Physics and application of impurity plume dispersal as an edge plasma flow diagnostic on the Alcator C-Mod tokamak

A unique system has been developed for studying impurity transport in the edge plasma of Alcator C-Mod. Impurity gas (which for these experiments is deuterated ethylene, C₂D₄) is injected locally into the scrape-off layer (SOL) through the end of a reciprocating fast-scanning probe, and the resultant emission ("plume") is imaged from two near-perpendicular views. Plumes are generated at various SOL depths, while views are obtained using coherent fiber bundles optically coupled to gated, intensified CCD cameras through beam-splitters, allowing for the simultaneous acquisition of C⁺¹ and C⁺² emission. Plume structure is observed to depend on local values of electron density and temperature, background parallel flow (v[sub][parallel]), and radial electric field (E[sub]r). Emission resulting from sputtering of carbon deposited on the probe surface also contributes to the structure. For plumes generated in the near SOL, emission contours are non-elliptical and the parallel extent relative to the ionization mean-free path is large, indicating that transport of impurities near the probe is "jet"-like along B. Plume results also suggest an elongation of the impurity ion density down the probe axis. A Monte Carlo impurity transport code (LIM) was used to simulate the plumes. Results indicate that contributions to the emission from sputtering explain the cross-field plume width, and that the parallel extent of emission generated in the far SOL is well-described using a sputter launch-energy distribution for the impurities. In the near SOL, the presence of a localized parallel electric field arising from background ion recycling off the probe surface is necessary to explain the parallel extent of emission generated in this region.

Department dc:contributor.department

Massachusetts Institute of Technology. Department of Nuclear Engineering

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2003

Author dc:creator

• Gangadhara, Sanjay, 1972-

Advisor dc:contributor.advisor

• Brian LaBombard.

• Nuclear Engineering.

Statement dc:rights

• M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/1721.1/16602

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/16602