Laser-driven proton beams: Mechanisms for spectral control and efficiency enhancement

This thesis reports on investigations of proton acceleration driven by the interaction of short, intense laser pulses with thin, solid targets. Laser-driven plasma interactions are used to establish accelerating quasi-electrostatic field gradients, on the rear surface of the target, that are orders of magnitude higher than the current limit of conventional, radio-frequency-based accelerator technology. The resulting high energy (multi-MeV) proton beams are highly laminar, have ultra-low emittance, and the inherently broad energy spectrum is particularly effective for use in proton imaging, heating and transmutation applications. This thesis reports on a series of investigations carried out to explore routes towards control of the spectral properties of laser-driven proton sources and optimisation of laser-to-proton energy conversion efficiency. The dependence of laser accelerated proton beam properties on laser energy and focal spot size in the interaction of an intense laser pulse with an ultra-thin foil is explored at laser intensities of 1016-1018 W/cm2. The results indicate that whilst the maximum proton energy is dependent on both these laser pulse parameters, the total number of protons accelerated is primarily related to the laser pulse energy. A modification to current analytical models of the proton acceleration, to take account of lateral transport of electrons on the target rear surface, is suggested to account for the experimental findings. The thesis also reports on an investigation of optical control of laser-driven proton acceleration, in which two relativistically intense laser pulses, narrowly separated in time, are used. This novel approach is shown to deliver a significant enhancement in the coupling of laser energy to medium energy (5-30 MeV) protons, compared to single pulse irradiation. The 'double-pulse' mechanism of proton acceleration is investigated in combination with thin targets, for which refluxing of hot electrons between the target surfaces can lead to optimal conditions for coupling laser drive energy into the proton beam. A high laser-to-proton conversion efficiency is measured when the delay between the pulses is optimised at 1 ps. The subsequent effect of double-pulse drive on the angular distribution of the proton beam is also explored for thick targets.