The Alp-Pals Project: Optimal Coupling For Laser Propulsion

Since the early 1970s ablative laser propulsion (ALP) has promised to revolutionize space travel by reducing the 30:1 propellant/payload ratio needed for near-earth orbit by up to a factor of 50, by leaving the power source on the ground. But the necessary sub-ns high average power lasers were not available. Dramatic recent progress in laser diodes for pumping solid-state lasers is changing that. Recent results from military laser weapons R&D programs, combined with progress on ceramic disk lasers, suddenly promise lasers powerful enough for automobile-size, if not space shuttle-size payloads, not only the 4-10 kg "microsatellites" foreseen just a few years ago. For ALP, the 1.6-mu m Er:YAG laser resonantly pumped by InP diode lasers is especially promising. Prior coupling experiments have demonstrated adequate coupling coefficients and specific impulses, but were done with too long pulses and too low pulse energies. The properties of ions produced and the ablated surface were generally not measured but are necessary for understanding and modeling propulsion properties. ALP-PALS will realistically measure ALP parameters using the Prague Asterix Laser System (PALS) high power photodissociation iodine laser (lambda = 1.315 mu m, E-L <= I kJ, tau similar to 400 ps, beam diameter similar to 29 cm, flat beam profile) whose parameters match those required for application. PALS' 1.3-mu m lambda is a little short (vs. 1.53-1.72 gm) but is the closest available and PALS' 2 omega/3 omega capability allows wavelength dependence to be studied.