Specific Impulse and Other Characteristics of Elementary Propellants for Ablative Laser Propulsion

Abstract

This work was performed as a continuation of ablative laser propulsion (ALP) research recently revived by the authors. An ALP-based space vehicle would be driven by high-energy, short-duration (10 -10 s and less) laser pulses, focused on a solid propellant. Under such irradiation conditions, plasma and laser pulses are temporally separated, and the direct ablation of solid propellant dominates other possible momentum transfer mechanisms. The studies were performed for various elemental targets, ablated at irradiances up to ∼3 × 10 13 W/cm 2 delivered by 100-ps laser pulses at a wavelength of 532 nm. The data, collected predominantly from time-of-flight technique, include ion velocities, number densities, ion density angular distributions, and mass-removal rates. From these data, specific impulses and net momenta exerted on targets by ablation were calculated. For the irradiation conditions used, the highest I sp and lowest mass-removal rates were achieved for low-Z targets, whereas high-Z elements provided higher momenta. The upper limits for I sp derived from ion velocity reached 2 x 10 4 s with carbon (graphite). Ion energies varied from element to element in the range of ∼0.4-2.0 keV, with ion fractions of ∼0.8-3.8% and mass-removal rates in the range of 0.1-3.0 μg per laser pulse. Ion velocities exhibit Maxwellian distributions, whereas ion number densities in the plasma plume were fitted to a cosine function. All studied characteristics indicate that high-I sp ALP must be based on solid low-Z propellants such as a carbon or aluminum.