Destabilization of reversed shear Alfvén eigenmodes driven by energetic ions during NBI in HL-2A plasmas with<i>q</i>_min∼ 1

Abstract

Two groups of frequency sweeping modes are observed and interpreted in the HL-2 A plasmas with qmin ~ 1. The tokamak simulation code calculations indicate the presence of a reversed shear q-profile during the existence of these modes. The mode frequencies lie in between TAE and BAE frequencies, i.e. ωBAE 1 and nqmin − m > 0, and another group of modes characterized by up-sweeping frequency with qmin drop, owing to qmin < 1 and nqmin − m < 0 before sawtooth crash. The kinetic Alfven eigenmode code analysis supports that the down-sweeping modes are kinetic reverse shear Alfven eigenmodes (KRSAEs), and the up-sweeping modes are RSAEs, which exist in the ideal or kinetic MHD limit. In addition, the down- and up-sweeping RSAEs both have fast nonlinear frequency behaviour in the process of slow frequency sweeping, i.e. producing pitch-fork phenomena. These studies provide valuable constraint conditions for the q-profile measurements.