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

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 (C 2 D 4 ) 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 v E × 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 ( E r ) in the near SOL and volume-averaged values of the parallel Mach number ( M ∥ ) in the far SOL. The E r values obtained from plume data compare favourably with estimates of E r 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.