Experimental validation of a diagnostic technique for tuning the fourth shock timing on National Ignition Facility

Abstract

Capsule implosions on the National Ignition Facility (NIF) [Lindl et al., Phys. Plasmas 11, 339 (2004)] will be driven with a carefully tailored sequence of four shock waves that must be timed to very high precision in order to keep the fuel on a low adiabat. The Hohlraum conditions present during the first three shocks allow for a very accurate and direct diagnosis of the strength and timing of each individual shock by velocity interferometry. Experimental validation of this diagnostic technique on the OMEGA Laser Facility [Boehly et al., Opt. Commun. 133, 495 (1997)] has been reported in [Boehly et al., Phys. Plasmas 16, 056302 (2009)]. The Hohlraum environment present during the launch and propagation of the final shock, by contrast, is much more severe and will not permit diagnosis by the same technique. A new, closely related technique has been proposed for measuring and tuning the strength and timing of the fourth shock. Experiments to test this technique under NIF-relevant conditions have also been performed on OMEGA. The result of these experiments and a comparison to numerical simulations is presented, validating this concept.