High-energy ions from a CO2 laser-produced plasma

Abstract

A time‐of‐flight ion collector was used to measure the energy of ions from the plasma produced by a high‐power CO₂ laser pulse focused on a flat polyethylene target. At laser irradiances exceeding 5×10¹² W/cm² at the target, high‐energy ion current spikes appeared in addition to the ion current from an expanding thermalized plasma. The measured arrival time of these spikes was consistent with the assumption that ions of different charge and mass were accelerated by a high potential during the laser pulse which was produced by a steep pressure gradient at the plasma surface. The accelerating potential (equal to the measured energy of a singly charged ion) was proportional to the laser irradiance at the target to the 2/3 power. At an irradiance of 2×10¹⁴ W/cm², the H⁺ ion energy was 140 keV. About 90% of the total ion energy appeared as high‐energy ions.