Alcator C-Mod Weekly Highlights May 16, 2005 FY2005 weeks of research operations Planned: 17 weeks Completed: 7.5 weeks Operations ---------- Last week was a maintenance week at Alcator C-Mod. No run days were scheduled. On Wednesday the vessel was vented in order to remove the lower hybrid launcher, which had developed a leak. The vessel was pressurized in argon, and considerable effort was made to minimize infiltration of air into the chamber. Over 10 grams of dust was also removed from the vessel in the vicinity of C-port, where the LH launcher had been installed, and documentation photos were made. The dust is being analyzed; preliminary results indicate the presence of titanium and trace amounts of boron. Inspection of the LH launcher indicated substantial degradation of the front section of the grill, as well as the presence of additional dust-like material in the waveguides. The old C-port horizontal flange was then installed and the vessel pumped down and leak checked on Thursday. The vessel is presently undergoing a 120C bake and ECDC in D2. Plasma operations are planned to resume this week. Physics ------- Analysis of the highest plasma pressure discharge from April 26 (reported previously) has revealed the presence of a large low frequency MHD mode, apparently triggered by a "monster" sawtooth during the high pressure phase, when beta_N~1.77. The mode, which persisted for about 50ms after the sawtooth collapse had even toroidal parity indicating a tentative mode identification as an m=3, n=2 mode. The sawtooth trigger suggests that it could be a Neoclassical Tearing Mode (NTM). However, in comparison with MHD modes that have previously occured at high beta in C-Mod, the normalized collisionality nu*i/omega*e was in the same range, around 0.13 at the start of the mode. After the sawtooth collapse, the RF power tripped and so beta dropped during the mode. There was no clear dependence of the mode amplitude on the changing beta value. This point lies in the same range as the previous high beta MHD modes, somewhat below the expected threshold in terms of the DIII-D/AUG NTM scaling. Later in time during the same discharge, the density remains high at about 4.5 x 10^20 m^-3 while the temperature drops, so the collisionality increases dramatically during the mode. An analysis of the MHD stability parameter delta' remains to be done to see how close this mode may be to classical tearing instability. The manuscript "Microturbulent drift mode stability before internal transport barrier formation in the Alcator C-Mod radio frequency heated H-mode" by M. H. Redi, W. Dorland, C. L. Fiore, J. A. Baumgaertel, E. M. Belli, T. S. Hahm and G. W. Hammett has been accepted for publication by Physics of Plasmas. Long Pulse Diagnostic Neutral Beam System ----------------------------------------- The interlocks required to be satisfied for the the beam to fire short pulses into C-Mod plasmas were tested last week. The interlocks are based on four conditions: the source gate valve position, the calorimeter position, the C-Mod gate valve position, and the beam vacuum pressure. For a plasma shot, both gate valves must be open, the calorimeter must be out of the beam line, and the beam-line pressure must be less than 1x10(-4) Torr. The mass flow controller (MFC) on-time for the anode gas was reduced and the pre-fill time for the both the anode and cathode gas was reduced in order to decrease the overall amount of gas input to the DNB during each shot. The beam cryo pumps were regenerated, and heater tapes were added to the cryo pump LN2 vents to reduce ice formation. Most of the DNB signals between the Russian CAMAC and the beam hardware were documented and checked. Many of these signals have been transferred from Russian CAMAC control to local PLC control. CAMAC modules were added to provide fast data acquisition for beam signals into the local C-Mod CAMAC crate. This addition will allow beam data to be taken with each shot and stored in MDSPlus. A CAMAC analog output module was installed to provide signals for the anode and cathode MFC's. The MFC's are temporarily controlled by triggering function generators located near the beam control rack. This analog output module will allow remote control of the mass flow rate of hydrogen gas to the source anode and cathode, as well as allowing the on/off time of the MFC's to be strictly controlled to the minimum length of time required for each shot to help reduce the gas load into the beam tank. ICRF Systems ------------ Modifications were made to the water cooling system on FMIT transmitter #2, and additional maintenance to eliminate intermittent overflow problems encountered during recent weeks. Power Systems -------------- Preventive maintenance and inspections were performed on the OH1 and OH2 commutation switches. One failed fuse (out of twelve) was located and replaced in the OH2U switch. Travel and Visitors ------------------- Bruce Lipschultz attended the US Plasma Facing Components meeting at PPPL May 9-10. He presented a summary of C-Mod results from the current run period comparing un-boronized machine to post-boronization. The PFC steering committee endorsed a refocussing of their work towards more high-Z issues based on C-Mod results and the information that ITER needs on these issues. On Monday and Tuesday, Earl Marmar participated at PPPL in the IEA Executive Committee meetings for IEA Implementing Agreements on Large Tokamak facilities (LT) and on Tokamaks with Poloidal Field Divertors (PD). On Thursday and Friday he returned to PPPL for the first meeting of the U.S. ITER Project Program Advisory Committee. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly