Wide Pedestal Quiescent H-mode Plasmas in DIII-D Tokamak
Quiescent H-mode (QH-mode) is a naturally ELM-stable high performance operation mode which has been obtained in DIII-D, ASDEX Upgrade, JET, and JT-60U [1 and references within]. The additional transport to maintain constant density and radiation is provided by the benign coherent edge harmonic oscillations mode (EHO) [2] which is thought to be a kink/peeling mode [3]. A new QH-mode regime with enhanced pedestal has been discovered in DIII-D at reactor-relevant low torque and collisionality [4]. The regime was originally found in conventional QH-mode when the counter-Ip neutral beam torque drops to ~2Nm in double null plasma shapes. It is referred to as ‘wide-pedestal QH-mode’ because the pedestal width exceeds the EPED kinetic ballooning mode (KBM) limit [5]. Across the transition from QH to wide-pedestal QH, the pedestal electron pressure generally increases by 60% and widens by 50% and the plasma confinement (ITER Hy98y2) rises by 40%. The onset of broadband edge MHD modes and micro-turbulence accompanied with a lower ExB shear in this region is a common feature of the wide-pedestal QH, instead of EHO. It is conjectured that the increased transport provided by these edge modes reduces the pedestal gradients resulting in a higher pedestal while still remaining below the ELM-limit.