Experimental and theoretical studies of laser propulsion phenomenology

The work performed in this 2-year effort involved two tasks: one dealing with issues related to laser energy absorption in a pulsed thruster and the other dealing with issues related to laser energy absorption in a CW thruster. For the pulsed studies, the first year's effort involved theoretical and experimental investigations of the threshold requirements for achieving laser-induced gas breakdown at short wavelengths (< 1 micrometer). With these breakdown studies completed, the next step, initiated in the second year, was to determine the degree of pulsed laser energy absorption that can be achieved in the laser-initiated plasma. Toward this end pulsed laser energy deposition studies were performed using a pulsed Nd:glass laser as the energy source (wavelength = 1.05 micrometers, pulse duration = 20 ns) and high pressure hydrogen and argon gases as the absorption media. The percentage of laser energy absorbed in the laser-produced plasma was determined from measurements of the fraction of laser energy transmitted through the breakdown region, as well as from optical interferometric measurements to determine the strength of the laser-produced blast wave. The initially deposited energy was inferred from the measured blast wave trajectories by comparing them with predicted trajectories calculated by a detailed hydrodynamic model. Results are presented for the energy deposition efficiency achieved in argon and hydrogen as a function of initial gas pressure (0.3 atm < P < 10 atm).