Measurement of Simultaneous PDV and VISAR Velocity Histories for Projectile-Driven Pressure Amplifiers

Experiments to measure the shock release velocity of pressure amplifiers are described, utilising First Light Fusion's (FLF) 38mm bore, two-stage light-gas gun. The amplifier is a key component of FLF's ability to convert the impact from a hypervelocity projectile to the pressures and velocities needed to drive fusion targets, and thus represents a critical system to understand in order to ensure the predictive capability of our hydrodynamic codes. Owing to the significant intensity emitted by the shock wave releasing from the output of the amplifier, a push-pull point VISAR system was developed to maximise the measurement signal-noise levels. An optical probe was additionally designed to allow PDV and VISAR to be fielded simultaneously, leveraging a dichroic mirror to make spatially averaged velocity measurements of the same 200µm spot. Samples of both quartz and water were used as window materials at the output of the amplifier, which produce an optically reflective surface comoving with the shock wave due to the pressures achieved. Velocity data using both VISAR and PDV were obtained across the experimental campaign, with inferred velocities in good agreement with hydrodynamic simulations. Finally, the limitations of this combined diagnostic are discussed, alongside future improvements to our velocimetry capability.