Investigation of reversed shear Alfvén eigenmodes (RSAE) in relation to internal transport barrier (ITB) for high-qmin scenario on EAST

The reversed shear Alfvén eigenmode (RSAE) and the relationship with the internal transport barrier (ITB) have been investigated in the Experimental Advanced Superconducting Tokamak for the high qmin scenario. The RSAE with low toroidal mode numbers (1 ≤n< 5) is distributed within the normalized minor radius range of 0.2 ≤ρ≤ 0.4. Furthermore, two distinct RSAE branches with similar toroidal mode numbers but different radial coverage and phase profiles are observed. The excitation of RSAE leads to a redistribution of fast ions, with the density change predominantly in the central region enclosed by the minimum safety factor (qmin), as measured by the radial neutron camera. A micro-instability characterized by thermal particle transport has been detected by the POLarimeter-INTerferometer diagnostic, which is excited concurrently after the upward sweeping frequency of RSAE. The inward shift of the inner q = 2 surface is indicated by a sawteeth-like collapse event (Δr≈ 2 cm, Δr/a≈ 5%, a is the plasma minor radius), accompanied by a comparable outward shift in the R/LTi profile [R/LTi is the normalized ion temperature gradient (ITG) scale length]. These profile modifications, which subsequently suppress thermal particle transport, are correlated with a decrease in magnetic shear. The formation of Ti-ITB, characterized by R/LTi≥ 10, is observed following the RSAE instability under conditions of high neutral beam injection and ion cyclotron resonance heating power. A negative radial electric field (Er) is observed at the position of qmin≈2, and the E×B shear rate significantly increases after the formation of ITB. Furthermore, the RSAE experiences partial suppression due to the improved ITG; however, it becomes fully suppressed after ITB formation and subsequently recovers following the ITB collapse.