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 (“plumes”) 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 3-D reconstruction of the impurity dispersal with ∼ 1 mm spatial resolution.