Low energy proton beams from laser-generated plasma

Low energy proton beams are produced by ns pulsed Nd:Yag laser operating in repetition rate at intensities of the order of 10¹⁰ W/cm ². Laser pulses interacting with thick and thin solid hydrogenated targets, placed in high vacuum, produce non-equilibrium plasmas with ion emission in backward and forward directions along the normal to the target surface. The ion emission is analyzed with time-of-flight techniques by using ion collectors and ion deflecting spectrometers. The spectra analysis permits the evaluation of the plasma temperature, density, proton energy and current, ion energy and charge state distributions. Special targets, based on thin polymers coupled to metals or to conducting nanostructures, induce high electric field in the plasma and confer high kinetic energy to the protons, up to about 200 eV. By using a post-acceleration system, placed along the target normal direction, it is possible to accelerate further the protons up to the energy depending on the applied acceleration voltage. The maximum proton energy of 30 keV, the current density of about10 nA/cm² and the beam quality can be improved, as discussed.