Impurity plume experiments in the edge plasma of the Alcator C-Mod tokamak

Abstract

The physics of impurity transport in response to a local gas injection in the scrape-off layer (SOL) of Alcator C-Mod is investigated. Carbon ‘plumes’ are formed at variable locations in the SOL—up to the separatrix—by puffing deuterated ethylene gas (C2D4) through the end of a reciprocating fast-scanning probe. CCD cameras are used to simultaneously record CII and CIII emission patterns from two near-perpendicular views. The plume dispersal patterns are found to yield direct qualitative information about plasma flow, including the direction of vE × B near the separatrix. Impurity transport and plasma–surface interaction physics implicit in the three-dimensional plume structure are explored in detail using a Monte Carlo impurity transport code, with the aim of extracting background plasma-flow quantities. A number of important local effects involving plasma–probe interactions have been identified: a vertical E × B drift near the probe surface, a parallel electric field above the probe tip arising from plasma recycling off the probe surface, and sputtering of a carbon layer that dynamically forms on the probe surface. The emission patterns are also found to yield important information on flows in the SOL: the radial electric field (Er) in the near SOL and volume-averaged values of the parallel Mach number (M∥) in the far SOL. The Er values obtained from plume data compare favourably with estimates of Er based on the poloidal propagation velocity of edge plasma fluctuations measured using the scanning probe. Emission patterns obtained in EDA H-mode suggest that the radial electric field changes sign at the location of the associated quasi-coherent mode layer. Comparisons between parallel Mach numbers obtained from the plume data and probe measurements indicate that the probe over-estimates the parallel flow towards the divertor in the far SOL. This result supports the picture of particle balance in the SOL of Alcator C-Mod being dominated by main-chamber recycling, with weak plasma flow into the divertor.