Expanding the parameter space of the wide-pedestal QH-mode towards ITER conditions

Abstract

Significant parameter space expansion of the wide-pedestal quiescent H-mode (QH-mode) towards more ITER and reactor relevant conditions has been achieved in the DIII-D tokamak. Recent experiments reveal this naturally ELM-free regime can operate over a range of neutral beam injected (NBI) torque spanning from the co- to counter-Ip direction. The accessed torques completely cover the scaled ITER-equivalent applied torque range. Wide-pedestal QH-mode features an edge region where the ExB shear is lower than that of standard QH-mode and H-mode, as well as unique low- to intermediate-k turbulence and a flattening of the pedestal profiles in the low ExB shear region. Wide-pedestal QH-mode has also been extended to lower single null shaping matching ITER average triangularity. A NBI power scan at moderate density and net-zero NBI torque shows no upper power limit to wide-pedestal QH-mode while the confinement quality H98y2 increases with power. At the highest power, H98y2 reaches 1.6 and normalized pressure βN reaches 2.3 simultaneously. The wide-pedestal QH-mode also exhibits improved confinement when (1) transitioning from standard QH-mode, (2) adding core ECH power, and (3) by transitioning from LSN to DN plasma shape. Turbulence suppression by increased ExB shear inboard of the pedestal top plays an important role in all three of these observations. These recent advances make wide-pedestal QH-mode a stronger candidate for an ELM-free, high performance regime for ITER and future reactors.