LLE Review 108

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Highlights

This volume of the LLE Review, covering July–September 2006, features a progress report on cryogenic DT and D₂ targets for inertial confinement fusion. The author reports on the development of cryogenic targets since the need was first recognized more than 30 years ago. Virtually all ignition target designs for the National Ignition Facility (NIF) are based on a spherical low-Z ablator containing a solid, cryogenic-fuel layer of deuterium and tritium. Techniques developed at LLE can produce targets with an inner ice surface that meets the surface-smoothness requirement for ignition (<1-µm rms in all modes). Significant progress with the characterization of such targets is also reviewed along with results from the most recent cryogenic implosions that simulations suggest have yielded values for ρR_peak as high as 190±20 mg/cm².

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

• Results are presented for a study of the displacement of cryogenic targets from target chamber center (TCC). While beam smoothing and power balancing can ensure highly uniform illumination at TCC, target displacements of 5 µm or more from TCC can unbalance the illumination enough to initiate hydrodynamic instability. Correlation between target vibration at TCC and the response characteristics obtained in their study indicate that the modes of the spider silk are primary cause of displacement.
• A novel imaging technology for measuring E and B fields in laser-produced plasmas using monoenergetic proton radiography is presented. The generation of electromagnetic fields by the interaction of laser light with matter is a process of fundamental interest in high-energy-density physics. They present high-resolution, time-gated radiography images of a plastic foil driven by 10¹⁴ W/cm² laser that imply B fields of 0.5 MG and E fields of 1.5 x 10⁸ V/m. Furthermore, their measurements demonstrate the beneficial focal smoothing effects produced by distributed phase plates for substantially reducing medium-scale chaotic field structure.
• An evaluation of cleaning methods for multilayer-dielectric (MLD) diffraction gratings is presented. MLD diffraction gratings are essential components for the OMEGA EP short-pulse, high-energy laser system, and as such they must have both high optical-diffraction efficiency and high laser-damage threshold. The authors report on a study of chemical processes for cleaning MLD gratings that identifies techniques for removing contaminants left during fabrication in a way that does not compromise a grating’s efficiency or damage threshold.
• A study into the design and analysis of binary beam shapers for high-power laser systems is presented. OMEGA EP uses square beams with high-order super-Gaussian profiles to maximize the fill factor of amplifiers without exceeding the damage fluence of the laser components. The spatially dependent gain of the amplifiers can be, to a large extent, precompensated by attenuating regions of the input beam according to the gain they receive in the amplifiers. The authors have applied an error diffusion algorithm to the design of binary beam shapers consisting of a nonuniform array of 10-µm-sq pixels, which can be produced using standard lithographic techniques on high damage-threshold, metal-on-glass substrates.