of non-equilibrium plasma flows

Abstract

This thesis considers the analysis and modelling of uids based on the laws of conservation. The uids are thermal ionizing plasma ows, and recombining plasmas expanding in large vessels at low pressure. These plasmas show deviations from local thermal equilibrium (LTE), i.e. from ionizationrecombination equilibrium and from temperature equilibrium; in the latter case the electrons and the heavy particles (atoms, ions and molecules) di??er in temperature. The analysis contains research on the deviations from the two equilibria, and on the ow and the ionization behavior of plasmas. With respect to the modelling of uids, a series of alternative models have been developed. These models have in common that they are able to describe the mentioned deviations from LTE, and the mentioned ow and ionization behavior of plasmas. They di??er however in how they handle the laws of conservation of mass, momentum and energy. In this manner, models may focus on one of the three conservation balances. The models di??er further in how they handle second-order terms and how geometry is included. For the modelling of thermal ionizing plasma ows, we made also use of a general two-dimensional plasma model, to which development contributions were made. Further, this thesis contributes to the so-called 'approximate modelling' approach, in which the variables to describe the plasma state are no longer taken from the traditional choice of pressure, temperatures and densities. Instead, other variables are chosen such as a normalized electron density, which describe better the plasma state. The alternative models are aimed at improving speed and comprehensibility of the model rather than accuracy. Therefore the dominating processes for the speci??c plasma situation are analyzed, and notion is acquired how to extend the models to obtain more accurate results. Notion about the fundamental processes, their coherence and their hierarchy is increased by comparing the di??erent approaches of the di??erent models with each other. The analysis of the mentioned plasmas is of interest for several applications. The primary aim in this work is to derive the optimum conditions for a plasma source to be used for experiments in which materials are tested via the bombardment by high energy uxes. This is especially relevant in fusion experiments, as Tokamaks. The ??rst wall in divertor plates must be able to cope with very high particle and energy uxes. More speci??c, the plasma-wall interaction needs to be studied for a plasma in contact with the divertor plates, where extremely energetic hydrogen ion uxes bombard the wall in a low pressure environment. Besides testing materials, high energy uxes in a low pressure environment are of interest for the modi??cation of materials. Furthermore, optimizing plasma sources is relevant for deposition and etching with plasmas, and for the development of ion sources. For all these applications a plasma source with a high ionization degree is a ??rst requirement. In the expansion from the source the ionization degree and the energy content should be maintained as much as possible. By changing the design of the plasma source an increase in the ion ux coming out of the plasma source has been obtained. An analysis of the behavior of expanding plasmas has been started. Finally, fundamental aspects of this thesis concern various general aspects of the ow and ionization properties of high density plasmas, which are not in full equilibrium.