Target and Laser Pulse Optimization for Laser-Driven Ion Acceleration

The research presented in this thesis is primarily focused on experimental investigations of laser-driven ion acceleration from solid targets via the target normal sheath acceleration mechanism. In particular, ways of optimizing the absorption of the laser pulse energy by free plasma electrons in the target, or modifying the shape of the accelerating electron sheath were addressed. The aim of this work was to increase the efficiency, and maximum proton energy that could be obtained with a given laser system, and to reduce the divergence of the beams of accelerated protons.

The shape of the electrostatic sheath was indirectly influenced by using laser micromachining to modify the front surface of the target, on which the laser pulse is incident. The absorption of the laser pulse was enhanced by either placing nanostructures on the front side of the foil target, or by manipulating the temporal profile of the ultrafast part of the laser pulse before its interaction with an ultrathin target.

It is important to ensure the survival of the target by using a laser pulse with very high temporal contrast. A double plasma mirror (DPM) was designed and implemented for this purpose. Design considerations and the optimization of the performance of the DPM, which is now used routinely at the Lund High-Power Laser Facility during laser-solid interaction studies, are discussed. Sufficiently high temporal contrast was achieved, and an increase was seen in the maximum proton kinetic energy when using targets with nanowire and foam structures on the surface. Efficient ion acceleration from ultrathin targets with a thickness down to 10 nm was observed as well.

When an ultrafast laser pulse interacts with an ultrathin foil, the temporal shape of the electric field of the pulse affects the laser--solid interaction, and a slightly positively chirped pulse was found to increase the maximum kinetic energy of the accelerated protons.

Laser-solid interactions at very high intensities are known to have shot-to-shot instabilities, motivating the use of single-shot diagnostics. The ion spectra in the forward direction were recorded using a Thomson parabola spectrometer, and in the backward direction with a magnetic dipole spectrometer. The intensities of the reflected and transmitted fractions of the laser pulse were also recorded on a shot-to-shot basis. In addition, a proton spatial profile monitor could be inserted to spatially characterize the proton bunch accelerated in the forward direction.