High-speed velocimetry inside imploding cylindrical liners*

Dynamic planar compression is conceptually simple but difficult to maintain at extreme pressure ($>$5~Mbar). Higher pressures are attainable with imploding cylindrical liners, using Photonic Doppler velocimetry (PDV) to track the liner interior. PDV measures Doppler shift directly---1~GHz of beat frequency for every 1~km/s of velocity---requiring a special ``leapfrog'' approach for liners traveling in excess of 20~km/s. Single-point and multi-point PDV measurements have been performed in aluminum, beryllium, and tantalum liners under ramp compression, and the technique can readily applied to other implosion experiments. Combined with electrical current diagnostics, these measurements test thermodynamic equations of state at pressures up to 10~MBar and beyond.