LLE Review 135

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Highlights

This volume of the LLE Review, covering April–June 2013, features “Improving Cryogenic DT Implosion Performance on OMEGA,” which describes a flexible direct-drive target platform that is used to implode cryogenic deuterium–tritium (DT) capsules on the OMEGA laser. The goal of these experiments is to demonstrate ignition hydrodynamically equivalent performance where the laser drive intensity, the implosion velocity, the fuel adiabat, and the in-flight aspect ratio (IFAR) are the same as those for a 1.5-MJ target designed to ignite on the National Ignition Facility. The implosions span a broad region of design space to study target performance as a function of shell stability (adiabat) and implosion velocity.

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

• Polar-drive experiments with shimmed targets on OMEGA demonstrate the efficacy of using a shimmed target (one with a contoured shell thickness) to improve the symmetry of the implosion.
• Localized time-resolved electron-temperature measurements show that ω/2 spectra and images provide powerful direct-drive, coronal plasma diagnostic for inertial confinement fusion. Spatially and temporally resolved half-harmonic spectra and images of laser-driven implosions demonstrate evidence of local, multi-beam driven two-plasmon–decay (TPD) instability.
• The divergence of the fast electrons is deduced from a series of shots on the OMEGA Laser System, where Mo-coated shells of increasing diameter (D) were embedded within an outer CH shell. The intensity of the Mo-K_α line and the hard x-ray radiation were found to increase approximately as ~D², indicating wide divergence of the fast electrons.
• The published measurements and models of the cross section for electrons causing K_α emission from copper are reviewed to find a suitable expression for analyzing K_α-emission measurements in laser–solid experiments at peak intensities above 10¹⁸ W/cm². Two different limiting forms for the cross section at relativistic energies are used and existing measurements do not give a clear indication as to which is correct. Comparison with the limiting form of electron stopping power indicates an alternative relativistic form and indicates that the density effect correction will be important in copper above 10 MeV.
• During the fabrication of multilayer-dielectric (MLD) thin-film coated optics, such as the diffraction gratings used in OMEGA EP’s pulse compressors, acid piranha cleaning can lead to the formation of chemically induced delamination defects. The causes of these defects are investigated and a mechanism for the deformation and failure of the MLD coating in response to hydrogen peroxide in the cleaning solution is described.