Seeing Inside Detonating Explosives

Although the theory of ideal detonations posits a stable detonation that runs freely through the explosive, real detonations can encounter a variety of perturbations; among others, interactions with other shock waves, which reveal how chemically-sustained detonations relax after two shocks meet, and mechanical inhomogeneities in plastic-bonded explosives that can create so-called hot spots from which detonations can bloom. We have developed a tabletop high-throughput shock experiment that uses laser-launched hypervelocity flyer plates (0-6 km/s) to generate shocks in tiny (microgram) samples of explosives while optical diagnostics with high time (ns) and space (micrometer) resolution measure velocities and temperatures while nanosecond video images shock interactions. In this talk I will briefly describe this shock microscope and give two examples. In the first, liquid nitromethane (NM) is driven to detonation at 13 GPa and 3500K. A powerful shock causes the NM to explode in two stages. As the shock propagates it collides with two successive shocks generated by the NM explosion forming a cellular structure. The size of the cells shows us when and where these two shocks were generated. In the second, a thin wafer of a plastic-bonded explosive based on HMX ( 1,3,5,7-Tetranitro-1,3,5,7-tetrazocane) crystals with a polymer binder is embedded in a transparent polymer. When a powerful shock passes over this wafer it produces hot spots that can grow into a deflagration. Using the thin wafer we can see every HMX crystal in the explosive so we can determine where hot spots are created, their temperatures, and how they grow or extinguish.