Optimisation of laser driven proton beams and their applications to plasma radiography

The interaction of ultra-intense laser pulses >1018W/cm2 with thin foils drives the acceleration of protons to multi-MeV energies. Proton beam has unique properties such as low emittance, short pulse duration and high particle flux. However, the beams usually • exhibit large envelope divergence and quasi-Maxwellian energy spectra, which might be undesirable for a range of foreseen applications. Hot electrons generated during the interaction of intense lasers with foil targets escape the foil, charging it to potential of the order of the hot electron temperature. It is observed that the charge flows towards the ground with a velocity close to the speed of light in a localised pulse of a Gaussian profile with 6ps rise and 15ps decay, which retains its temporal profile over a few centimetres. Based on these findings, novel target geometry is envisioned to create an electrostatic lens which simultaneously focuses/collimates the proton beam and allows energy selection. This electrostatic lens demonstrates a reduction in beam diameter by 75% and an enhancement of the proton flux by an order of magnitude for 6.5 MeV protons in comparison to typical divergent proton beam. Particle tracing simulations corroborate the dynamics nature of the lens. Laser driven proton beam is employed as proton radiography technique to investigate the expansion of ablated plasma created by the interaction of intense laser >1015W/cm2 with solid target, into low density background plasma. High temporal and spatial resolution of the technique allows detection of the precursory stages that lead to formation of an electrostatic collision less shock at the boundary of blast shell of the expanding laser ablated plasma. The evolution of the electrostatic potential associated with the shock unveils the transition from a double layer into a symmetric shock structure, stabilized by ion reflection at the shock front. A PlC simulation supports the existence of super-critical electrostatic shocks.