The adoption of a non-uniform dopant profile has substantially increased the tolerance to high mode deformations of our baseline indirect-drive design. In addition, a low deuterium-tritium (DT) gas density, obtained by 'dynamic quenching' at 2.3 K below triple point, could partly compensate for the decrease in robustness due to DT ageing. Finally, the net margin regarding all laser and target technological defects is about 2. As soon as a sufficient amount of laser beams and diagnostics is available, we will shoot pre-ignition experiments to tune the point design. We are studying new targets which need less energy for these campaigns.We have estimated different direct-drive schemes using indirect-drive beams. The optimal LMJ polar direct-drive configuration is a 2-cone one and leads to marginally igniting targets. A new 2-cone direct-drive scheme, associated with focal spot zooming, allows us to reach ignition with enough margin.
Recent changes in the manner of performing hohlraum drive experiments have significantly advanced the ability to diagnose, understand and control the x-radiation flux ͑or drive͒ inside a laser heated hohlraum. Comparison of modeling and data from a very broad range of hohlraum experiments indicates that radiation hydrodynamics simulation codes reproduce measurements of time dependent x-radiation flux to about Ϯ10%. This, in turn, indicates that x-ray production and capsule coupling in ignition hohlraums will be very close to expectations. This article discusses the changes to experimental procedures and the broad variety of measurements and tests leading to these findings.
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.
The Radiom model [M. Busquet, Phys Fluids B 5, 4191 (1993)] is designed to provide a radiative-hydrodynamic code with non-local thermodynamic equilibrium (non-LTE) data efficiently by using LTE tables. Comparison with benchmark data [M. Klapisch and A. Bar-Shalom, J. Quant. Spectrosc. Radiat. Transf. 58, 687 (1997)] has shown Radiom to be inaccurate far from LTE and for heavy ions. In particular, the emissivity was found to be strongly underestimated. A recent algorithm, Gondor [C. Bowen and P. Kaiser, J. Quant. Spectrosc. Radiat. Transf. 81, 85 (2003)], was introduced to improve the gold non-LTE ionization and corresponding opacity. It relies on fitting the collisional ionization rate to reproduce benchmark data given by the Averroès superconfiguration code [O. Peyrusse, J. Phys. B 33, 4303 (2000)]. Gondor is extended here to gold emissivity calculations, with two simple modifications of the two-level atom line source function used by Radiom: (a) a larger collisional excitation rate and (b) the addition of a Planckian source term, fitted to spectrally integrated Averroès emissivity data. This approach improves the agreement between experiments and hydrodynamic simulations.
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