Study of plume dynamics from laser-ablated elemental materials for propulsion application

The efficacy of utilizing material ablation from picosecond laser pulses onto elemental metal targets for propulsion applications is studied by a variety of measurement methods to characterize laser ablation plume dynamics. A 100 ps, 532 nm laser is used for ablating elemental metals, and the resulting ablation plume dynamics are measured by time-of-flight (TOF) energy analyzer probes, a piezoelectric force sensor, and an intensified imager. Results indicate that plume dynamics exhibit a material dependence and behave in a manner indicative of laser-induced plasma formation and expansion, and indicate that ablation-based laser propulsion is characterized by high specific impulses (on the order of 10^3 s) due to the high plume velocities measured. An efficiency analysis is performed comparing input laser energy to plume kinetic energy, showing that aluminum is the best-suited material for laser ablation propulsion. A demonstration is constructed and performed showing proof of this concept. Further experiments are performed to measure the optimal pulse length and to use pump-probe pulse configurations to enhance the ablation plume dynamics. Results indicate that despite some ion population and velocity gains using delayed pump-probe pulse sequencing, measured forces show a corresponding reduction in applied thrust, and hence any enhancements are unlikely. In the course of these measurements, an ablation energy loss mechanism known as time-delayed phase explosions was discovered, and results are shown to challenge existing phase explosion theory.