Toward steady-state high beta-N operation based on low qmin approach in KSTAR*

Recently, various operation scenario with low q-min characteristics have been studied in KSTAR to achieve high beta-N, steady state operation. Particularly, the so-called high poloidal beta mode, characterized by its high value of poloidal beta, has been verified to be capable of fully non-inductive operation at high BT(~2.5T). Interestingly, we found that obtained high poloidal beta plasmas had a dominant change in fast ion confinement while little change in thermal confinement. The improved fast ion confinement was related to low-n high frequency MHD stabilization by central EC, with a narrow window of BT. In addition, a large effort was made to improve the plasma performance, while keeping the betap high (~3.0) for steady state operation. As a result the plasma performance was able to increase up to betaN~2.8 with reduced BT. In parallel, another effort has been made to improve the thermal confinement with improved fast ion confinement. We found that low frequency low-n MHD observed in high poloidal beta plasma could be responsible for the little change of thermal confinement. It was associated with density evolution and NBI scheduling in the early phase of discharges.