Dynamics of low–intermediate–high-confinement transitions in the HL-2A tokamak

Abstract

In recent experiments at the HL-2A tokamak, dynamic features across the low–intermediate–high (L–I–H) confinement transition have been investigated in detail. Experimental evidence shows two types of opposite limit cycles (dubbed type-Y and type-J) between the radial electric field (Er) and turbulence evolution during the intermediate I-phase. Whereas for type-Y the turbulence grows prior to the change in Er, for type-J the oscillation in Er leads turbulence. It has been found that the type-Y usually appears first after an L–I transition, followed by type-J before the transition to the H-mode phase. Possible roles played by zonal flows and the enhanced pressure-gradient-induced flow shear in suppressing turbulence, respectively, in the type-Y and type-J periods have been identified. In addition, during the I-phase of the L–I–H discharges a kink-type MHD mode routinely occurs and crashes rapidly just prior to the I → H transition. The mode crash evokes substantial energy release from the core to plasma boundary and further increases the edge pressure gradient and Er shear, which eventually results in confinement improvement into the H-mode.