Forward Modeling Fringe Analysis For Improved High Speed Velocimetry

When near-instantaneous shocks are recorded by a Doppler velocity interferometer (VISAR) they typically exceed the detector’s ability to react, and “skipped fringes" result where its visibility briefly reduces. Traditionally, replacing skipped fringes required guesswork in analysis, which increased arrival time errors. Secondly, the use of long but velocity-sensitive interferometer delays with fast detectors which can resolve the delay has traditionally been avoided, because of the fear of confusing the arrival time signal. But shorter delays produce smaller fringe phase shift per velocity and thus can decrease velocity precision. We realize that while some loss of fringe information occurs at shock events, this is often just a partial loss, and the residual fringe information can still hold valuable information. We describe a forward model (FM) of the interferometer action and detector blurring that assists with VISAR fringe analysis at skipping events: (1) more precise shock arrival times, (2) even with long delays, and (3) improved ghost subtraction which improves accuracy over a broad time region. We demonstrate the utility of FM on National Ignition Facility (NIF) and Laboratory for Laser Energetics (LLE) Omega shots.