Alcator C-Mod Weekly Highlights Oct. 2, 2000 Plasma operations continued on Alcator C-Mod last week. Five run days were scheduled and completed. A total of 84 plasma shots were obtained, with a startup reliability of approximately 80%. Experiments carried out included an investigation of main chamber recycling, completion of the recombination experiment using the divertor TV views, scanning probe studies of the quasi-coherent mode in low power (ohmic) EDA H-modes, and further work on the four-strap ICRF antenna. After the previous week's vacuum incident, a 110C bake followed by electron cyclotron discharge cleaning (ECDC) in D2 were carried out. Machine conditions were evaluated during the runs Monday and Tuesday; aside from a relatively high H/D ratio, initially over 20%, the vacuum conditions were satisfactory. On Tuesday evening a fresh boronization was carried out; about a 1600 Angstrom thick layer was deposited. H/D ratios on Wednesday were below 5%, in the range for efficient minority heating, and EDA H-modes were readily obtained with both ohmic and ICRF heating. Physics ------------ Experiments were run to get additional data on edge fluctuations and profiles in EDA H-mode. The EDAs were more clear than in some previous OH runs with Quasi-Coherent fluctuations clearly seen on the PCI, reflectometer, and electrostatic probes. We have some good data for analysis of fluctuation driven flux. We should also have Ly_alpha data to look at the ionization source, neutral density and Deff. Some progress was made on localizing the QC mode. As before, it is clearly seen in the steep density gradient region. Images of carbon plumes from the gas puffer in FSP showed a flow reversal just outside the region where the QC mode began. Since the flow reversal implies a change of sign of Er, this observation might be interpreted to localize the mode relative to the Er well. On several shots it appeared that the north floating probe of the ASP traversed the entire mode. If so, the structure would be not much more than 2mm in width. Plume studies were continued to investigate the variation of plume structure with injection depth during EDA H-mode. Plumes are generated via gas-injection of deuterated ethylene (C2D4) through the F-bottom scanning probe. The plumes are imaged from two near-perpendicular locations (F-side, F-top) with coherent fiber bundles, which are optically coupled to gated, intensified CCD cameras through beam-splitter optics and bandpass interference filters. The optical system allows for imaging of C^+1 and C^+2 emission simultaneously at each view location. During the run, the injection depth was varied over a fine scale ranging from right at the separatrix to just a few mm outside. Some interesting results were obtained, including: (1) Plumes generated at the separatrix exhibit cross-field dispersion indicative of a radial electric field pointed inwards. Similar results were obtained for plumes generated at this depth during ELM-free H-mode; (2) Plumes generated ~ 2.5-3.5 mm outside the separatrix exhibit a shearing in cross-field dispersion (as seen from the F-side view), in which sections of the plume cross-field dispersion indicate a radially inward Er and other sections indicate a radially outward Er; (3) Plumes generated ~ 4 mm or beyond the separatrix exhibit cross-field dispersion consistent with Er directed radially outward. These results suggest a shear layer which exists for a thickness of ~ 4 mm. Measurements from the FSP of the QC fluctuation associated with EDA indicate that the mode exists only within the region of negative (radially-inward) Er. Analysis is on-going. ICRF Systems ------------ Following reconfiguration of the feeds to the J-port four-strap antenna, based on an improved transmission line model, the antenna performance was markedly improved. For an upper x-point L-mode target plasma, J-port heated as effectively as the D and E-port dipoles with no major impurity injections. Comparison shots were taken at a power level of 1.5MW. Additional experiments to evaluate the maximum power capability of the antenna in the present configuration, and the heating efficiency into H-mode targets, are planned for this week. DNB Systems ----------- We continued our investigation of the DNB drift duct failure. Beam reionization calcuations for the full range of conditions in C-Mod, including disruptive discharges and fizzles, were completed. Measurements of the magnetic field in the vicinity of the damaged area were completed. Measurements of the beam profile were correlated with the background gas pressure in C-Mod. The implications of failure of the diverting magnet to turn on during the pulse were investigated. The results of these investigations will be discussed in a meeting on Tuesday with the expectation that a plan can be finalized for repair and modification to prevent a recurrence of the failure. Travel and Visitors ------------------- Jim Irby attended the first meeting of the Fusion Facilities Operations Committee at PPPL last week. He discussed machine design, machine operations, and safety issues with representative from PPPL, GA, and DoE. A list of experts from each facility is being developed to help all sites with technical problems.