The advances achieved using X-ray Free Electron Lasers such as the Linac Coherent Light Source (LCLS), have revolutionised the routine production of uniform solid-density plasmas. Pulses of X-rays above 1 keV and with durations shorter than 100 fs attaining intensities on target of around 10<sup>17</sup> Wcm<sup>-2</sup> are now routinely created. Through simple single-photon photoionization events with atoms in ambient solid conditions, it is possible to create uniform samples that are simultaneously hot, dense, and highly ionized which may be easily modelled. This thesis describes measurements of the spectrally-resolved X-rays emitted from solid-density magnesium targets of varying sub-μm thicknesses isochorically heated by an X-ray laser. The data exhibit a thickness-independent source function, allowing the extraction of a measure of the opacity to K-shell X-rays within well-defined regimes of electron density and temperature, extremely close to Local Thermodynamic Equilibrium conditions by fitting to the simple 1D slab solution of the equation of radiative transfer. The deduced opacities at the peak of the K-α transitions of the ions are consistent with those predicted by detailed atomic-kinetics calculations. The extracted opacities transpire to be robust to a plethora of variations in X-ray drive conditions, including the shape, pulse-length, and energy content. Furthermore the approximations in using the 1D slab solution are examined in detail and found to be good. A full three-dimensional model of the plasma is advanced which includes attenuation, line-of-sight effects, full longitudinal and transverse gradients, and photon time-of-flight effects. The results from this model are found to also agree with the simpler 1D slab solution. This novel method of elucidating opacities may complement other methods based on absorption and could be important for further benchmarking of opacities in solar-interior relevant conditions.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:729880 |
| Date | January 2017 |
| Creators | Preston, Thomas Robert |
| Contributors | Wark, Justin |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://ora.ox.ac.uk/objects/uuid:1983895a-b336-45e9-a5d0-b6badc8efb47 |
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