LLE Review 99

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Highlights

This volume of the LLE Review, covering April–June 2004, features “Multidimensional Simulations of Plastic-Shell Implosions on the OMEGA Laser.” The multidimensional hydrodynamic code DRACO is applied to studies of shell stability during the acceleration phase in the presence of nonuniform illumination and target roughness. Simulations show that for thick shells remaining integral during acceleration, the target yield is reduced by a combination of long-wavelength modes due to surface roughness and beam-to-beam imbalance as well as intermediate modes due to single-beam nonuniformities. Compared to 1-D predictions, the neutron-production rate for these shells truncates. Diminished yield for thin shells is mainly due to shell breakup at short-wavelength scales of the order to the in-flight shell thickness. DRACO simulation results are consistent with experimental observations.

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

• A comprehensive overview is given of the methodology of, and issues involved in, the preparation of deuterium-ice layers in OMEGA targets. The process of first forming and then smoothing the ice layer is governed by multiple parameters that, when optimally controlled, yield ice layers approaching a 1-μm-rms roughness in low-spatial-frequency modes.
• A new, modular x-ray streaked imager has been developed and fielded that combines a four-mirror Kirkpatrick–Baez microscope with a high-current PJX streak tube. Performance optimized for use at 1.5 keV, this instrument provides better-than-5-μm spatial resolution over its central 200-μm field of view. When equipped with a mix of iridium-coated and multilayer mirrors, simultaneous images in distinct x-ray energy bands can be obtained.
• Growing 25-mm-diam Nd:YLF rods with high optical-wavefront quality is extremely challenging. Bulk wavefront distortions of several microns are typically found in state-of-the-art rods. These have now been successfully compensated by magnetorheological single-surface correction. Large-aperture rods corrected in this manner render nearly diffraction-limited output-beam performance.
• Several prototypes of NIF neutron-time-of-flight detectors have been developed and tested on OMEGA with DD and DT implosions. For low and moderate NIF neutron yields, systems comprising fast plastic scintillators and fast photomultiplier tubes have proven successful. For higher yields, a third type based on chemical-vapor-deposition (CVD) diamonds provides superior response. Taken together, these detectors cover a neutron yield range between 10⁹ and 10¹⁹.
• Ultrafast current sensing has reached a new level of sensitivity and speed by taking advantage of the magneto-optic Faraday effect in CdMnTe single crystals. Based on pump–probe magneto-optic sampling measurements at 10 K, response times of a few hundred femtoseconds can be realized at a current sensitivity of ~0.1 mA. According to the wavelength dependence of this sensitivity, excitonic contributions enhance this material’s dynamic Verdet constant.
• The intrinsic high steady-state peak electron velocity in GaN offers promise for fast UV detectors that is exploited in an interdigitized-finger, metal–semiconductor–metal photodiode design yielding 50-ps response times for subpicojoule optical-energy inputs. At higher input energies, space-charge screening leads to broadening of the electrical output pulse. These experimentally observed phenomena closely resemble whole-package simulations involving a single, adjustable parameter for external quantum efficiency.
• Creating micrometer-scale structures through the use of electrometric stamps experienced a further refinement: controlled deformation of the stamp by overpressure enables pattern formation of nanoparticles or self-assembled monolayers on a scale length up to an order of magnitude smaller than the original stamp as well as patterns that do not exist in the original masters. As one example, magnetic ring and anti-ring structures are being fabricated for memory-device applications.