LLE Review 105

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Highlights

This volume of the LLE Review, covering October–December 2005, features results of measured dependence of nuclear burn region size on implosion parameters in inertial confinement fusion experiments. Radial profiles of nuclear burn in directly driven implosions have been systematically studied for the first time using a proton emission imaging system at the OMEGA Laser Facility. The system is sensitive to energetic 14.7-MeV protons from the fusion of deuterium (D) and 3-helium (³He). Clear relationships have been identified between variations in the size of the burn region and variations in such experimental parameters as capsule size, shell composition and thickness, gas-fill pressure, and laser energy.

Additional highlights of research presented in this issue include the following:

• The Rayleigh–Taylor growth measurements of 3-D modulations in a nonlinear regime are presented. The measured modulation Fourier spectra and nonlinear growth velocities are in excellent agreement with Haan’s model. In real-space analysis, the bubble merger was quantified by a self-similar evolution of bubble size distributions, in agreement with the Alon–Oron–Shvarts theoretical predictions.
• Results of studies of isotopic fractionation during solidification of H₂-HD-D₂ mixtures are reported. An isotopic fractionation reduces the efficiency of the fusion reaction in future cryogenic D-T targets. It is found that frozen H-D mixtures have spatial concentration gradients of the order 0.02 to 0.05 molecular fraction per millimeter, which points to little separation of isotopes.
• Implications of hydrogen fractionation in ICF ignition target designs are discussed. A numerical investigation of the effects that fractionation has on hot-spot formation, ignition, and burn in ICF target designs indicates that small levels of fractionation (~10%) are acceptable for ignition performance on the NIF.
• Polar-direct-drive (PDD) simulations and experiments on the OMEGA Laser System are described. Forty OMEGA beams arranged in six rings to emulate the NIF x-ray–drive configuration are used to perform direct-drive implosions of CH shells filled with D₂ gas. Simulations performed with DRACO code are in good agreement with experimental x-ray radiographs and show ignition with a gain of 20 and the development of a 40-µm-radius, 10-keV region with a neutron-averaged ρr of 1270 mg/cm² near stagnation.
• Surface features of tungsten carbide composites processed by bound abrasive microgrinding and magnetorheological finishing (MRF) are analyzed. It was found that the peak-to-valley microroughness of the surface gives a measure of the deformed layer depth. MRF spots revealed the true depth of the grinding-induced deformed surface layer.