Pedestal Studies in DIII-D

Studies of the H-mode pedestal in the DIII-D tokamak are presented. The global energy confinement increases as the plasma pressure on top of the pedestal increases. The best empirical description for a pedestal width parameter is _pe [proportional to] (_pol^PED)^0.4, where _pe is the width of the electron pressure pedestal and _pol^PED is the poloidal beta at the top of the pedestal. The edge profiles of electron density n_e, electron temperature T_e, and ion temperature T_i can all have different shapes. Thus, a simple width scaling for the edge might not exist, and studies of the physics of individual profiles have been initiated. A model for the n_e profile, based on self-consistent treatment of edge particle sources and edge particle transport, agrees with several experimental observations. The steep gradient region for the T_e profile often extends farther into the plasma than the n_e pedestal step. Magnetohydrodynamic stability provides the ultimate limits to the evolution of the pedestal and usually leads to edge instabilities called edge-localized modes (ELMs). However, the absence of ELMs in a regime called the Quiescent H-mode shows that large pedestals can be produced without ELMs.