LLE Review 89

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Highlights

LLE Review 89, covering October to December 2001, begins with an article describing a project that was initiated by LLE’s staff photographer when he began taking visible light photographs of imploding OMEGA targets for aesthetic rather than scientific reasons. These beautiful images were used to communicate LLE’s mission to the general public. A closer examination of the images revealed a one-to-one correspondence between the bright spots in the image and each of the 60 laser beams. The intensity of the bright spots has been related to refraction and absorption in the plasma surrounding the imploding target. These photographs are now proving to be the basis of a new laser—plasma interaction diagnostic.

Additional research presented in this volume includes the following:

• An analytical model of the nonlinear bubble evolution of single-mode, classical Rayleigh-Taylor (RT) instability at arbitrary Atwood numbers is described. The model follows the continuous evolution of bubbles from the early exponential growth to the nonlinear regime when the bubble velocity saturates.
• A reduced-autocorrelation phase plate, calculated with a novel algorithm, for OMEGA and the National Ignition Facility improves the time-averages rms laser nonuniformity. The reduced autocorrelation phase design shifts the speckle energy up into the higher spatial frequencies where smoothing by spectral dispersion (SSD) and thermal smoothing in the target corona are most efficient.
• LLE’s Tritium Recovery System is used to clean up the various exhaust streams and to control tritium activity in the gloveboxes. This system uses the best-available technologies to extract tritium from inert gas streams in the elemental form and reduces the volumes of effluent to the maximum extent practical.
• A SiO₂-thin-film system with absorbing gold nanoparticles was used to study the connection between the pulsed-laser energy absorption process and film damage morphology. The initial absorption was confined to the nanoscale defect. Energy absorbed by the defect quickly heats the surrounding matrix, changing it from a transparent to an absorbing media, which creates a positive-feedback mechanism that leads to crater formation.
• The time-resolved dynamics of the superconducting-to-resistive transition in dc-based epitaxial YBa₂Cu₃O_7—x (YBCO) microbridges, excited by nanosecond-long current pulses, leads to resistive switching. The experimental dynamics were in agreement with the Geier and Schön theory, modified to include the dc bias.
• A series of thin, hydrogenated amorphous carbon films have been deposited using the saddle-field deposition configuration. These films are a precursor to depositing tritiated films. Smooth, low-porosity films up to 15 µm thick and with densities up to 2 g/cc have been grown