Alcator C-Mod Weekly Highlights May 23, 2005 FY2005 weeks of research operations Planned: 17 weeks Completed: 7.5 weeks Operations ---------- Plasma operations resumed after completion of a 120 degree bake and discharge cleaning period. The tokamak was cooled to operating temperature by Wednesday morning, marking a one week turnaround from the beginning of the vent. Three run days were scheduled and two were accomplished. Wednesday's scheduled run was cancelled in order to repair a transformer feeding a contactor in the alternator drive motor system, which caused an interlock trip during the morning setup procedure. All tokamak systems functioned nominally on Thursday and Friday. The runs on Thursday and Friday were devoted to plasma conditioning in recovery from the previous week's vent, during which the lower hybrid launcher was removed from C-port. The reconditioning process is proceeding well. Full length (up to 2 seconds) plasmas with over 1MA plasma current were produced, although startup reliability was relatively low. Over the two days, 16 plasma discharges were produced. This behavior is fairly typical as we clean up following an opening. ICRF power up to 3MW has been coupled to these target plasmas, and brief H-modes were obtained. The H/D ratios were as low as 6% in ohmic plasmas and less than 15% with ICRF, indicating that the argon backfill and isolated flange removal operation was successful in minimizing influx of room air into the vessel during the vent. Core titanium levels, monitored by VUV spectroscopy, are reduced relative to those observed during the week ending May 6, and dust injection events are greatly reduced, as determined from bolometry as well as by inspection of video images. Flattop disruptivity on Friday was zero, with all plasmas extending well into ramp-down. Molybdenum levels were also low, near typical post-boronization levels. Deuterium fueling, believed to be associated with outgassing from the remaining in-vessel titanium dust, was still present, but decreasing with accumulated plasma time. Ohmic target densities below 8.5e19/m^3 were obtained during Friday's run. Plasma operations are scheduled to continue this week. Lower Hybrid System ------------------- Analysis of the lower hybrid launcher and of the dust removed from the vessel is proceeding. Serious erosion of the coupler surfaces has been found. Erosion of the channels extended deep into the waveguide, and in some cases dust was found piled up against the alumina vacuum windows, ~10cm from the coupler face. Using SEM/EDS techniques it has been determined that essentially all of the dust consists of the titanium alloy used for the couplers (alloy 6242). Approximately 15 grams of titanium dust was vacuumed from the vessel, and larger amounts are expected to be removed from the launcher as analysis of the coupler components proceeds. Additional analysis techniques are being employed to quantify the amount of deuterium in the samples, and ascertain how much of the dust is titanium deuteride. It is not yet clear what the relative contributions from D2 backfills, boronizations, routine ECDC, or tokamak plasma operations were to the damage observed. It is known that titanium will form hydrides when the oxide layers protecting the surface are removed and the material is exposed to hydrogen. Tests of alloy samples which have been etched to remove the oxide, placed in vacuum, baked, and then exposed to 500 Torr of D2 for 24 hours at 60C showed no deleterious effects. However, the in-vessel components may have experienced more effective removal of an oxide layer due to ECDC and plasma operation over time, which could have rendered it more susceptible to the deuterium. Though understanding the nature of the damage is of interest, our primary effort now is concentrated on replacing the couplers. A meeting was held at MIT on Tuesday, May 17, to establish guidelines and initiate activities in support of this goal. Attending from MIT were Bill Beck, Bob Childs, Dave Gwinn, Jim Irby, John Liptak, Shyri Marazita, Earl Marmar, Ron Parker, Miklos Porkolab (by telephone), Vincent Tang, Dave Terry, Rui Vieira, Steve Wolfe, and Jim Zaks; from PPPL, Gerd Schilling and Randy Wilson. Also attending via teleconference at PPPL were Stefano Bernabei and Rich Hawryluk. Several design concepts were discussed, and action items for pursuing these were agreed. No titanium will be permitted in the new design. A follow-up meeting has been scheduled for this week. A more detailed summary of the launcher history and status, with photographs, is available at http://www.psfc.mit.edu/research/alcator/data/20050523_lh_launcher_summary.pdf Long Pulse Diagnostic Neutral Beam System ------------------------------------------ The new DNB operated into C-Mod plasmas for the first time on Friday. Short (30msec) pulses were injected, and spectra used for assessing beam performance were recorded. Beam parameters were 46kV, 4 amps. In other DNB system work last week, The anode and cathode gas mass flow controller gains were adjusted to minimize the settling time during the step in command from the closed state to a steady state flow condition. This allowed a reduction in pre-fill time from 1 second to 500 ms. Work is also proceeding on the interlock circuitry required before long beam pulses into the C-Mod vessel can be permitted. Travel and Visitors ------------------- Randy Wilson and Gerd Schilling were at MIT 5/17 for discussions on repair of the Lower Hybrid coupler. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly