Nanosecond Scale Optical Pulse Separations for Holographic Investigation of High-Speed Transient Events

The dynamics of detonation for dispersed solid particulate explosives are not well understood. These explosives, used for mine neutralization, are comprised of a fine, solid particulate dust which is dispersed as a cloud over a given area. When detonation is initiated in some portion of the cloud, the ensuing detonation wave propagates throughout the entire cloud and results in an explosion, generating a tremendous pressure which serves to destroy any land mines present. However, the mechanism with which individual explosive particles interact to sustain detonation in these solid dispersed particle explosives is not clear. In liquids, for example, it is known that coupling between a propagating shock front and the chemical reaction front which follows is maintained by a “stripping” action of the shock front on the explosive droplets so that an ultrafine mist is produced and swept along in the convective flow behind the shock front [1]. Thus, upon ignition and combustion of this ultrafine mist, coupling is maintained between the shock and reaction fronts. In the case of solid particulate explosives, it is unclear whether it is the shock front alone or a combination of the shock and reaction fronts which initiate subsequent particle detonation. In an effort to more closely examine the generation and propagation of the shock and reaction fronts arising from single particle detonation, and the interaction of these fronts with neighboring particles, high-speed holography is being used to visualize and study these transient phenomena. The shock front generated by detonation of a single explosive particle (100p.m diameter) may exist as a true shock front only 10-50 particle diameters away, and may propagate at velocities approaching 5000m/s. As a result, the holographic techniques used to study these dispersed particle explosives need be capable of capturing events with lifetimes shorter than 1gs, thus requiring optical pulse separations between 10 and 200 ns.