Study of shock-coalescence on the LIL laser facility

Abstract

Abstract. We use the LIL (Ligne d'Intégration Laser) facility to study the coalescence of two planar shocks in an indirectly-driven planar sample of polystyrene. This experiment represents the preliminary stage of the future shock-timing campaign for the Laser Megajoule (LMJ). The main objectives are to validate the experimental concept and to test the numerical simulations. We used a gold spherical hohlraum to convert into X-ray the 351 nm wavelength laser pulse and to initiate the two shocks in the sample. To access time resolved shock velocities and temperature, we used two rear-side diagnostics: a VISAR (Velocity Interferometer System for Any Reflection) working at two different wavelengths and a streaked optical selfemission diagnostic. We observed the coalesced shock, in good agreement with the numerical simulations. We also observed a loss of signal during the first nanoseconds probably due to sample heating from the hohlraum X-ray flux. CONTEXTThe LMJ, like the National Ignition Facility (NIF) in the USA, is designed to achieve thermonuclear ignition in an indirect drive scheme, by imploding a spherical capsule filled with a deuterium-tritium (DT) mixture. The best way to efficiently compress the fuel is quasi-isentropic compression. To approximate this compression, a series of four successive centripetal shocks are sent into the sample [1]. The shocks need to be accurately controlled in timing and strength to coalesce at the same time in the gaseous DT close to the inner surface of the cryogenic DT. The first three shocks allow compressing the capsule quasi-isentropically, keeping the cryogenic DT on a low adiabat. The fourth shock drives the final implosion until the ignition. This scheme requires to adjust the laser power time history entering in the gold hohlraum.In this experiment, we seeked to validate the experimental concept in planar geometry and indirect drive for the first two shocks using a truncated LMJ-like pulse shape and to compare the results with numerical simulations. EXPERIMENTAL SETUPFor this experiment we used the LIL laser facility at 3 (351 nm) with an energy of 10 kJ in 12 ns. A spherical gold hohlraum was used to obtain a radiation temperature similar to the first part of the nominal LMJ-drive. Two planar shocks are sent in a planar sample of polystyrene (PS -C 8 H 8 ) fixed on a

Stabilization of a single-frequency generation of the injection-seeded pulsed solid-state laser

“…This allows using single−frequency radiation sources in creating unequal−arm interferometers. Such interferometers are used as measuring systems in experiments devoted to studying the effect of megabar shock waves on matter in the Physics of High Energy Densities [3][4][5][6]. In this case, short duration of the processes under study (∼ 10 ns) requires using as radiation sources only pulsed lasers, since only they can provide the target illumination sufficient for detection.…”