A model based on magnetohydrodynamic (MHD) stability of the tokamak plasma edge region is presented, which describes characteristics of edge localized modes (ELMs) and the pedestal. The model emphasizes the dual role played by large bootstrap currents driven by the sharp pressure gradients in the pedestal region. Pedestal currents reduce the edge magnetic shear, stabilizing high toroidal mode number (n) ballooning modes, while at the same time providing drive for intermediate to low n peeling modes. The result is that coupled peeling–ballooning modes at intermediate n (3<n<20) are often the limiting instability which constrains the pedestal and triggers ELMs. These modes are characterized in shaped tokamak equilibria using an efficient new numerical code, and simplified models are developed for pedestal limits and the ELM cycle. Results are compared to several experiments, and nonideal MHD effects are briefly discussed.

Cross-field particle transport in the scrape-off layer ͑SOL͒ of Alcator C-Mod ͓Phys. Plasmas 1, 1511 ͑1994͔͒ can be characterized by an effective particle diffusivity (D eff) that increases markedly with distance from the separatrix. As a consequence, recycling onto the main-chamber walls is large compared to plasma flows into the divertor volume. The SOL exhibits a two-layer structure: Steep gradients and moderate fluctuation levels are typically found in a ϳ5 mm region near the separatrix ͑near SOL͒ where parallel electron conduction typically dominates energy losses. Small gradients and larger fluctuation levels with longer correlation times are found outside this region ͑far SOL͒. D eff in the near SOL increases strongly with local plasma collisionality normalized to the magnetic connection length. As the discharge density limit is approached, D eff and associated fluctuation levels become large across the entire SOL and cross-field heat convection everywhere exceeds parallel conduction losses, impacting the power balance of the discharge.

We review and test the peeling-ballooning model for edge localized modes (ELMs) and pedestal constraints, a model based upon theoretical analysis of magnetohydrodynamic (MHD) instabilities that can limit the pedestal height and drive ELMs. A highly efficient MHD stability code, ELITE, is used to calculate quantitative stability constraints on the pedestal, including constraints on the pedestal height. Because of the impact of collisionality on the bootstrap current, these pedestal constraints are dependent on the density and temperature separately, rather than simply on the pressure. ELITE stability calculations are directly compared with experimental data for a series of plasmas in which the density is varied and ELM characteristics change. In addition, a technique is developed whereby peelingballooning pedestal constraints are calculated as a function of key equilibrium parameters via ELITE calculations using series of model equilibria. This technique is used to successfully compare the expected pedestal height as a function of density, triangularity and plasma current with experimental data. Furthermore, the technique can be applied for parameter ranges beyond the purview of present experiments, and we present a brief projection of peeling-ballooning pedestal constraints for burning plasma tokamak designs.

Cross-field particle transport increases sharply with distance into the SOL and plays a dominant role in the `main-chamber recycling' regime in Alcator C-Mod, a regime in which most of the plasma particle efflux recycles on the main-chamber walls rather than flows into the divertor volume. This observation has potentially important implications for a reactor: contrary to the ideal picture of divertor operation, a tightly baffled divertor may not offer control of the neutral density in the main-chamber such that charge exchange heat losses and sputtering of the main-chamber walls can be reduced. The conditions that give rise to the main-chamber recycling regime can be understood by considering the plasma-neutral particle balance: when the flux surface averaged neutral density exceeds a critical value, flows to the divertor can no longer compete with the ionization source and particle fluxes must increase with distance into the SOL. This critical neutral density condition can be recast into a critical cross-field plasma flux condition: particle fluxes must increase with distance into the SOL when the plasma flux crossing a given flux surface exceeds a critical value. Thus, the existence of the main-chamber recycling regime is intrinsically tied to the level of anomalous cross-field particle transport. Direct measurement of the effective cross-field particle diffusivities Deff in a number of ohmic L mode discharges indicates that Deff near the separatrix strongly increases as plasma collisionality increases. Convected heat fluxes correspondingly increase, implying that there exists a critical plasma density (or perhaps collisionality) beyond which no steady state plasma can be maintained, even in the absence of radiation.