Abstract

Ground-based lasers can be used to propel rockets into space. The double-pulse laser-supported detonation¹ (LSD) wave concept for a thruster involves the use of a first laser pulse to vaporize the propellant followed by a delayed second pulse that ignites a plasma. Large specific impulses can then be achieved by expansion of the gas. The means of igniting plasmas at low fluences have been analyzed. An aluminum flake embedded in the (solid) propellant of 0.2-μm thickness will vaporize at fluences as low as 2 J/cm², and the metallic vapor will break down at relatively low irradiances (/ ≳ 5 × 10⁷ W/cm² for λ = 10.6 μm). Maintenance of an LSD wave requires complete absorption of the laser beam in an absorption front traveling supersonically with respect to the upstream gas.¹ Gas heating obtains by electron-neutral and electron-ion inverse bremsstrahlung absorption of laser radiation. Calculations of propellant heating and ignition from conditions behind the shock to Chapman jouguet conditions have been carried out for H₂O. The presence of a small amount of easily ionizable seed can lower the maintenance thresholds. We find // ρ ≳ 5 × 10⁸W cm/g for 1% lithium seed in H₂O over a density range of 10^–4 < ρ < 10^–2 g/cm³.

© 1988 Optical Society of America