Progress in Understanding H-mode Pedestal Structure*

This talk reviews progress in understanding key physics that forms the H-mode pedestal structure, a narrow region of high pressure gradient at the edge of magnetically confined plasmas. Concerted experimental, theoretical and modeling research has developed a validated predictive capability for the achievable pedestal pressure in tokamaks as well as identified sources of residual transport that may limit pedestal gradients. An over-arching result is that magnetohydrodynamic instabilities set a hard limit to the achievable pedestal pressure. Well validated models show that peeling-ballooning (PB) modes provide this limit. A reduced model to predict the pressure (EPED) combines a PB model with a model for kinetic ballooning modes (KBM) limiting the pressure gradient. This model predicts the observed pressure in several tokamaks, with the pressure varying by a factor of ~70. The PB and EPED models provide predictive capability that is used for modeling present and future machines. Much research shows that several processes likely cause residual transport that limits gradients of density and temperature. Neoclassical physics explains some, but not all, of particle transport. MHD modes, such as KBMs, are plausible candidates but have not been definitively identified as the source. Ion thermal transport is often close to neoclassical levels. In some regimes, ion temperature gradient or trapped electron modes may provide transport for ion heat as well as other channels. Experiment and modeling provide compelling evidence that microtearing modes and electron temperature gradient modes are present and provide important electron thermal transport. Models of several processes have reproduced all pedestal transport in some cases. We have made much progress to unlock secrets of the pedestal; much work remains for the ultimate goal of predicting the full pedestal structure.