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Title: Studies of the expansion phase of a laser plasma
Author: Abou-Koura, G. H.
Awarding Body: University of Wales Swansea
Current Institution: Swansea University
Date of Award: 1998
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This thesis describes a theoretical investigation of the expansion phase of a laser-produced plasma. The flow of ions and electrons in the expansion phase was modelled as a "displaced Maxwellian velocity distribution" i.e. the particles were assumed to possess a Maxwellian distribution of velocities relative to an average velocity of mass motion along the whole of the flight path. Using this model equations have been set up to describe the decrease in the electron and ion temperatures and the average charge carried by the ions as the expansion proceeds. This set of non-linear equations describing the flow of particles has been solved numerically using a Runge-Kutta method. A major process in the plasma expansion is three-body recombination and its strong influence on the nature of the plasma flow has been confirmed in this work. Three-body recombination influences the plasma flow by (i) reducing the average charge state of the plasma ions when electrons are captured by ions and (ii) releasing the excess energy of recombination to the plasma electrons. The influence of these two effects is clearly demonstrated by the numerical results. For example, in the absence of three-body recombination from the calculations the decrease of the electron and ion temperatures with time t follow a t-2 law, whereas, in the presence of three-body recombination the electron temperature is found to follow a law that is close to t-1. The average charge state of the plasma ions has also been calculated during the expansion and was found to decrease with time at a rate which depended on the particle concentration in the initial plasma, decreasing more rapidly as the initial concentration increased. Results are presented for both carbon and gold plasmas. The calculations made also provide a means of studying the influence of three-body recombination on the form of the ion signals that would be produced at the output of a mass spectrometer type of instrument after the ions had travelled along a long flight path. These ion signals are then used to create ion velocity distributions for each type of ion present at the entrance aperture of the mass spectrometer. The use of this type of information as a diagnostic tool in studies of the initial plasma conditions is discussed.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available