Measurement of CO2-laser-induced shock pressures above and below LSD-wave thresholds

Abstract

TEA‐CO₂‐laser‐induced shock pressures in aluminum, cellulose acetate, and polymethyl methacrylate (PMMA) targets were measured by both backsurface and in‐material techniques as functions of peak laser power density and ambient air pressure. We report the first experimental observation of shock‐pressure profiles in metallic and nonmetallic targets above and below the thresholds for laser‐supported‐detonation‐wave (LSD‐wave) initiation. For aluminum irradiations above threshold conditions, the measured peak pressure is consistently below that predicted by cylindrical blast‐wave theory, which can be explained in terms of LSD initiation at local surface defect sites and imperfections. Surface‐vapor blow‐off dominates the shock‐pressure response below threshold conditions for both aluminum and the plastics. LSD‐wave blockage of the surface vaporization process leads to a decrease in the peak pressure above the initiation threshold for cellulose acetate. The largest peak pressures (accompanied by severe backsurface spallation) are thus recorded just below threshold irradiance conditions for LSD‐wave initiation in plastics. LSD initiation times inferred from these shock‐pressure measurements are in reasonable agreement with values determined by other methods and support the contention that initiation on practical metallic and nonmetallic surfaces is a highly local phenomenon.