Bremsstrahlung radiation and fast electron transport in laser-plasma interactions

  • Chris Armstrong

Student thesis: Doctoral Thesis

Abstract

This thesis reports on experimental investigations using high-intensity (> 1018W/cm2) lasers pulses to ionise and accelerate electrons from thick solid targets in order to generate a bright bremsstrahlung source. Throughout the work the focus has been, firstly, to better understand the process behind the generation of x-rays within solid target interactions and, secondly, to apply this understanding to radiography of industrial samples. Across three experimental investigations aspects of the x-ray source are addressed; in the first the spatial evolution of the x-ray source is investigated; in the second a mechanism to increase converse from laser into electrons is considered; and in the third a simple method to increase the conversion from electrons to x-rays is explored.As part of the research presented within this thesis, a novel diagnostic was developed to overcome limitations in current diagnostics that measure the spatial characteristics of the emitted x-ray source. This diagnostic was a curved foil, designed to ensure that over a large field of view the x-ray experienced a uniform transmission length – reducing the inherent uncertainty in knife edge techniques. The foil was calibrated to ensure accurate measurements at x-ray energies > 100 keV and applied throughout the research herein to characterise the emitted profile. The first experimental chapter explores the spatial characteristics of the xray profile. Using the curved foil, a two source structure was measured consistently in solid target interactions that emerges due to the recirculation of electrons. A narrow central source, and a broad secondary source. By defocusing the laser (and reducing the intensity on target) the x-ray flux produced by the small central source dominates over the secondary source and produces a sharper radiograph.The flux of this second source was investigated through an analytical model that treated the sheath as a threshold to drive recirculation of electrons through the target. This model found good agreement with the measured results, and can be applied to predict the optimum defocus for different target materials and thicknesses.The second chapter utilises a capillary target to trap the laser light and increase the conversion into electrons, the initial concept was to invoke numerous low intensity interactions as the laser propagated through the capillary.This would, in theory, produce a high flux – low temperature electron population that in turn would produce a high flux of x-rays. However, it was found that the flux increased but the temperature remained similar to that of a solid target. PIC simulations demonstrate that the electrons experience a series of accelerations within the capillary, undergoing direct-laser-acceleration (DLA)as lateral fields emerge within the channel.The final chapter outlines a simple targetry change to confine and enhance the x-ray emission from solid targets. By using a standing wire geometry, instead of a foil target, the electron expansion is confined to the lateral extent of the wire. The resultant field that emerges on the surface of the wire is greater in magnitude than that of a foil and develops sooner into the acceleration window recirculating more of the electrons within the target. This results in a significant, 50%, increase in total x-ray flux from the same laser conditions,and an increase in imaging quality of 2:6 due to the increase in flux and the confinement of the lateral x-ray source.
Date of Award19 Sep 2019
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SupervisorPaul McKenna (Supervisor) & Zheng-Ming Sheng (Supervisor)

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