LLE Review 102

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Highlights

This volume of the LLE Review, covering January–March 2005, features the new “Saturn” target design concept for use in polar direct drive on National Ignition Facility (NIF) while the facility is in its initial, indirect-drive configuration. The concept differs from earlier polar-direct-drive designs by adding a low-Z ring around the capsule equator. Refraction in the plasma formed around this ring permits time-dependent tuning of the capsule drive uniformity. Proof-of-principle, polar-direct-drive (PDD) experiments on OMEGA using 40 repointed beams of the 60-beam OMEGA Laser System to approximate the NIF PDD configuration have been carried out. Backlit x-ray framing-camera images of D₂-filled spherical CH capsules show a characteristic nonuniformity pattern that is in close agreement with predictions. Saturn targets increase the drive on the equator, suggesting that highly symmetric PDD implosions may be possible with appropriate tuning. Two-dimensional simulations reproduced the approximately threefold reduction in yield found for the non-Saturn PDD capsules. Preliminary simulations for a NIF Saturn target design predict a high gain close to the 1-D prediction. These results increase the prospects of obtaining direct-drive ignition with the initial NIF configuration.

Additional research developments presented in this issue include the following:

• Direct-drive, spherical, cryogenic, D₂-filled capsules have been illuminated using the 60-beam OMEGA Laser System. The targets are energy scaled from the baseline ignition design developed for NIF. Thin-walled (~4-µm), ~860-µm-diam deuterated (CD) polymer shells are permeation filled with D₂ gas and cooled to the triple point (~18.7 K). Cryogenic ice layers with a uniformity of ~2-µm rms are formed and maintained. The targets are imploded with high-contrast pulse shapes using full single-beam smoothing (1-THz bandwidth, two-dimensional smoothing by spectral dispersion with polarization smoothing) to study the effects of the acceleration- and deceleration-phase Rayleigh–Taylor growth on target performance. These experiments have produced fuel areal densities up to ~100 mg/cm², primary neutron yields ~4 x 10¹⁰, and secondary neutron yields 1% to 2% of the primary yield.
• The interaction of directed energetic electrons with hydrogenic plasmas are analytically modeled from fundamental principles. The effects of stopping, straggling, and beam blooming are rigorously treated in a unified approach for the first time. Enhanced energy deposition, which occurs in the latter portion of beam penetration, is inextricably linked to straggling and beam blooming. Both effects asymptotically scale with the square root of the linear penetration. Eventually, they dominate over all other sources of beam divergence. Understanding their effects is critical for evaluating the requirements of fast ignition.
• Direct-drive, plastic-shell implosions on OMEGA with a 1-ns square pulse have been simulated using the multidimensional hydrodynamic code DRACO. Yield degradation in “thin” shells is primarily caused by shell breakup during the acceleration phase because of short-wavelength perturbation growth, whereas “thick” shell performance is influenced primarily by long and intermediate modes. Simulation yields, temporal history of neutron production, areal densities, and x-ray images of the core compare well with experimental observations. Thin-shell neutron production history falls off less steeply than one-dimensional predictions because of shell breakup induced under compression and delayed stagnation. Thicker, more-stable shells show burn truncation due to instability-induced mass flow into the colder bubbles. Estimates of small-scale mix indicate that turbulent mixing does not influence primary neutron yields.
• Effects of temporal density variation and spherical convergence on the nonlinear bubble evolution of single-mode, classical Rayleigh–Taylor instability are studied using an analytical model based on Layzer’s theory. When the temporal density variation is included, the bubble amplitude in the planar geometry asymptotes to a fixed value that depends on the Layzer bubble velocity, the fluid density, and a factor to account for the two- and three-dimensional geometries. The model can be applied to spherical geometries to predict the nonlinear bubble amplitude.