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Title: An experimental investigation of the azimuthal structures formed during the ablation phase of wire array z-pinches
Author: Swadling, George Forrester
ISNI:       0000 0004 2716 008X
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2012
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A series of wire array implosion experiments have been carried out on MAGPIE, a 1.4MA peak current, 240ns rise-time pulsed power generator located at Imperial College London. The aim of these experiments was to investigate the azimuthal structure of ablation plasma produced during the early stages of wire array implosions. Previous work [Lebedev, S.V., et al., PoP, 2001] has demonstrated that a significant portion of the implosion time is dominated by the ablation phase, during which the dense wire cores remain at their initial radius and ablate streams of plasma towards the array axis. This ablation plasma prefills the array, and its distribution forms the initial conditions for the eventual snowplough-like implosion. In order to carry out this investigation a new two-colour end-on interferometry system was constructed. A custom software suite was developed in order to analyse the resulting data, enabling the conversion of the raw interferograms into 2D electron density maps. The experiments focused on cylindrical tungsten and aluminium arrays of between 8 and 128 wires. The structures observed in aluminium arrays were dominated by collisional features. Networks of oblique shocks were formed as the ablation streams interacted with their neighbours. Analysis of these shock structures allowed an estimate to be made for the Mach number of the ablation streams, M=7.9-8.2. In contrast, the structures observed in the tungsten experiments were characterised by smooth density profiles, consistent with relatively long ion mean free paths. The quantitative nature of the dataset makes it ideal for comparison with the results of numerical simulations. Also reported in this thesis is the development of two CW laser diagnostics, a quadrature interferometer, and a Faraday current diagnostic. The design of these diagnostics is discussed and some examples of the analysis are presented.
Supervisor: Lebedev, Sergey Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral