Compact laser driven ion sources have inspired cautious optimism that they may provide an alternative to conventional accelerators for existing applications, such as in medicine, or aid the realisation of new ones such as fusion energy. However, the sources must be developed, with increased conversion efficiency of laser to proton energy being high on the list of requirements. Recent reports in the literature have shown that record conversion efficiencies can be achieved with double pulse interactions, and this thesis proceeds with this theme. The double pulse operation of the plasma mirror is characterised for the first time, in terms of the post interaction far field quality, and integrated reflectivity. The main pulse reflectivity is significantly enhanced to 96% and the far field remains of high optical quality up to five picoseconds after the prepulse interaction, within the regime for conversion efficiency enhancement. These observations are explained by perturbations of the quasi-near field intensity distribution seeding nonuniformities in the plasma expansion of the plasma mirror surface. A novel plasma half cavity target geometry is investigated which utilises the high fraction of laser energy reflected from an ionised surface and refocuses it such that a double pulse interaction is attained. This new geometry is found to double the laser to proton energy conversion efficiency, compared with planar foil interactions and to modify the low energy region of the proton spectrum. For pulse separations of tens of picoseconds, a long time delay regime is identified for planar foil interactions, where a significant reduction in maximum proton energy and conversion efficiency is reversed, and return to that expected for single pulse interactions. This is explained by the main pulse interacting with bulk target expansion induced by the prepulse. Increased electron temperatures from enhanced absorption in the preplasma are found to mitigate the detrimental effects on ion acceleration, associated with rear surface density scale lengths.
|Date of Award||25 Nov 2014|
- University Of Strathclyde