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The understanding and demonstration of novel laser driven ion acceleration mechanisms is key to overcoming the limitations of both well understood laser based processes and becoming competitive with conventional particle accelerators. This thesis presents two methods of enhancing the ion energies achievable in thin solid targets irradiated by intense laser pulses. In the first approach, the transition to the Light-Sail Radiation Pressure Acceleration mechanism was explored with the use of ultra-thin foils, and a contrast enhanced circular polarised pulse with a peak intensity of 5.5×10 Wcm⁻². The experimental data indicates the presence of an (intensity dependent) optimum thickness of 15 nm for C⁶⁺ (33 MeV/nucleon) where a local minimum of 18 MeV is instead observed for H⁺. At this thickness, each species’ maximum energy is shown to scale differently with laser intensity; C⁶⁺ scales favourably in the radiation pressure scheme (I¹.²) with protons scaling much slower, indicating acceleration by plasma heating. This species dependence is attributed to the precursor laser intensity impinging onto the target. This is shown to result in a multi-species expansion which allows for the preferential acceleration of bulk C⁶⁺ ions over sheath accelerated H⁺ which are displaced from the target prior to the arrival of the main intensity peak. This is shown for the first time and is of major significance to the development of laser based ion accelerators at currently available, and future, high power lasers. The second approach indicates the enhancement of radiation pressure accelerated ions in temporally stretched pulses, with a lower peak intensity, by measuring both higher ion energies and a greater red-shift in the back reflected light than for a fully compressed, higher intensity pulse. This is against what would be expected in a typical interaction. It is shown that proton energies are increased from 30 MeV to 50 MeV by stretching the pulse by over a factor of 2 with the maximum red-shift of the 2nd harmonic increasing from 55 nm to 75 nm, indicating an enhanced hole boring velocity from 0.1c to 0.14c. Numerical simulations indicate that this effect is due to the development of a hot electron current, and subsequently equalising return currents, in the overdense bulk plasma, which result in the growth of significant (10’s kT) magnetic fields within the target. These currents repel and cause a significant drop in target density reducing the internal plasma pressure and allowing radiation pressure to dominate on the front surface over what is observed with the higher intensity pulses. The experimental trends are qualitatively reproduced and the enhancement of front surface acceleration is demonstrated.

Over the past two decades, a number of experiments have been performed demonstrating the acceleration of ions from the interaction of an intense laser pulse with a thin, solid density target. These ions are accelerated by quasi-static electric fields generated by energetic electrons produced at the front of the target, resulting in ion energies up to tens of MeV. These ions have been widely studied for a variety of potential applications ranging from treatment of cancer to the production of neutrons for advanced radiography techniques. However, realization of these applications will require further optimization of the maximum energy, spectrum, or species of the accelerated ions, which has been a primary focus of research to date. This thesis presents two experiments designed to optimize several characteristics of the accelerated ion beam. The first of these experiments took place on the GHOST laser system at the University of Texas at Austin, and was designed to demonstrate reliable acceleration of deuterium ions, as needed for the most efficient methods of neutron generation from accelerated ions. This experiment leveraged cryogenically cooled targets coated in D2O ice to suppress the protons which typically dominate the accelerated ions, producing as many as 2 x 10^10 deuterium ions per 1 J laser shot, exceeding the proton yield by an average ratio of 5:1.The second major experiment in this work was performed on the Scarlet laser system at The Ohio State University, and studied the accelerated ion energy, yield, and spatial distribution as a function of the target thickness. In principle, the peak energy increases with decreasing target thickness, with the thinnest targets accessing additional acceleration mechanisms which provide favorable scaling with the laser intensity. However, laser prepulse characteristics provide a lower bound for the target thickness, yielding an optimum target thickness for ion acceleration which is dependent on the laser system. This experiment utilized new liquid crystal film targets developed at OSU, which may be formed at variable thicknesses from tens of nanometers to several microns. On this experiment, an optimum ion energy and flux was reached for targets of 600-900 nm, providing a peak proton energy of 24 MeV, and total ion flux of >10^9 protons over 3.4 MeV from 5.5 J of laser energy at an intensity of 1 x 10^20 W/cm^2.The primary ion diagnostics for these two experiments are described in detail, including the analysis techniques needed to extract absolutely calibrated spatial and spectral distributions of the accelerated ions. Additionally, a new technique for target alignment is presented, providing repeatable target alignment on the micron scale. This allows for a repeatable laser intensity on target, allowing improved shot to shot consistency on high intensity experiments.In addition to these two experiments, work on the upgrade and characterization of the 400 TW Scarlet laser is discussed, including several calculations critical to the design and upgrade of the laser system, as well as prepulse characterization needed for experiments on thin targets.