Theory Weekly Highlights for August 2002

August 30 2002

A new facility was added to the ONETWO transport code to limit the growth rate of the safety factor q near the magnetic axis. This is done by dynamically adjusting a seed current to flatten the toroidal electric field when the current density is decreasing. This option prevents the formation of multiple magnetic axes, which would otherwise cause the transport calculations to break down.

August 23 2002

A new working version of IDL 5.5 for OSF V5.1 was implemented by the Data Analysis Group. This eliminates the problems with the previous unpatched version of IDL, which locked up under certain circumstances. Several LSF hosts have already been upgraded and are using the patched version of IDL; other hosts will be upgraded in the next few weeks.

The GA Theory group successfully hosted the Magnetofluid Modeling Workshop on August 19 and 20. This was followed by several satellite workshops August 21 through 23, devoted to detailed discussions for the major national extended MHD modeling efforts: the Center for Magnetic Reconnection Studies (Amitava Bhattacharjee, U Iowa); the Center for Extended MHD Modeling (Steve Jardin, PPPL); and the NIMROD Project (Dalton Schnack, SAIC). See for details.

August 16 2002

Analytic solutions for “doublet FRC” equilibria with double magnetic axes and two teardrop-shaped private flux regions, have verified an earlier speculation that these configurations possess more favorable stability properties than standard FRCs. General expressions for the interchange functions V′ and V′′ at the O-point - the least interchange stable point - were derived, and the stability criteria compared, for the standard and doublet FRCs; this also revealed a correction to the published expression for V′′ in L. Sparks, R. Sudan, Phys Fluids 27, 626 (1984). For the proposed Swarthmore SSX-FRC experiment (elongation ~ 1.5), stability requires a minimum ratio of edge separatrix-to-O-point pressure of 0.4, compared to 0.58 for a conventional FRC with the same pressure profile. However, more oblate doublet FRCs, or increased indentation at the waist, can lower the edge pressure required for interchange stability, making doublet FRCs a more viable fusion configuration.

August 9 2002

Several DIII-D discharges have in the past exhibited apparent phase reversals or 'phase folding' in the Mirnov signals on the inboard side; that is, a signal that superficially appeared to be a simple m = 3 structure but with a region along the inboard side where the mode seemed to be m = -1. These apparent phase reversals were a long-standing puzzle that had prompted a number of somewhat complicated and non-standard explanations. In collaboration with the FARtech group, a re-analysis of the comparison of the Mirnov prediction from the ideal mode computed by the GATO code with the measured signal for DIII-D discharge #87009, however, revealed that there is, in fact, no mystery and no non-ideal processes need be invoked. The phase reversal is only apparent, and is due to the rich poloidal spectrum of the unstable ideal mode, coupled with the noncircular discharge geometry and relative placement of the Mirnov loops.

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