Theory Weekly Highlights for May 2002

May 31 2002

A shared visualization demonstration was performed for the DIII-D Experimental Team using tiled display walls at General Atomics and the Argonne National Laboratory. The demonstration included simultaneous viewing of a simple IDL visualization program (ReviewPlus) and a complex 3D program (SciRUN) based on OpenGL. The shared visualization capability was well received both for use in the DIII-D control room and in smaller working meetings. Work will continue to develop this technology for beta testing deployment with the DIII-D team.

May 24 2002

Modifications to the GA ideal MHD stability code GATO were made to enable reading of equilibria from the PPPL inverse equilibrium code JSOLVER. This will now enable the GATO code to directly evaluate the stability of the FIRE and IGNITOR equilibria being analyzed in preparation for SNOWMASS. Previously, this evaluation required recalculation of the equilibria from either the TOQ or EFIT code.

May 17 2002

A detailed benchmark of calculated MHD growth rates and mode structures between the GATO ideal MHD stability code and the recently developed ELITE MHD stability code found excellent agreement for two equilibria, one with a high edge pedestal that is strongly unstable to ballooning-like modes over a broad range of n, and one that is unstable to edge localized external peeling modes driven by the edge current density. The agreement in the calculated growth rates is within 3% over the entire region of overlap in toroidal mode numbers n=4, 5, 6, 7, 8, 9. Also, comparison of the calculated eigenmodes for n=8 finds them to be virtually identical. This benchmark is an important test since the GATO and ELITE codes employ very different formulations. GATO is a well established, global finite element code designed to study low to intermediate n modes, whereas ELITE directly solves the Euler equations for the Fourier mode amplitudes, including an expansion through two orders in 1/n, allowing for efficient study of intermediate to high n (n>~5) modes localized in the outer regions of the plasma.

May 10 2002

In the well-known work of Glasser, Green and Johnson, Phys. Fluids 18, 875 (1975), the existence of overstable modes or “modified tearing modes” in tokamak discharges is posed as a possibility, but these modes have not yet been shown to exist in experiments. In recent NIMROD analyses of DIII-D discharge 98549, high-m modes localized near the X-points and edge show a linear growth rate that oscillates in time with a constant frequency - a trademark of overstable modes. A detailed analysis of diamagnetic stabilization effects on these modes is ongoing which is expected to answer why these modes are not observed in the experiment.

May 03 2002

The time dependent and time independent finite difference forms of the coupled set of transport equations in Onetwo were recently both rewritten to account for the application of boundary conditions at arbitrary values of the minor radius (normalized flux). In particular, in simulation mode, each equation can now have an independent boundary specified as a function of time. This not only allows studies of the effects of various boundary conditions but it also makes possible the L to H transition modeling with GLF23 and provides for a flexible core/edge coupling methodology.

These highlights are reports of research work in progress and are accordingly subject to change or modification