Healing of magnetic islands in stellarators by plasma flow

Abstract

Recent experiments from the Large Helical Device indicate that plasma flow can play a primary role in ‘healing’ vacuum magnetic islands in stellarators. The observed elimination of magnetic islands tends to occur at low collisionality and high plasma β. A model explaining this phenomenon is developed reminiscent of ‘mode locking/unlocking’ physics of tokamak and reversed field pinch experiments. The theory describes transitions between two asymptotic solutions, a state with a large non-rotating island and a state where rotation shielding suppresses island formation. Transitions between these two states are governed by coupled torque balance and island evolution equations. In conventional stellarators, neoclassical damping physics plays an important role in establishing the flow profiles. The balance of neoclassical damping and cross-field viscosity produces a radial boundary layer for the plasma rotation profile outside the separatrix of a locked magnetic island. The width of this boundary layer decreases as the plasma becomes less collisional. This has the consequence of enhancing the viscous torque at low collisionality making healing magnetic islands occur more readily in high temperature conventional stellarators.