Common Features of Core Electron-Root Confinement in Helical Devices

Abstract

The characteristics of core electron-root confinement (CERC) in helical devices are illustrated using results from four different experiments: the Compact Helical System, Large Helical Device, TJ-II, and Wendelstein 7-AS. Common features include strongly peaked electron temperature profiles and large positive radial electric fields E_r in the core region for discharges with sufficient central electron cyclotron heating (ECH). Such observations are consistent with a transition to the electron-root solution of the ambipolarity condition for E_r, a feature of neoclassical theory that is unique to nonaxisymmetric configurations. The magnetic topology of the configuration plays a role in this transition, and thresholds are found for the particle density and ECH power, in accordance with neoclassical expectations. Neoclassical theory alone cannot explain all observations, however, as CERC formation can also be influenced by ECH-driven convective fluxes of localized electrons and by the presence of magnetic islands in the core region. This is the first report describing collaborative activities within the framework of the International Stellarator Profile Data Base.