A Study of Extreme Ultraviolet Capillary Discharge Lasers and the Ablation of Solid Targets

This thesis discusses the use of capillary discharge laser output in the Extreme Ultraviolet (EUV) as a means of ablating solid targets. The EUV capillary discharge laser uses a neon like argon plasma as the lasing medium contained within a ceramic capillary. The laser produces a pulse of duration 1.2ns, at a wavelength of 46.9nm, with a repetition rate of up to 10Hz. A review of EUV production and the optical properties of EUV radiation at 46.9nm is given. From this a review of potential focusing methods for the laser is presented. An on-axis spherical mirror has been experimentally tested to give focal spot diameters of 3μm. The characterisation and optimisation of the capillary discharge laser is discussed. Optical spectra have been shown to enable a new method of measuring the average electron temperature of the plasma medium of the laser by modelling it as a black body. Plasma temperatures of approximately 3eV are measured. The capillary discharge laser has been used to ablate solid targets of aluminium, gold, copper and Poly-methyl methacrylate (PMMA). The ablation craters for each target material were measured using an atomic force microscope. Single shot depths of ablation of 1.3μm (Al), 0.9μm (Au), 0.6μm (Cu) and 0.3μm (PMMA) using fluences of approximately 200Jcm-2 were obtained. Ablation depths for aluminium are well modelled assuming ablation only occurs over the EUV attenuation length in the solid. For targets with short attenuation lengths another model based on a propagating ablation wave fits the experimental ablation depths. Both models allow an estimate of the ablated plasma temperature which is typically in the range of 3eV to 50eV, meaning the ablating plasma can be classed as a warm dense plasma.