LLE Review 100

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Highlights

This volume of LLE Review, covering July–September 2004, features “Shock Propagation in Deuterium–Tritium–Saturated Foam.” Testing the assumption of homogeneous mixing in fibrous foams saturated with cryogenic deuterium and tritium, shock passage in wetted-foam mixtures is simulated by the adaptive-mesh, two-dimensional hydrodynamic code AstroBEAR. For foam fibers of diameter ~1/10 µm and for relevant foam densities, the mixing length behind the shock is found to be of the order of microns. Transverse motion decays quickly such that, at the mixing region’s edge farthest from the shock, Rankin–Hugoniot jump conditions are obeyed to within a few percent and shock speeds are also within a few percent of their homogeneous values. Also considered is the question of feedthrough and the decay time of shock-front perturbations seeded by the foam. Because shock fronts are stable, perturbations rapidly decay once the shock has entered the homogeneous DT-ice layer, rendering mute the feedthrough concern. As a result, full-scale simulations of foam-target implosions may model the wetted foam as a homogeneous mixture

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

• A status update is provided on the OMEGA EP tiled-grating-compressor technology. Similar to the scheme known from astronomy where very large telescopes are constructed using an array of mirrors, the large gratings required for petawatt-class, pulsed compression can be assembled from multiple smaller gratings. For the first time, real-time, closed-loop, computer-controlled phasing of a multilayer-dielectric-grating triplet, using interferometric feedback via nanopositioners, is demonstrated and a transform-limited, far-field spot is achieved.
• A method for simultaneously measuring beam position, shape, and relative intensity of all 60 of OMEGA’s beams is introduced by CID-recorded, focal-spot x-ray-image analysis. A 4-mm, Au-coated pointing surrogate is used as a target. This method offers pivotal input into efforts to improve target-illumination uniformity by improving beam pointing and reducing variations from average in beam intensities.
• Chemical durability is an important issue for water-sensitive laser glasses. A survey of commercial glasses—both cast and continuously melted—explores their hazing tendency under high relative humidity as a function of different polishing and cleaning conditions. While for any of the glass types tested no unambiguous correlation exists between initial finished surface quality and quantifiable magnitude of humidity-driven degradation, aggressive aqueous cleaning does result in hazing: more so for pitch and pad–finished surfaces than for MRF-processed ones.
• Single-molecule spectroscopy is used to determine the electronic and structural properties of individual, single-walled carbon nanotubes. Specifically, tip-enhanced, near-field Raman imaging and spectroscopy afford spatial resolution of 10 to 20 nm. Unlike most other molecules studied to date, single-walled nanotubes’ fluorescence intensity does not fluctuate. In addition, a nonuniform distribution of Raman bands is found along the tube axis.
• Polymeric cholesteric liquid crystals reflect light of a specific wavelength and circular polarization state. When broken into small flakes and suspended in a dielectric host fluid, they can be collectively oriented by an external field as useful, for instance, in a display application. An account is given of experimental measurements as well as theoretical simulations of flake rotation in an ac field. As an underlying mechanism, the field coupling to an induced dipole moment is identified—a dipole that arises on the flake surface due to Maxwell–Wagner polarization.
• This volume also includes a summary of the LLE Summer High School Research Program, the FY03 Laser Facility Report, and the National Laser Users’ Facility and External Users’ Programs.