Physics and application of impurity plume dispersal as an edge plasma flow diagnostic on the Alcator C-Mod tokamak
Author(s)Gangadhara, Sanjay, 1972-
Massachusetts Institute of Technology. Dept. of Nuclear Engineering.
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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.(cont.) This electric field accelerates impurity ions formed near the probe tip away from the probe, causing jet-like behavior. LIM was also used to investigate causes for the vertical elongation of the impurity emission. Results suggest the existence of a probe-induced E x B drift, of order [approximately] 1000 m/s in the near SOL. This drift may be responsible for the transport of both impurity and bulk plasma ions down the probe axis. Values for v in the far SOL and E[sub]r in the near SOL have been extracted from the plume structure. A comparison between plume and probe results for E[sub]r suggests that calculations which employ a probe-sheath model may be in error, and that measurement of the poloidal propagation velocity of edge plasma fluctuations may be a more reliable means of inferring E[sub]r from probe data. Comparisons between plume- and probe-inferred values for the parallel Mach number suggest that the probe over-estimates parallel flow to the divertor in the far SOL, where effects of short field line connection to the divertor are important. This result strengthens the argument for the main-chamber recycling view of particle flows in the Alcator C-Mod SOL.
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2003.Includes bibliographical references (p. 265-273).This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
DepartmentMassachusetts Institute of Technology. Dept. of Nuclear Engineering.
Massachusetts Institute of Technology