Transport and stability in sustained high q_min, high β_N discharges on DIII-D

To address the needs for a fusion pilot plan design, DIII-D/EAST joint experiments on DIII-D have demonstrated high normalized beta β_N>4, poloidal beta β_P>3, toroidal beta β_T>3% with q_min>2, q₉₅≤8 sustained for more than six energy confinement times. The excellent energy confinement quality (H_98y2~1.7) is achieved with an internal transport barrier (ITB) at high line-averaged Greenwald density fraction, f_GW>0.9 (~7×10¹⁹ m^-3). Gyrofluid (TGLF) modeling of the transport characteristics shows that the beam driven rotation does not play an important role in the high confinement quality. This is consistent with the paradigm that turbulent transport within the ITB region is suppressed by strong alpha-stabilization effect at high β_P. The high performance phase can be terminated by fast growing modes triggered near the n=1 ideal-wall kink stability limit by large edge localized modes (ELMs). While the achieved bootstrap current fraction on DIII-D is high (up to 80%), it will be even higher in a reactor at the same β_N, q₉₅ and f_GW due to the lower collisionality from higher B_T and I_P. Future experiments will aim to make up the difference in bootstrap current and achieve fully noninductive conditions by using helicon and top launch EC wave injection for additional, efficient off-axis current drive at high density and beta.