Extreme ultraviolet probing of laser ablation

This thesis details the use of extreme ultraviolet (EUV) sources in probing laser ablation and material opacity.

The radiation hydrodynamic codes POLLUX and h2d are used to study the Rayleigh-Taylor instability in a regime relevant to inertial fusion energy and, in support of experimental effort, to test the feasibility of methods for measurement of iron opacity. Simulations of EUV radiography using the POLLUX code show how the presence of warm, dense material leads to strong absorption of the probe beam. Methods using both broad and narrowband EUV wavelength probes for benchmarking theoretical models of opacity are presented. An iron opacity experiment conducted at the Bhabha Atomic Research Centre is modelled, where K_alpha emission from an aluminium layer is used to probe an iron layer beneath. H2d simulations of laser heated iron, conducted to determine the suitability of experimental results obtained at the Rutherford Appleton Laboratory for iron opacity, show the difficulty in preventing large temperature and density gradients from forming.

Interferometry has been used to measure both transmission and phase information for a 21.2 nm zinc EUV laser beam probing longitudinally through laser ablated CH plastic at the Prague Asterix Laser System. By conducting interferometric probing with EUV laser light, the region of warm dense matter between the critical surface and ablation surface in a laser ablated plasma is diagnosed. Analysis of phase shifts reveals refractive indices below solid and plasma values arising in CH plastic, due to bound-free absorption in C+, a model for which is presented. The transmission of the EUV probe beam provides a measure of the rate of ablation, matching previous experimental scaling laws.