LLE Review 125

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Highlights

This volume of the LLE Review, covering October–December 2010, features “Measurements and Timing of Multiple Spherically Converging Shock Waves in Liquid Deuterium”. The article reports on experiments that measure the velocity and timing of multiple converging shock waves inside spherical targets filled with liquid (cryogenic) deuterium. The experiments were performed on the OMEGA Laser System and the velocity interferometry system for any reflector (VISAR) was used to detect a Doppler shift of an optical probe beam that reflects off the shock front. A streaked optical pyrometer was also used to observe the optical self-emission from shocks in the deuterium, providing an independent measurement of shock timing. The results show that shock timing can be measured to better than the ±50-ps precision required for ignition targets, and this technique is being directly applied to full-scale experiments to tune hohlraum-driven ignition targets on the NIF laser system.

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

• Experimental data from OMEGA on the first time-gated proton radiography of the spatial structure and temporal evolution of how the fill gas compresses the wall-blowoff, inhibits plasma-jet formation, and impedes plasma stagnation in the hohlraum interior. The results demonstrate the important roles of spontaneously generated electric and magnetic fields in the hohlraum dynamics and capsule implosion and provide novel insight into the effects of fill gas on x-ray driven implosions. This will have an important impact on upcoming ignition experiments on the NIF.
• Cone-in-shell fast-ignitor experiments conducted at the OMEGA Laser Facility using the short-pulse OMEGA EP beam are a promising approach to bring the fast-electron source location as close as possible to the fuel assembly. The re-entrant cone allows for a high-energy petawatt laser pulse to propagate as close as possible to the dense core, avoiding the need to channel the laser beam through a large region of plasma material. For the smaller 10-µm tip diameter targets and a properly timed short pulse, a factor-of-4 increase in neutron yield was observed by short-pulse heating in a narrow (~100-ps) time window close to peak compression. This work addresses a few key issues for fast ignition including cone survivability and the trade-off of matching the cone thickness to the fast-electron energy.
• Photothermal heterodyne imaging (PHI) was used to evaluate the spatial distribution of absorbers in hafnia monolayers. Insight into the nature and distribution of damage precursors is valuable to further improve the material’s damage resistance. Using PHI data and AFM mapping of damage-initiation sites, an upper limit for absorber separation was calculated to be ~0.1 µm. By comparing heterodyne signals for different film thicknesses, it was determined that hafnia/silica interfacial absorption is not a major factor in damage initiation, but the main contribution comes from absorption inside the hafnia film.
• The large tunable, terahertz electro-optic response in cadmium manganese telluride (Cd,Mn)Te single crystals was measured. The measurements demonstrated CMT’s exceptionally large EO Pockels effect and showed that the EO sensitivity can be magnified for a particular probe wavelength using band-gap engineering.
• Improvements to optimize the long-pulse, on-target energy of the OMEGA EP laser are summarized. The improvement efforts involve procuring higher UV-damage-threshold beam-transport optics, obtaining better near-field beam profiles, and developing simulation tools to provide rapid prediction of laser-system performance during shot operations. These enhancements have resulted in better beam quality, greater reliability, and improved efficiency during shot operations.