Effects of MHD island on bootstrap current and turbulence in spherical tokamak plasmas

Magnetic islands, by altering the topological structure of the confining magnetic field, may strongly impact both neoclassical and turbulent dynamics in fusion devices such as spherical tokamaks. Major effects are associated with island-induced three-dimensional ambipolar electric field. First, a magnetic island induces a localized Er-well across island inner boundary; further, strong turbulence-driven Reynolds stress gradient contributes to continued growth of E×B flow beyond the neoclassical level. The resultant E×B shear layer is shown of capable of triggering the formation of an internal transport barrier inside the island rational surface for an NSTX-U L-mode plasma. A magnetic island also drives non-resonant electric potential islands at the island edges, which may introduce a major change in plasma self-driven current through an efficient nonlinear parallel acceleration of electrons, resulting in a significant global reduction of electron current with respect to the neoclassical bootstrap current in large aspect-ratio tokamaks. The reduction of the axisymmetric current scales with the square of island width. Remarkably, this effect of bootstrap current reduction by MHD islands is significantly weaker in STs, in particular, in the reactor-relevant high-βp regime where NTM islands are more likely to develop.