Special Plasma Seminar
Thursday, May 23
4274 Chamberlin Hall
Speaker: Dr. Linda Sugiyama, Massachusetts Institute of Technology
"Fast MHD Sawtooth Crash"
Abstract: The m=1, n=1 sawtooth oscillation in a toroidal plasma is important for fusion burning, yet remains poorly understood. In the linear 1/1 internal kink mode in MHD (Bussac, 1976), toroidal mode coupling links the unstable m=1 kink over q<1 to more stable harmonics with m>1 over q>1. Nonlinear sawtooth theories, however, almost all use the m=1 harmonic alone. The result, reduced MHD, is too slow to explain observed sawtooth crashes at low resistivity; instead, non-MHD effects are usually invoked. 3D simulations have been difficult to interpret, so the incompressible RMHD result has been accepted for 3D MHD as well. Large scale numerical simulations, however, show that compressible and finite aspect ratio effects in full MHD completely change the picture, leading to a fast crash much closer to experiments. The crash can be qualitatively described by a large aspect ratio expansion, but not by RMHD. Sweet-Parker-type reconnection does not occur. Both sound wave and compressional Alfven wave terms contribute and the crash does not follow the Kadomtsev model. Similar effects occur in other cross-field MHD instabilities in a torus and in fast reconnection in a slab configuration, as in space physics.