Physics and scaling of the H-mode pedestal

Abstract

The transport barrier which exists at the plasma edge in the high-confinement (H-mode) regime is now recognized as important to global energy confinement. Recently, detailed measurements of the width and height of barriers in electron and ion density and/or temperature have been made on several divertor tokamaks. These results are reviewed, with an emphasis on scalings of the pedestal height and width. Many experiments observe situations in which the limiting pedestal pressure gradient is well described by ideal ballooning stability. In some cases the extreme edge is in the `second stable' regime. Differences between the widths Δ of pedestals in electron and ion temperature and densities are noted in several experiments. Some reported scalings are Δ_Ti = 3.3(ε)^1/2ρ_i,pol on JT-60U, and Δ_Pe/R∝β_pol,ped^0.4 on DIII-D. Δ_Te is independent of plasma current in many experiments. Theoretical modelling of the fully developed H-mode pedestal is reviewed. Several recent predictions of the pedestal parameters and their scalings are outlined and compared with experiments.