Characterization of divertor footprints and the pedestal plasmas in the presence of applied n = 3 fields for the attached and detached conditions in NSTX

Abstract

Recent progress in the study of 3D field effects on the divertor and pedestal plasmas is reported with the use of a new set of diagnostics. A wide angle visible camera provides 2D data of lower divertor surface covering almost the full range of radius (r) and toroidal angle (Φ), a significant advantage over the conventional 1D radial profile in examining non-axisymmetric effects of 3D fields on the divertor footprints. The spatial distribution of connection lengths (Lc) calculated by vacuum field line tracing in the presence of 3D fields (n = 3) agrees with the footprint pattern observed in the 2D wide angle camera images. The full (r, Φ) image data with high temporal resolution revealed that the spatial structure of modified divertor footprints is maintained even during the edge-localized modes (ELMs) triggered by applied n = 3 fields, when the ELM size is sufficiently small, i.e. the ELMs are ‘phase locked’ to the imposed perturbation field structure. This phase-lock is lost during the ELM rise time for ELMs with large energy loss, e.g. ΔWELM/WMHD > 4–5%. Divertor gas puff was used to create detached divertor condition and the effect of 3D fields on the detachment was investigated. The divertor remains partially detached with the 3D field application when a sufficient amount of gas is injected into the divertor region, which is accompanied by a noticeable drop of pedestal electron temperature (Te). However, with a lower gas puff, the divertor plasma re-attaches, when 3D fields were applied to the detached plasma, and the pedestal Te rises back up. There observed no other change in the pedestal profile associated with the re-attachment, indicating that this is likely to be dominated by a change in the electron thermal transport processes. A TRANSP analysis shows that the drop of pedestal electron heat diffusivity (χe) is responsible for this change but the source of this reduction is yet unclear.