An exploding foil shockwave technique for magnetic flux compression and high-voltage pulse generation
This thesis describes a novel electromagnetic shockwave technique for use in compressing magnetic flux and to serve as the basis for a new approach to producing fast-rising voltage pulses with amplitudes of several hundred kV. The shockwave is produced by an exploding foil driven electric gun that accelerates a Mylar flyer to impact with a sample of aluminium powder. Both Japanese and Russian researchers have previously published experimental results for shockwave magnetic flux compression using an explosive driver. The present research considers replacing the explosive energy of this driver by the electrostatic energy stored in a capacitor bank, thereby enabling experiments to be performed in a laboratory enviromnent. Differences in performance that arise from the use of explosive and electrical driver are examined. A conventional electric gun system in planar geometry is developed to study the insulator-to-metallic transition in shock-compressed aluminium powder. This provides data on the conducting shock front in powder that can be used for flux compression and high-voltage pulse generation. A prototype cylindrical geometry system is described for proof-of-principle experiments, in which an imploding shockwave compresses flux towards the central axis of a system. A highcvoltage pulse can then be produced by the rapid time-change in the flux linking a suitably situated coil. Design calculation, constructional details and experimental results for the new system are all presented. The experimental programme is augmented by a detailed study of the fundamental shockwave processes. A new mathematical model for an electric gun is developed, that provides detailed description of the foil explosion and flyer acceleration processes. A hydrodynamic code including an equation of _state model for the powder is developed, and is shown to reproduce with reasonable accuracy the shock compression of aluminium powder by flyer impact, including the elastic precursor phenomenon. A magnetohydrodynamic code with an electrical conductivity model for the shockcompressed powder is developed for the study of flux compression and high-voltage pulse generation techniques. This provides a critical insight into the shockwave processes and facilitates a systematic design and performance prediction for future experimentation.