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

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 (E-r) and turbulence evolution during the intermediate I-phase. Whereas for type-Y the turbulence grows prior to the change in E-r, for type-J the oscillation in E-r 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 E-r shear, which eventually results in confinement improvement into the H-mode.