Measurement and simulation of extreme ultraviolet ablation of solids

This thesis reports an experimental and modelling investigation of ablation of solid targets by Extreme Ultra-Violet (EUV) laser light at 46.9 nm. The increased photon energy at these wavelengths enables photo-ionization and increases the critical density above solid density; setting EUV ablation apart from optical wavelength ablation. Experimental ablation features on aluminium, gold, and copper targets are created by focussing radiation from a capillary discharge laser emitting at 46.9 nm to produce on-target fluences from 50 J/cm^2 to 500 J/cm^2. The ablation features have depths in the region of 0.5 to 1.2 microns and crater radii between 2 and 3.5 microns, measured by post-shot atomic force microscopy. Four models are compared to the metal ablation depths; an optical femtosecond pulse model proposed by Gamaly, a `bleaching wave' model, a hydrostatic transmission model with spatial resolution, and the radiation hydrodynamics code MULTI-IFE. The transmission model is developed in this thesis; it employs a Saha-Boltzmann ionization model and uses classical expressions for photo-ionization and inverse bremsstrahlung absorption. Additional physics including degenerate ionization models and absorption coefficients, Gaunt factors, and ionization potential lowering are tested; only the latter is found to significantly affect ablation depth. The model is also adapted to use an existing ionization tool, FLYCHK, to calculate ablation depths with increased accuracy. Investigations into the `bleaching' of the plasma implied by the bleaching wave model show little evidence to support it.

The ablation depth behaviour of targets was found to depend on the attenuation length of the solid target material to the EUV radiation. For targets with long attenuation lengths such as aluminium, the femtosecond optical ablation model and the hydrostatic transmission model predict ablation depths close to those measured experimentally. For short attenuation length targets such as gold and copper, the one-dimensional hydrodynamic model MULTI-IFE predicts similar ablation depths to those measured experimentally - suggesting lateral transport is not significant. This shows that EUV ablation interactions can be modelled using one-dimensional fluid codes intended for optical laser interactions.