Ablative laser propulsion: determination of specific impulse from plasma imaging

Abstract

This work summarizes the combination of experimental and digital image processing technique developed for determination of plasma expansion velocity angular profiles. Such profiles were used further for assessment of specific impulses for ablative laser propulsion. The technique uses time-resolved intensified charge-coupled device (ICCD) camera with 18 ns minimum time delay, 100 &#956;m spatial resolution, and 5 ns gating speed. The plasma was formed in vacuum (~ 3x10^-3 Torr) by focusing output pulses of a laser system (100-ps pulsewidth at 532 nm wavelength and ~35 mJ energy) on surfaces of C (graphite), Al, Si, Fe, Cu, Zn, Sn, and Pb targets. Plasma expansion velocity profiles were derived from plume edge contours. Specific impulse (<i>I_sp</i>) was then deduced from the profiles. New <i>I_sp</i> data appeared in excellent agreement with specific impulses derived from force measurements, conducted earlier. Observed angular profiles of plasma edge velocity and integral intensity are reported and discussed.