Didier Galmiche

Rayleigh–Taylor instability evolution in ablatively driven cylindrical implosions

The Rayleigh–Taylor instability is an important limitation in inertial confinement fusion capsule designs. Significant work both theoretically and experimentally has been done to demonstrate the stabilizing effects of material flow through the unstable region. The experimental verification has been done predominantly in planar geometry. Convergent geometry introduces effects not present in planar geometry such as shell thickening and accelerationless growth of modal amplitudes (e.g., Bell–Plesset growth). Amplitude thresholds for the nonlinear regime are reduced, since the wavelength λ of a mode m decreases with convergence λ∼R/m, where R is the radius. Convergent effects have been investigated using an imploding cylinder driven by x-ray ablation on the NOVA laser [J. L. Emmet, W. F. Krupke, and J. B. Trenholme, Sov. J. Quantum Electron. 13, 1 (1983)]. By doping sections of the cylinder with opaque materials, in conjunction with x-ray backlighting, the growth and feedthrough of the perturbations from the ...

Ablation Front Rayleigh- Taylor Growth Experiments in Spherically Convergent Geometry

Experiments were performed on the Nova laser [E. M. Campbell et al., Rev. Sci. Instrum. 57, 2101 (1986)], using indirectly driven capsules mounted in cylindrical gold hohlraums, to measure the Rayleigh–Taylor growth at the ablation front by time-resolved radiography. Modulations were preformed on the surface of Ge-doped plastic capsules. With initial modulation amplitude of 2–2.5 μm, growth factors of about six in optical depth were seen, in agreement with simulations using the radiation hydrocode FCI2 [G. Schurtz, La fusion thermonucleaire inertielle par laser, edited by R. Dautray et al. (Eyrolles, Paris, 1994), Vol. 2]. With initial modulation amplitude of 0.5 μm and a longer drive, growth factors of about 100–150 in optical depth were seen. Comparable planar experiments showed growth factors of about 40 in optical depth. Analytical models predict the observed growth factors.

Gold emissivities for hydrocode applications

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 Averroes 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 ...

Target design for the LMJ

Abstract We recall the main features of the LMJ. By using a simple but global model we determined different shells able to give a thermonuclear yield larger than 15 MJ; this model delimited an operating domain for the laser with a 25% margin to take into account the poorly understood phenomena. The different issues are related to the physics of the shell and to the physics of the hohlraum: optimisation of the shell implosion; laser–plasma interaction in the hohlraum; irradiation uniformity given by the hohlraum (2D simulations); implosion with non-uniformities; robustness against the experimental uncertainties; hydrodynamic instabilities during implosion.

Numerical analysis of spherically convergent Rayleigh-Taylor experiments

In the frame of a CEA/US DOE collaboration, radiation driven spherically convergent experiments were performed on the Nova laser in order to measure the Rayleigh-Taylor growth at the ablation front. Numerical simulations using het 2D Lagrangian code PCI2 have correctly reproduced experiments in moderate convergent geometry. Experiments have addressed convergence ratios up to 4 by considering larger capsules, larger hohlraum and longer laser pulses. Numerical analysis of these high convergence implosions is presented, and the effect of convergence on the Rayleigh-Taylor growth is investigated.