Internal transport barrier dynamics at high poloidal beta on DIII-D and extensions towards high confinement scenarios for a fusion reactor

Towards the ITER steady-state goal of Q=5, recent DIII-D/EAST joint experiments on DIII-D have demonstrated sustainment of excellent energy confinement quality (H_98y2>1.5) via a large radius ITB at high normalized beta (β_N~3.5) and reactor-relevant q₉₅~6.0. At high β_P (>2), the large Shafranov shift can stabilize turbulence and create a bifurcation in kinetic ballooning mode (KBM) transport at large minor radius, leading to an improved confinement state. Large transient perturbations such as type-I ELMs can trigger this transition from the H-mode confinement state with a high edge pedestal to the higher confinement state with a lower pedestal and an ITB. At lower β_p (≤2), negative magnetic shear in the plasma core contributes to turbulence suppression and can compensate for reduced Shafranov shift to still produce a large radius ITB and excellent confinement with low plasma rotation, consistent with results of gyrofluid transport simulations. However, resistive wall modes can be a limitation at simultaneous high βN, low internal inductance, and low rotation. Additional off-axis external current drive will further provide a more stable path towards operation at reduced q₉₅.