Strategy for Developing Internal Transport Barriers at Large Radius in High Poloidal Beta Plasmas on EAST

Gyrokinetic modeling shows possible approaches to develop internal transport barriers (ITBs) at large minor radius in EAST high β_P plasmas. EAST achieved steady state operation for 60 s with high β_P (~2.0). However, no large radius ITBs (a feature of the high β_P scenario on DIII-D) are observed on EAST. Analysis of large-radius ITB formation on DIII-D shows that the joint effects of low magnetic shear and high α-stabilization keep the plasma away from the kinetic ballooning mode (KBM) instability boundary and drive the plasma into a second stability region featured with low transport. Similar gyrokinetic analysis applied on EAST high β_P plasmas shows that the ion temperature gradient (ITG) is the dominant micro-turbulence mode that prevents ITB formation at large radius. The analysis suggests several potential approaches for breakthrough: 1) strong off-axis current drive from external sources; 2) discharge initiation with higher magnetic shear at higher q₉₅; 3) additional ion heating at large radius or higher pedestal ion temperature; 4) impurity injection at large radius. In addition, applying a large edge perturbation is expected to bypass the KBM instability, triggering ITB formation. Joint experiments in the ongoing EAST campaign plan to test these approaches.