Short Shock Experiments and Modeling of Initiation in the HMX Based Explosive PBX 9501

We present results from a series of gas-gun driven plate impact experiments designed to measure the initiation response of PBX 9501 (95 wt.\% HMX, 2.5 wt.\% estane and 2.5 wt.\% nitroplasticizer) to short duration shocks. Embedded electromagnetic particle velocity gauges measured the reactive growth and initiation progress. Photonic Doppler Velocimetry (PDV) measured a particle velocity wave profile at the interface of the $\approx$ 23 mm thick PBX 9501 sample and a Lithium Fluoride (LiF) window. Impact stress in all three experiments was 4.4 GPa. Pulse durations of 0.5, 0.36, and 0.25 $\mu$s were created using 1.0, 0.75, and 0.5 mm thick Kel-F81 flyers backed by syntactic foam. The 0.5 $\mu$s pulse transited to detonation at $t_D=$ 2.08 $\mu$s, $x_D =$ 9.32 mm, considerably beyond the coordinates of $t_D=$ 1.4 $\mu$s, $x_D =$ 6.2 mm, expected for a long pulse. The 0.25 $\mu$s pulse failed to transition to detonation while the 0.36 $\mu$s pulse transitioned to a detonation at a position slightly less than the sample thickness of 23 mm. These experiments provide a more stringent test for reactive burn models than do the long pulse experiments used to generate the Pop-plot.