The Influence of Environmental Pressure on Momentum and Thermal Coupling for Microsecond Laser Propulsion with Metals

Abstract

This experimental research study tested the effect of varying ambient air pressure on the normal incidence laser propulsive performance of various flat target element samples (Al, Bi, C, Cu, In, Mg, Mo, Ni, Si, Sn, grade 2 Ti, and V) as well as several metal alloys (mild steel, 316 stainless steel, grade 5 Ti, and 93% WHA). A spherical vacuum chamber was pumped to <0.1 Pa, then an Nd:YAG laser was operated at a fixed fluence at the focus position to vaporize fresh areas of the target with single shots of laser energy. The ambient pressure was incremented between shots by partial venting of the chamber with ambient air from ~10^-2 to ~10⁵ Pa. Momentum coupling and thermal absorptance were evaluated as a function of pressure for each material at a calculated beam fluence at ~10⁴ J/cm². The results were interpreted with the assistance of open shutter DSLR photography of the ablation events at each pressure. The data are generally consistent with literature predictions and should be helpful for design of a wide range of future laser propulsion missions involving metal ablatants.