Computation of pinch dynamics in plasma focus discharge using Lee model / Lim Ling Hong

Lim , Ling Hong (2021) Computation of pinch dynamics in plasma focus discharge using Lee model / Lim Ling Hong. PhD thesis, Universiti Malaya.

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Abstract

The dynamics of plasma focus discharge modelled by Lee code is investigated with the objective to improve the accuracy of the model in the radial phase. Lee model code has been developed based on the motion and circuit equations of a plasma focus discharge and the empirical data obtained in Mather type plasma focus. Lee model simulates the dynamics of the plasma focus discharge in five phases; the axial phase, radial inward shock phase, radial reflected shock phase, slow compression (pinch) phase and expanded column phase. The radial reflected shock phase is a phase in between the radial inward shock phase and pinch phase. This phase starts when the collisional inward shock hits the central axis of anode and a reflected shock is produced. This phase ends when the outwards moving reflected shock meets with the incoming magnetic piston. The dynamics of the radial trajectories in the Lee model is investigated in this thesis. Comparison between the computation and measured radial trajectories for two different types of electrode of plasma focus machines is done. The comparison shows a good agreement between the computation and measured results. Optimisation of neutron yield operated in deuterium and radiation yield operated in Argon, Neon and Nitrogen for several different plasma focus machines is done and the radial trajectories are investigated. Radiation cooling and collapse of the focus pinches are studied. It is found that radiation cooling generally occurs when plasma focus machines are operated in Neon. The radiation cooling becomes severe enough to induce radiation collapse when plasma focus machines are operated in Argon. Amendments to the radial reflected shock speed and temperature in the Lee model are presented. The present Lee code models the radial reflected shock with a constant speed. Because of this simplistic assumption, there is a discontinuity in temperature at the point of transition from the radial reflected shock phase to the pinch phase that has no physical basis. The temperature of the reflected shock phase is calculated from the reflected shock speed whilst the pinch temperature is calculated by Bennett balance. The amendment algorithm presented in this thesis adjusts the radial outward reflected shock speed at any point in accordance with the radial inward flow speed at that point during the radial inward shock phase. This amendment algorithm results in the reflected shock temperature profile transitioning smoothly into the pinch temperature profile when the outwardly moving reflected shock hits the incoming driving piston. This amendment to the reflected shock phase successfully removes the discrepant discontinuity in temperature profile at the transition point from reflected phase to pinch phase. Several plasma focus machines are then used with this amended Lee model and the radial trajectories are investigated. A step-by- step derivation of the Lee model is also presented in this thesis together with the model flowcharts. Normalisation of the radial reflected shock phase and the radial compression (pinch) phase has not been done before and is presented in this thesis for the first time.

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