Effects of rotating magnetic island on the transport of trapped fast ions

Abstract

The enhanced transport of trapped energetic ions (TEIs) in the presence of resonant interactions between trapped fast ions and a rotating magnetic island is investigated within a drift-kinetic framework. Gyro-orbit banana center model equations of resonances between the island rotation, the bounce motion of trapped fast ions, and their precession frequency (poloidal precession and precession in the helical direction) are constructed. There are two solutions for resonances in phase space for different mode numbers, with only one solution having low-energy resonant lines (<100 keV); the other has not only low-energy resonant lines but also high-energy lines (≥100 keV). Island rotation plays an important role in the low-energy region, especially near the trapped-passing boundary. The precession frequency is more important when resonances occur in the high-energy area. Thus, the effect of islands on TEI transport in a low-energy region is the focus of this paper. Transport fluxes caused by collisions, resonances, and symmetry breaking induced by an island are obtained. We divide transport fluxes into two types: Γc arising from magnetic drift and Γb arising from the island rotation. There is a discontinuity in Γc with different island widths near the island separatrix. On the right-hand side of the (m = 2, n = 1) rational surface, Γc is more important than Γb, and at the plasma boundary, the flux due to drift can suppress Γb, which makes fast ions move toward inner plasma. On the left-hand side of the rational surface, Γb is dominant. When the island width is larger than a certain threshold, the fluxes oscillate, and Γb is far larger than Γc.