Enhanced Pedestal H-mode Regime on NSTX

The Enhanced Pedestal (EP) H-mode on NSTX features the development of a wider pedestal with a significant increase in the ion temperature and carbon rotation gradients in an ELM- and MHD-free period following an ELM. These discharges achieve improved energy and momentum confinement with a beneficial decrease in the impurity accumulation relative to a standard ELM-free H-mode regime. The bifurcation to improved confinement is triggered by a transient period of lower collisionality during the ELM recovery where the ∇T_i increases consistent with neoclassical ion thermal transport. The larger ∇T_i reinforces the lower edge collisionality via a reduction of the neoclassical impurity pinch and moving the region of large E_r shearing rate and the density pedestal toward the core. The larger ∇T_i also acts to stabilize electron-scale turbulence, improving the electron thermal confinement. The favorable positive feedback initiated by transiently accessing lower ion collisionality leading to a significant improvement in the thermal confinement and reduced impurity accumulation motivates the development of actuators for controlling the edge density that are compatible with large core density and heat flux mitigation.