Direction-selective free expansion of laser-produced plasmas from planar targets

Direction-selective expansion of laser-produced plasmas from planar slab targets of Al, Ni, Mo, and Ta are reported. Angular distributions of the particles emitted from the targets, produced by a 130 mJ, 5 nsec, Nd:YAG laser, were obtained by means of a retarding potential analyzer and a quartz crystal. It was observed that the angular distributions of the particles had mainly three characteristics. For a given laser energy and a given target element, the angular distribution showed more preferential focusing toward the target normal as the value of the focal spot size B increased. Second, for a given laser energy and a given focal spot size, the focusing was more pronounced toward the target normal as the atomic mass number of the target materials increased. Third, for a given energy, a given focal spot size and a given element, the particles with higher ionization states were much more focused toward the target normal. Our experimental results confirm the Monte Carlo simulation results of the earlier works taking into account collision and recombination processes.