Extending the physics basis of quiescent H-mode toward ITER relevant parameters

Abstract

Recent experiments on DIII-D have addressed several long-standing issues needed to establish quiescent H-mode (QH-mode) as a viable operating scenario for ITER. In the past, QH-mode was associated with low density operation, but has now been extended to high normalized densities compatible with operation envisioned for ITER. Through the use of strong shaping, QH-mode plasmas have been maintained at high densities, both absolute ( ) and normalized Greenwald fraction (). In these plasmas, the pedestal can evolve to very high pressure and edge current as the density is increased. High density QH-mode operation with strong shaping has allowed access to a previously predicted regime of very high pedestal dubbed ‘Super H-mode’. Calculations of the pedestal height and width from the EPED model are quantitatively consistent with the experimentally observed density evolution. The confirmation of the shape dependence of the maximum density threshold for QH-mode helps validate the underlying theoretical model of peeling-ballooning modes for edge localized mode (ELM) stability. In general, QH-mode is found to achieve ELM-stable operation while maintaining adequate impurity exhaust, due to the enhanced impurity transport from an edge harmonic oscillation, thought to be a saturated kink-peeling mode driven by rotation shear. In addition, the impurity confinement time is not affected by rotation, even though the energy confinement time and measured shear are observed to increase at low toroidal rotation. Together with demonstrations of high beta, high confinement and low for many energy confinement times, these results suggest QH-mode as a potentially attractive operating scenario for the ITER mission.