Alcator C-Mod Weekly Highlights May 8, 2000 Operations: ----------- Plasma operations continued at Alcator C-Mod last week. Three run days were scheduled and completed, although one run was delayed by a power outage. A total of 40 plasma shots were obtained. Prior to plasma operation, the first boronization of the current campaign was carried out on Monday night. We used 127 psi of diborane which should produce a film about 1860 A thick on average. The boronization was followed by 15:40 hours of ECDC in helium at 5.e-5 Torr. The primary goals of the week's operation, re-establishing operation after boronization, continuation of the ICRF commissioning up to high power, and characterization of the interactions of the diagnostic neutral beam with tokamak plasmas and initial checkout of DNB diagnostics, were all accomplished. Over 5MW of ICRF power was injected into plasmas using all three antennas by the end of Friday's run. EDA H-modes lasting up to 0.5sec were obtained using the D- and E-port dipole antennas. H-mode was also obtained using the J-port antenna alone. We are now ready to begin the physics phase of the 2000 Campaign. Plasma operations are scheduled to continue next week. Physics and Diagnostics ----------------------- Results from simple 1-D modelling of the neutral behavior in the C-Mod divertor, compared with recent divertor bypass experiments, suggest that a large fraction (~ 50%) of ions that strike the divertor plates recycle directly to the midplane as neutrals via leaks in the outer divertor plate structure (or through the bypass when open). The primary leakage path with the bypass closed is the open divertor ports. The modelling explains the experimental observation that increasing the present leakage conductance by opening the bypass doesn't change the leakage flux significantly. The resulting drop in divertor gas pressure is also explained by the model. David Winslow (UTx) completed installation of the Texas probe; bakeout of this probe vacuum system is in progress. Bakeout of the re-installed A-side scanning probe (ASP), which had experienced an internal short last month, has been completed, and this system is being readied for operation during this week's runs. ICRF Systems ------------ Commissioning D, E, and J-port antennas continued this past week after the first boronization of the campaign. D and E-port are operating up to 1.7 MW each. For H-mode target plasmas, the maximum injected power is limited by the antenna voltage to typically 1.7 MW and 1.2 MW for D and E-port, respectively. We are currently investigating the reason for the differing voltage limits. J-port phase feedback has become operational and the interference with diagnostics and EF4 have been reduced. Impurity production by the J-port antenna has been greatly reduced compared to the previous campaign. No large impurity events, seen repeatedly last campaign above 1.2 MW, have been observed up to 2.5MW. The camera views of the J-port antenna also suggest that the arcing seen between front tiles has been eliminated. We have obtained an H-mode with J-port alone and obtained 5 MW with D, E, and J-port antennas (with brief pulses up to 5.7 MW). Further investigation of the heating efficiency and impurity production for the different antennas needs to be performed. DNB Systems ----------- The DNB was operated into plasma discharges this week. Deuterium beams were fired into deuterium plasmas. The typical operating voltages were between 40 and 45 kV. There was no indication that these were the available upper limits for the beam operation. Improvements in cooling for both the suppressor and the mod/reg led to more reliable operation. The beam operation continues to suffer from reduced arc current during C-Mod shots which is likely caused by sagging of the 480VAC due to other power supplies on the same service. This will be followed up this week. The BES diagnostic observed the interaction of the low pressure D2 gas in the C-Mod vacuum vessel during fizzles and during a gas only shot. In the latter case, the beam was also observed with the LANL fast framing camera. Beam plasma interaction was observed on the BES diagnostic, but was not so clear -- if present -- on the CXRS diagnostic. The CXRS problems may be due to the use of deuterium beams compounded by the arc current problem mentioned above. Travel and Visitors ------------------- Bob Granetz, Joe Snipes, and Jesus Ramos participated in the FIRE Workshop at PPPL last week. Miklos Porkolab and Rejean Boivin attended the APS Division of Plasma Physics Executive Committee Meeting in Long Beach, Ca., on April 29th.