Significant role of the recombination effects for a laser ion source

This study is devoted to characterize a free expansion laser plasma produced by an excimer laser with respect to recombination phenomena. The plasma was ejected from a Cu target after the laser pulse. The diagnostic system was a Faraday cup located at different distances from the target in order to collect the ions carried by the plasma. Ion recombination effects were clearly observed at distances close to the target, due to the high density of the laser-produced plasma at the beginning of its expansion. The recombination range and critical distance were experimentally estimated measuring the ion charge variation on the distance from the target. Beyond it, the charge loss due to the recombination effects was negligible and the freezing of the charge states set in. In this region the plasma dilution, which was due to its free expansion into the vacuum, occurred, and the collected ion charge, Q, followed the law QL-2 as a function of the distance from the target, L. The assessment of recombination processes is very important for the optimization of a laser ion source.