The influence of charge transfer on ion probe measurements of laser-produced plasmas

Abstract

Charge transfer to the background gas from ions expanding in a laser-produced nickel plasma created by a neodymium laser focused to intensities in the range 1016-1018 W cm-2 is examined by comparing the ion charge per steradian measured by an ion probe at 0.33 m from the target to that measured by an identical probe signals for background gas pressures less than about 10-4 Torr. A simple theoretical analysis (i) confirms this experimental result, (ii) indicates that the reduction in ion current due to charge transfer is the same for ions of different initial charge and atomic number, (iii) shows that 'superthermal' ions with energy up to approximately=100 keV per unit atomic mass are equally affected by charge transfer as are slower thermal ions, and (iv) indicates that at higher background gas pressure (>10-4 Torr), the experimental results are affected by an additional mechanism for ion charge loss, probably ion scatter off the background gas.