Time-resolved and Integrated Laser Ablative Impulse Performance of Polyacetal

Abstract

Impulse generation mechanisms and characteristics in pulsed laser ablation were experimentally studied. Using the Velocity Interferometer System for Any Reflector (VISAR) and framing Schlieren visualization, the processes of the impulse generation were diagnosed with the time resolution of 4 ns and 10 ns, respectively. The fluence was from 13 to 24 J/cm. In most cases, the propulsive force generated even after the primary laser power peak significantly contributed to the total impulse. When a TEA CO2 laser pulse was directed onto the polyacetal target, the impulse increased by a factor of ten in comparison with the aluminum target, yielding a ‘local’ momentum coupling coefficient at the spot center exceeding 400 μN-s/J. When P0 was at atmospheric, 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 Pa. Impulse induced by repetative CO2 laser pulses was measured using a torsion-type impulse balance. In the first several laser pulses the impulse and ablation rate were strongly influenced by the initial conditions of the target surface. After ten cleaning pulses, one hundred pulses were irradiated in various burst modes. Successive laser pulses in a burst were irradiated at a repetition frequency of 50 Hz. The ‘spatially-integrated’ momentum coupling coefficient was almost independent of burst mode. With a fluence of 18.8 J/cm, Cm gradually increased with increasing total number of pulses, reaching 220 μN-s/J at an ambient pressure of 10 Pa, and 145 μN-s/J in the atmosphere. When the fluence was 31.8 J/cm, Cm began to decrease after about 50 pulses. Cm was smaller for a smaller spot diameter. Those impulse characteristics were closely associated with target surface morphology and fluid dynamics of the ablation plume and the ambient air.