Initiation of laser-supported-detonation (LSD) waves

Abstract

Experimental results of plasma diagnostic measurements taken during the early development of laser‐supported‐detonation (LSD) waves in TEA‐CO₂ laser irradiations of solid surfaces in air are reported. Photographic and sample examination results for irradiations of practical aluminum surfaces indicate that the initiation process is highly local at surface features which are heated preferentially. The intensity dependence of time to plasma initiation was measured using target‐electron emission, target‐hole transmission, and electrostatic‐probe response as indicators of the initiation event timing. The initiation time for aluminum was found to be in the range 25–70 nsec for peak‐power densities in the range 0.4 to 4×10⁸ W/cm². Emission‐spectroscopy results indicate the presence of neutral aluminum vapor at late times in the pulse, but no ionized aluminum was detected at atmospheric pressure. The experimental results for aluminum are found to be consistent with a simple theoretical model, wherein electrons are thermionically emitted at preferentially heated surface features. The emitted electrons serve as priming electrons in a cascade air breakdown. A limited amount of data on metals other than aluminum are also presented.