Ablative Impulse Characteristics of Polyacetal with Repetitive CO2 Laser Pulses

Abstract

Impulse characteristics for a polyacetal target repetitively irradiated with pulses from a transversely excited atmospheric CO 2 laser were experimentally investigated. About 10-J laser pulses were repetitively irradiated up to 110 times onto a 6.6- or 8.6-mm-diam spot on the target, which was mounted on 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, 100 pulses were irradiated in various burst modes. Successive laser pulses in a burst were irradiated at a repetition frequency of 50 Hz. The time interval between successive bursts was greater than 3 min. The momentum coupling coefficient C m was almost independent of the burst mode. With a fluence of 18.8 J/cm 2 , C m gradually increased with an increasing total number ofpulses, reaching 220 μN. s/J at an ambient pressure of 10 -2 Pa and 145 μN - ·s/J in the atmosphere. When the fluence was 31.8 J/cm 2 , C m began to decrease after about 50 pulses. C m 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.