Roles of turbulence– and pressure-gradient–induced flows in triggering H-mode at marginal heating power on HL-2A tokamak

A qualitative model which includes three loops of zonal flow vs. turbulence and turbulence vs. pressure gradient is proposed for the experimentally observed two types of limit cycle oscillations (LCOs) and low-intermediate-high (L-I-H) confinement transitions. The experimental evidences supporting the model and revealing the roles of turbulence- and pressure-gradient-induced flows in triggering the H-mode at marginal heating power in tokamak plasmas are presented. The dominant role played by pressure-gradient-induced flows in I-phase of type-J LCO as well as I-H transition is demonstrated, along with that done by the turbulent-Reynolds-stress-driven zonal flows in the L-I transition as well as I-phase of type-Y LCO. The trigger and conditions for the I-H transition are identified from the experimental observations. Copyright (C) EPLA, 2016