Session 7F - DIII-D: Edge and H-Mode.
ORAL session, Thursday morning, November 14
Vail Room, Adam's Mark
We report results of experiments designed to identify physical effects that define the tokamak n_e limit. Our results support the postulate that the n_e limit is defined by two generic groups of physical effects, power balance and MHD stability. Power balance limits of the core, boundary, and divertor plasmas result in a n_e limit that increases with heating power P, typically n_e\hbox(max)\propto P^0.5. MHD modes, such as ballooning or neoclassical tearing, limit P from above (through transport) with P_max\propto I_P^2. Thus the n_e limit increases with I_P. With typical H--mode density profiles, divertor and boundary plasmas dominate in the power balance. Using n_e profile control, by divertor pumping and pellet injection, we have increased the \bar n_e relative to the edge values. As a result we have succeeded in increasing the H--mode n_e limit by a factor of two and up to the core radiative limit. Densities up to 1.5 times the Greenwald limit with \tau_E^ \simeq0.9\times\tau_E (JET\slash \hboxDIII--D H--mode) have been obtained.