Alcator C-Mod Weekly Highlights July 24, 2000 The last full week of invessel activity has been completed. Alcator C-Mod will be pumped back down early this week. Modifications to all the antennas will be completed by 7/25 as will numerous cleanup and repair activities associated with invessel diagnostics. Maintenance and calibration activities continued on power and cryo systems. Invessel Work ------------- Work continued this week on rf shielding of the perimeter of the PPPL ICRF antenna. The last measurements performed at the end of June supported the idea that the low heating efficiency was due to rf power leaking around the back of the antenna and coupling to plasma surface waves. The top and bottom of the antenna have now been enclosed with stainless steel shim stock and spot welded to the antenna frame and vacuum vessel wall. The left and right sides of the antenna frame could not be directly grounded to the vessel wall because eddy currents generated during disruptions would have been too large. Instead, rf bypass capacitors have been installed between the antenna and the wall. These capacitors are essentially shorts at rf frequencies, but appear as a relatively high impedance to disruption currents. All molybdenum protective tiles on all antennas have been replaced with boron nitride tiles. In addition, a new set of boron nitride tiles has been installed between the D and E antennas to better protect their Faraday screens. The viewing telescope, optics, and coherent fiber bundle for the new PPPL 2-D turbulence imaging diagnostic have been installed inside the vacuum vessel. A test target at the position of the gas puff was clearly imaged onto an external video camera. Invessel calibration and alignment of the F-top and F-side optical systems used for viewing "plumes" generated by gas injection from the F-bottom scanning probe took place during the recent C-Mod opening. Both a relative pixel calibration and an absolute brightness calibration were obtained, the latter using the Labsphere. A back-illuminated aluminum plate with holes drilled on a precise grid pattern was used to determine pin-cushioning and other distortion effects. Absolute alignment was performed by viewing a small light bulb placed at various known locations in the vessel, e.g. in grooves between divertor tiles and at the tip of the scanning probe when inserted to a known vertical position. The system is now ready for operation during the upcoming run campaign. Other Engineering Activity -------------------------- Work continued last week on an upgrade to our glow discharge system. Besides a general cleanup of the layout and improvements to the PLC control electronics, we are also adding fast solid state controllers to the current going to the glow paddles. These controllers will be able to rollback the current to the electrodes in a few microseconds and limit the energy dumped into arcs to a few mJ. This energy will be insufficient to sputter material from the vessel walls. Our 13.8 kV current and pulse transformers have been re-calibrated. Results indicate both current and voltage measurements were accurate to within +-2%. Fuses and breakers associated with C-Mod power systems have been inspected. A fuse for the A-phase of the FMIT#1 transmitter had clearly been arcing at some point since the last inspection. This condition may explain some of the transient fault conditions experienced over the last few weeks of operation. Preventative maintenance to the CRYO system proportional valves continued last week. Work on the new cryostat exhaust valve also made progress. All CYRO system filters have been cleaned or replaced. The LN2 pump has been removed from the sump and is undergoing an inspection. Quarterly Review ---------------- A C-Mod Quarterly Review was held on July 20th. Rostom Dagazian and Warren Marton from DoE were at MIT for the review. Presentations by MIT, University of Texas at Austin, and PPPL personnel were given. The agenda and the presentations can be viewed in PDF format at http://www.psfc.mit.edu/cmod/sciprogram/q_rev_jul00/ Travel and Visitors ------------------- Stewart Zweben from PPPL was at C-Mod for the week, working with Jim Terry on the turbulence imaging installation and presenting a talk at the Quarterly Review. Miklos Porkolab attended the Fusion Power Associates Meeting on July 17 in San Diego and made the following presentation: Alcator C-Mod Program and Recent Results. The following 2 days he attended the FESAC meeting at General Atomics where the recent draft of the Integrated Program Planning Activity document was discussed. Earl Marmar also attended the FESAC meeting, as well as the first meeting of the Next Step Options Program Advisory Committee.