Time-resolved measurements of impulse generation in pulsed laser-ablative propulsion

Impulse generation mechanisms in pulsed laser ablation were experimentally studied using the velocity interferometer system for any reflector and framing Schlieren visualization. The impulse was estimated from the rear surface velocity at the center of the laser irradiated spot The fluence was from 13 to 24 J/cm². In most cases, the propulsive force generated even after the primary laser power peak significantly contributed to the total impulse. With the combination of transversely excited atmospheric C0₂ laser and aluminum target, only air breakdown was induced on the target surface without ablation and the impulse level was low. With decreasing ambient pressure P₀, the impulse also decreased, and eventually vanished. With the combination of Nd: YAG laser and aluminum target, the ablation jet contributed to impulse generation and the impulse did not vanish even at vanishing P ₀. When a transversely excited atmospheric C0₂ laser pulse was directed onto the polyacetal target, the impulse increased by a factor of 10 in comparison with the aluminum target, yielding a momentum coupling coefficient exceeding 400 μN · s/J. When P₀ was at atmospheric pressure, the laser plasma shielded the target surface against the proceeding laser power transmission and the impulse saturated at a lower value than at P₀ = 10^-2 Pa.