Temporally and spatially resolved ion dynamics of droplet-based laser-produced tin plasmas in lateral expansion direction

Abstract

The temporal and spatial plasma ion dynamics in the lateral direction generated by a Nd:YAG laser irradiated droplet target were studied with a hemispherical electrostatic probe array. The ion dynamics produced from 1.6 × 10+11 W/cm2 irradiation with a pulse duration of 23.9 ns FWHM were measured simultaneously from 50° to 130° from the laser axis with radial probe distances d from 1.5 to 7 cm to the plasma ignition point at an ambient argon gas pressure of 2 × 10−2 mbar. The collected ion charge and expansion velocities were derived from the ion profiles. It was found that the collected ion charge Q around the droplet scales with Q ∼ d−2 indicating that the main driving mechanism relates to the three-dimensional plasma expansion and not recombination processes. An anisotropic ion bulk expansion in the laser forward and backward propagation direction was deduced ranging from 2.9 cm/μs to 2.1 cm/μs, respectively. The gradients of the ion bulk expansion velocities along d were found to be constant within the error margin across the measurement range. The leading edge of the ion profiles showed an anisotropic behavior around the droplet, suggesting recombination effects scale differently in the laser forward and backward propagation direction which was linked to the higher expansion velocities in the laser forward direction. The broadening of the ion current waveform with increasing radial distance was studied and it was observed that the ion profile shape did not change for d > 5 cm, suggesting negligible recombination.