Target physics for the megajoule laser (LMJ)

Abstract

First we report two studies aimed at preparing laser integration line (LIL) experiments (LIL is the prototype of LMJ): deflection of a beam with and without 'longitudinal' smoothing (associated with focusing by gratings) and the radiation temperature, Tr, in a hohlraum with long pulses (10–20 ns). Experimentally, we did not see any Langmuir decay instability able to saturate the stimulated Raman scattering in a gas bag irradiated with the Omega laser. Next, in our hydro-code FCI2, we implemented an improved version of the non-LTE atomic physics model: the change in Tr in the hohlraum is negligible, but now the simulations are in agreement with experiments on x-ray conversion and on Rayleigh–Taylor instabilities (RTIs) in a spherical geometry. The RTIs in polyimide foil at 70 µm were understood, but not those at 30 µm. Finally, for the target design, we confirm the hydro-stability of the four targets of the operational domain of LMJ: the doped CH ablator of the nominal target can withstand a roughness in the range 50–100 nm. The robustness studies use 19 uncertainties coming from the laser power, the beam pointing and the target fabrication. Finally, the burning of DT has been studied in detail, identifying three regimes.