Flow measurements in the scrape-off layer of Alcator C-Mod using impurity plumes

Accurate measurements of plasma flows in the scrape-off layer (SOL) are a necessary requirement for understanding the physics of tokamak edge plasmas. A system is being developed on Alcator C-Mod for inferring flows parallel (v 1 ) and perpendicular (v E×B ) to local magnetic field lines from impurity emission patterns ('plumes') generated by local gas injection. Carbon plumes are generated at variable location in the SOL by puffing deuterated ethylene gas (C 2 D 4 ) through the end of a reciprocating fast-scanning probe. Two intensified CCD cameras are used to record C -1 and C +2 emission patterns simultaneously from near-perpendicular views. Plumes are modeled using a Monte Carlo impurity transport code, from which values for the background flows may be extracted. The sensitivity of the plume structure is investigated for a number of code inputs, including radial electric field (E r ) and the neutral launch dynamics. Initial modeling results indicate discrepancies between values of v || and E r extracted from the plumes and measurements obtained from probe data.