The effect of laser-produced plasma expansion on the ion population

Changes in the ion population during the expansion of Ag, Cu, Pb, and Ta laser-produced plasmas in vacuum were simultaneously measured with two coaxial ion collectors between 82 and 187 cm from a target. The plasma was produced with an iodine laser delivering a power density up to similar to 10(15) W/cm(2) onto the target. It was experimentally proved that the charge Q of expanding ions and the corresponding ion current density j are really decreasing with the distance L as Q proportional to L-2 and j proportional to L-3, respectively. It implies that the "freezing" of charge states dominates in that interval of L. (C) 1999 American Institute of Physics. [S0003- 6951(99)00943-2].