Self-driven current generation in spherical tokamak plasmas with magnetic island perturbations*

Magnetic islands, by altering the topological structure of the confining magnetic field, have varied and complex impacts on plasma transport and confinement in fusion experiments. One novel effect revealed by global gyrokinetic simulations results from island-induced three-dimensional ambipolar electric field. A magnetic island is shown to drive non-resonant electric potential islands centered at both the inner and outer edge of the magnetic island. Such potential islands may introduce a major change in plasma self-driven current through an efficient nonlinear parallel acceleration of electrons in conventional tokamak geometry, resulting in a significant global reduction of electron current with respect to the neoclassical bootstrap current. Remarkably, this potential island effect is weaker in low aspect ratio spherical tokamak regimes, and the overall island-induced bootstrap current reduction is significantly smaller than in large aspect tokamaks.  The reduction of the axisymmetric current scales with the square of island width, and in the meantime, a significant helical current is generated in the island region. On the other hand, turbulent fluctuations may drive a substantial anomalous bootstrap current in STs.