Abstract

Attempts have been made by several authors to investigate the ion concentrations within high density plasmas generated by laser ablation. Measurements have been confined to the detection of positive ions on negatively biased probes. Time of flight measurements have indicated that the ion velocities are of the order of 10⁴ m/s for target to probe distances in the range 3.9 to 11.7 cm.^1,2 Discrepancies appear in the measured ion densities, with values between 1.4% and 30% having been quoted.^3,4 It is, however, difficult to conclude if such inconsistencies are due either to the use of inappropriate electrostatic probe theories, for these nonstationary plasmas or merely due to differing experimental conditions. Since the ion densities are found to be strongly dependent on laser fluence and the lateral position of the probe within the plasma plume, the latter may well be the case. This paper tests the reliability of the electrostatic probe for use within such plasmas, concluding that charged particles can undoubtedly be collected on a biased probe but that other more complicated processes should not be ruled out.

© 1994 IEEE