Proton Acceleration by High-Intensity UV Laser Irradiation with Thin Foil Targets

Proton acceleration experiments by irradiation of intense ultra-violet lasers with thin foil targets were conducted. Energies and efficiencies of the accelerated protons were investigated over the target thickness from several mm to 50 nm using various materials. In order to irradiate the very thin foil targets, the discharge pre-amplifier in the previous system was removed to reduce amplified spontaneous emission which disturbed the main pulse interactions. A Thomson parabola ion spectrometer with CR39 plastic nuclear track detectors were used to observe spectra of the accelerated protons. The maximum energies and efficiencies of accelerated protons increased with decreasing the target thickness rather than the product of the density and thickness of the targets. These results were explained by a geometrical effect on hot electron recycling.