1) Institue for Fusion Studies, University of Texas, Austin , United States of America
Abstract. Magnetic perturbations with frequency much less than the Alfvén frequency interact strongly with the plasma near resonant surfaces where their wave-vector is parallel to the magnetic field, giving birth to magnetic islands and zonal flows. Examples of low-frequency MHD modes include the Neoclassical Tearing Mode (NTM) in tokamaks, conventional tearing modes in Reversed Field Pinches (RFP), and various Resonant Magnetic Perturbations (RMP), such as the Low-Density Locked Modes caused by error fields, the ergodic magnetic divertor in tokamaks, and the island divertor in Stellarators. All of these perturbations affect local confinement directly. Furthermore, they have an important influence on plasma rotation and thereby on global stability and confinement properties.
The theory of low-frequency Alfvén perturbations has advanced to include neoclassical damping of the plasma rotation outside of the resonant layer as well as the effects of symmetry breaking, diamagetic drifts, polarization currents, and turbulence inside the layer. At the same time the increasing resolution of the diagnostics has provided exquisitely detailed information on the profiles inside magnetic islands and on the effects of the perturbations on global plasma rotation. Experimental observations have confirmed expectations, based on two-fluid theory, that the relative velocity between the NTMs and the surrounding plasma lies between the ion and electron diamagnetic drift velocities. In this range of velocities the polarization current has a healing effect, in agreement with observations. Recent numerical investigations have also shown that turbulence leads to a slowing down of the island, contributing to NTM stabilization.
Resonant magnetic perturbations drive magnetic reconnection resulting in the formation of islands in otherwise tearing-stable plasmas. modelling of the effects of edge-resonant magnetic perturbations has explained the observations of density pump-out as being caused by convection cells that are significantly wider than the islands. Two-fluid effects contribute to broadening the cells through coupling with drift waves. Mode penetration can lead to island overlap and field-line stochasticity. Successful comparison between theory and experimental observations of electron transport in the RFX experiment has advanced the understanding of transport in stochastic fields.
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