Large-Eddy Simulations of Magnetohydrodynamic Turbulence in Heliophysics and Astrophysics

We live in an age in which high-performance computing is transforming the way we do science. Previously intractable problems are now becoming accessible by means of increasingly realistic numerical simulations. One of the most enduring and most challenging of these problems is turbulence. Yet, despite these advances, the extreme parameter regimes encountered in space physics and astrophysics (as in atmospheric and oceanic physics) still preclude direct numerical simulation. Numerical models must take a Large Eddy Simulation (LES) approach, explicitly computing only a fraction of the active dynamical scales. The success of such an approach hinges on how well the model can represent the subgrid-scales (SGS) that are not explicitly resolved. In addition to the parameter regime, heliophysical and astrophysical applications must also face an equally daunting challenge: magnetism. The presence of magnetic fields in a turbulent, electrically conducting fluid flow can dramatically alter the coupling between large and small scales, with potentially profound implications for LES/SGS modeling. In this review article, we summarize the state of the art in LES modeling of turbulent magnetohydrodynamic (MHD) ows. After discussing the nature of MHD turbulence and the small-scale processes that give rise to energy dissipation, plasma heating, and magnetic reconnection, we consider how these processes may best be captured within an LES/SGS framework. We then consider several special applications in heliophysics and astrophysics, assessing triumphs, challenges,and future directions

Range and damage distributions in ultra-low energy boron implantation into silicon

An ultra high vacuum, low energy ion implanter was used in conjunction with a range of analytical techniques to study the range and damage distributions of B/sup +/ ions implanted at normal incidence into Si(100) samples held at room temperature. Samples were implanted over a dose range from 1E14 ions/cm/sup 2/ with and without a surface oxide layer and those implanted at 1 keV and below were capped with a nominal 20 nm layer of /sup 28/Si by ion beam deposition in situ in order to produce an oxygen equilibration layer for subsequent secondary ion mass spectrometry depth profiling. The samples were analysed using secondary ion mass spectroscopy, medium energy ion scattering, spectroscopic ellipsometry, spreading resistance profiling and high resolution, cross section transmission electron microscopy to obtain the range and damage distributions and junction depths. The general observations were that channelling occurs at all energies studied, and that the relationship between the damage and range distributions depends strongly on bombardment energy. Comparison of the range and damage profiles was carried out to ascertain the role of the surface in determining the behaviour of defects produced very close to it by the low energy implants required for the production of junctions at depths in the 20 to 50 nm range. The role of the surface or silicon/silicon dioxide interface as a defect sink significantly influences the B redistribution behaviour during rapid thermal annealing