Impurity transport experiments in the edge plasma of Alcator C-Mod using gas injection plumes

Understanding impurity and bulk plasma transport in the scrape-off layer (SOL) of tokamak plasmas is critical for the design of future reactors. The development of a system on Alcator C-Mod for inferring impurity transport behaviour parallel and perpendicular to local magnetic field lines from impurity emission patterns (`plumesʹ) generated by local gas injection will be presented. Gas is injected at variable location in the SOL through the end of a reciprocating fast-scanning probe. Carbon plumes are generated by puffing ethylene gas (C2H4) through the probe over a period of ∼8–10 ms. Two intensified CCD cameras are used to record C+1 and C+2 emission patterns from near-perpendicular views. Flows parallel and perpendicular to the magnetic field have been observed in both views. In principle, the data allow a full 3D reconstruction of the impurity dispersal with ∼1 mm spatial resolution.