LLE Review 145

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Highlights

This volume of the LLE Review, covering October–December 2015, features “Direct Drive: Simulations and Results from the National Ignition Facility.” This article reports on LLE’s investigations of direct-drive implosions at the National Ignition Facility (NIF) in order to validate models related to implosion velocities and the magnitude of hot-electron preheat. Implosion experiments indicate that the energetics are well modeled when cross-beam energy transfer is included in the simulation. Trajectories from backlit images are accurately predicted; the velocities are lower than theory, but discrepancies likely result from nonuniformity growth seeded by laser imprint.

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

• Hydrodynamic simulations are used to develop an experimental platform for investigation of two-plasmon–decay and other laser–plasma instabilities. Proposed experiments utilize planar plastic targets with an embedded Mo layer to characterize the generation of hot electrons, approximating conditions of a polar direct-drive implosion.
• A first-principles study of the ionization and thermal conductivity of polystyrene (CH) over a wide range of plasma conditions is undertaken. Hydrodynamic simulations of cryogenic deuterium–tritium targets with CH ablators on OMEGA and the NIF predict an ~20% variation in target performance in terms of hot-spot pressure and neutron yield (gain) relative to traditional model simulations.
• Experiments on the OMEGA Laser System to evaluate cryogenic implosions that are hydrodynamically equivalent to spherical ignition designs for the NIF are reported. Current cryogenic implosions on OMEGA have reached 56 Gbar, but ignition in a direct-drive cryogenic implosion on the NIF will require central stagnation pressures in excess of 100 Gbar. Ongoing efforts to demonstrate hydrodynamic equivalence on OMEGA require lower coupling losses caused by cross-beam energy transfer and minimized long-wavelength nonuniformity.
• A next-generation neutron temporal diagnostic (NTD) was installed at the Omega Laser Facility to determine the hot-spot pressure achieved in inertial confinement fusion experiments and assess the implosion quality. This NTD is based on a plastic scintillator, which converts the neutron kinetic energy to light that is relayed to a streak camera. An ~200× reduction in neutron background was observed during the first high-yield DT cryogenic implosions compared to the current NTD installation on OMEGA, with an impulse response of ~40±10 ps.
• The contribution of thin-film interfaces to near-ultraviolet absorption is evaluated based on pulsed-laser–induced damage for ion-beam–sputtered and electron-beam–evaporated coatings. Film characterization shows a small contribution to total absorption from the interfaces relative to that of the HfO₂ film material, with a higher damage resistance in the seven-layer coating compared to that of a single-layer HfO₂ film.
• A simple diagnostic to characterize 1-D chromatic aberrations in a broadband beam is described. A Ronchi grating is placed in front of a spectrometer entrance slit to provide spatially coupled spectral phase information. The radial-group delay of a refractive system and the pulse-front delay of a wedged glass plate have been accurately characterized in a demonstration.