Center for Magnetic Self Organization
in Laboratory and Astrophysical Plasmas
Simulation of Dynamo and Flow Generation in the Reversed Field Pinch
Relaxation of the Reversed Field Pinch (RFP) to a self-organized state can be understood at the most basic level through single fluid magnetohydrodynamics, (MHD) but certain key features, such as the generation of flow, cannot be explained in MHD. We showed for the first time that these phenomena can be successfully reproduced by incorporating effects beyond MHD into the simulations.
Comparison of measured (stars) and simulated (solid curve) flow in the MST RFP; previous, single fluid simulations qualitatively and quantitatively disagreed with the data.
Our two-fluid simulation-based study of magnetic tearing and relaxation in pinch profiles has produced three new theoretical results. The first concerns the influence of finite gyroradius effects from warm ions, where stresses from the variation in the magnitude of magnetic field and from magnetic curvature in the poloidal direction lead to drifting and partial stabilization of linear tearing modes. The net effect is analogous to previous drift- tearing results, but others have considered configurations where drifts are from pressure gradients. The importance of “grad-B” and poloidal curvature drifts had not been appreciated for pinches. The second finding is that the ion gyroviscous stresses are also important for large, nonlinearly saturated magnetic islands. They are not diminished by nonlinear transport effects, unlike pressure-based drift effects, and the residual forces are balanced by net Lorentz forces that maintain a Hall dynamo effect. Our third result concerns warm-ion effects when multiple tearing fluctuations are active. The magnetic relaxation that results from the Hall dynamo effect from net Lorentz forces is accompanied by changes in the plasma flow profile, and the magnitude and direction of the computational results are consistent with measurements in the Madison Symmetric Torus RFP (Figure 3). These results confirm our understanding of the physics, and help to validate the NIMROD code. [This work is leveraged by support from U.S. Dept. of Energy grant DE-FG02-06ER54840 for theoretical research in fusion energy science.]