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.