Theory Weekly Highlights for December 2007

December 21, 2007

Analysis of the poloidal mode spectrum for NIMROD simulations of DIII-D plasmas with an applied n=3 resonant magnetic perturbation (RMP) shows that, in the absence of rotation, all resonant modes with n=3 and m < -3, are amplified by the plasma response. The amplification of resonant modes is predicted by the Fitzpatrick error field theory for tearing stable plasmas with tearing parameter -Δ' < 2m. The amplification factor is reduced, in some cases below unity, as the plasma rotation velocity is increased. At low separatrix rotation velocity, a nearly constant phase shift across all resonant modes is observed. At higher rotation, an oscillatory n=3 mode amplitude and phase for islands near the plasma edge are observed with a frequency equal to three times the plasma rotation frequency at the edge. These results for rotating plasmas are also consistent with the analytic Fitzpatrick error-field theory.

December 14, 2007

In collaboration with N. Gorelenkov of PPPL, Christian Konz from IPP Garching, and Ian Chapman of JET, a benchmark test between the GATO ideal MHD stability code, the NOVA code, and the Mishka code confirmed the earlier GATO result that the bean shape in the bean – oval DIII-D sawtooth experiments, is ideally stable during much of the sawtooth period, including right near the time of the crash (see April 6 highlight at Theory Weekly Highlights for April 2007). The previous results were surprising since they conflict with the widely held view that the sawtooth is triggered by an increasingly unstable ideal kink as q drops below unity at the axis. The bean shape, with much lower q on axis than the oval shape before the respective crashes, is actually more stable to the ideal mode. By carefully ensuring the same input equilibria were used, the benchmark between the Mishka and GATO codes for the oval case also found close agreement in the ideal growth rates to within a few percent.

December 07, 2007

In a recent Letter appearing in Nature Physics, (Vol 3 P609, 2007) an international collaboration led by the Chinese National Academy of Sciences has identified the presence of a pair of distinct magnetic field nulls, the null-null line associated with reconnection between them, and associated other structures, in data from the Earth’s magnetotail using a tetrad of local measurements from the four Cluster mission spacecraft, as first proposed by the late John Greene in 1992 (J. Comp. Phys. 98, 194, 1992). These and earlier measurements demonstrating the presence of single nulls in 3D confirm the general description of the topological structure of reconnecting field nulls in 3D described in a seminal paper by Greene in J. Geophys. Res. (Vol 93, 8583) in 1988. The researchers believe that these measurements will lead to a full understanding of the details of reconnection dynamics in 3D.

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