Reflectivity extinction in strongly-shocked liquid deuterium

Optical velocimetry from reflecting shock fronts in transparent media is a common application at high energy density facilities such as the National Ignition Facility (NIF). The primary application on the NIF is to tune the shock sequence for capsule implosions in inertial confinement fusion experiments. Shock tuning on the NIF is limited by reflectivity extinction or self-blanking to a maximum shock pressure of 3 TPa and velocity of 150 km/s in liquid deuterium. With the possibility for driving shocks well into the multi-TPa range for many materials at current and future facilities an understanding of the limits of optical velocimetry is becoming important. The underlying mechanism for reflectivity extinction is upstream photoionization of the unshocked material; however, the details involve a level of complexity that is not immediately evident. We describe a model for shock front reflectivity in liquid deuterium that matches observations and predicts a wavelength dependence. Application of the same modeling framework is possible for other common transparent shock materials such as quartz, fused silica, diamond and plastics.