XUV probing of warm dense matter

Abstract

Experiments were conducted to test models of free-free absorption in warm dense aluminium by Iglesias, Shaffer et al. and Hollebon et al., Radiative heating using L-shell x-rays from a high temperature palladium plasma was used to heat an aluminium foil to a temperature of 0.9±0.23eV. Before the sample could significantly decompress, it was then probed using an XUV beam generated by HHG from a 1.5ps optical laser, with photon energies of 21eV, 26eV and 31eV. This XUV beam propagated to a grating spectrometer, which allowed the opacity of each sample foil to be measured at multiple photon energies simultaneously. A modelling code was created to simulate the diffraction of the XUV beam around the sample, which allowed both the transmission and refraction-induced phase shift to be measured very precisely for each sample. From these, the real refractive index and attenuation coefficient can be measured, along with estimates of the experimental error.

The opacity of the WDM was found to be closest to the predictions by Iglesias and Hollebon, and followed a similar trend with photon energy, though was still lower than predicted by 2 error bars. The measurements were significantly higher than the predictions by Shaffer et al, and were not consistent with the predictions of a weak dependence on photon energy in this range. Refractive index measurements were inconclusive due to ambiguity in the measurements of the phase shift, though this proved valuable as a proof-of-concept for future work.

The analysis methods used here are a significant advancement over those used in previous work. One of these is to the diffraction modelling code; the ability to model phase shift of the light passing through the sample has been developed and allows the real part of the sample's refractive index to be measured for the first time. The error analysis methods are now done in a more systematic manner, allowing for more confidence in the error estimates at each stage of the process. The experiment design was also improved, resulting in additional data being collected that was not possible previously, more samples being measured, and more confidence in some of the assumptions that were required for the analysis. These factors allow for higher confidence in the final bulk property measurements than in the previous work on this project by Kettle et al., despite the similar error estimates.

Date of Award	Jul 2021
Original language	English
Awarding Institution	Queen's University Belfast
Supervisor	David Riley (Supervisor) & Marco Borghesi (Supervisor)