Comparison of Ablation Performance in Laser Lightcraft and Standardized Mini‐Thruster

Experiments on laser ablation of black and white Delrin® with a 10.6‐micron laser beam from a CO2 electric discharge laser are reported. Mass ablation and thrust generation (impulse) were accurately measured as a function of input laser energy in single‐shot experiments. The efficiency of conversion of laser energy to jet kinetic energy depended on the geometry of the energy absorption/conversion zone. The standard geometry, an axisymmetric mini‐thruster with 13‐degree conical half angle and 8:1 expansion ratio, produced ∼65 % conversion efficiency. The extensively‐studied 10‐cm focal diameter Lightcraft (with inverted paraboloid, plug‐nozzle geometry) produced ∼50% conversion efficiency. The upper limit to energy conversion was theoretically computed with a one‐dimensional chemical equilibrium code to be 73% for the well‐defined mini‐thruster geometry. Thus, total losses amount to ∼8% in the mini thruster and ∼23% in the Lightcraft. In these experiments a near‐exact match of coupling coefficients, ∼1%, w...