Alcator C-Mod Weekly Highlights July 25, 2005 FY2005 weeks of research operations Planned: 17 weeks Completed: 13 weeks Operations ---------- Plasma operations resumed last week at Alcator C-Mod. Five run days were planned, but only two were completed. A total of 43 plasma discharges were produced with a reliability of 80%. Two runs were cancelled as a result of weather-related excess demand on the local electric utility; one run was lost due to a computer problem. The planned experiments for these days have been re-scheduled. Operations are planned to continue this week. Run Summaries ------------- The run on Monday was devoted to MP#390 "D(He3) minority heating evaluation at 50MHz". A scan of He3 concentration was carried out at B=5T. H-modes were obtained at an ICRF power level of about 1MW, which is comparable to the threshold using D(H) minority heating at 80MHz in these plasmas. At increased He3 fraction, mode conversion was observed to produce localized electron heating as observed on the ECE diagnostics. Thursday's run was intended to support MP#424, on disruption mitigation using a high-pressure gas jet. Following the first plasma shot, the computer controlling the TF scanner, which monitors the turn-to-turn resistance of the TF magnet, was found to have malfunctioned. Repair and verification of this critical instrumentation took the remainder of the day, and the MP#424 experiment was postponed until Friday. On Friday, the gas jet disruption mitigation experiment was carried out successfully. Two noble gas species, neon and argon, were tested. The timing of the gas-jet-induced disruptions was very reproducible, resulting in a good sets of Thomson scattering profiles for both gases. The argon current quenches consistently started 1 ms later than the neon quenches, and the quench duration was slightly shorter. Halo currents with argon were also somewhat less than with neon. The TS temperature profiles for argon indicate that the gas barely gets into the plasma at all, whereas the neon penetrates to about r/a=0.85. Fast visible images of the neon and argon jets are quite different than those in helium (from the previous experiment carried out June 3). We obtained a good set of IR images of the divertor region, and this will be analyzed to determine the thermal energy deposition on the tile surfaces. The AXUV array with reduced gain worked well, eliminating the signal saturation observed in June. The time resolution for these measurements is 100usec, yielding detailed evolution of radiated power profiles during the disruption events. The profiles during the disruptions following neon and argon injections were quite different, and these results are being analyzed. Long Pulse Diagnostic Neutral Beam ----------------------------------- Preparations to repair and re-assemble the DNB cathode heater assembly are well underway. Tantalum sheet has been purchased for repair of the heater connection, and test coupons have been resistance welded successfully. The Russian team at Budker Institute in Novosibirsk, Russia has assembled a spare replacement heater assembly and it is currently in shipment to MIT. The DNB anode mass flow controller has been recalibrated by the vendor and is in shipment back to MIT. A new connection panel and power supply for both anode and cathode mass flow controllers has been assembled to replace the temporary set-up used for initial testing. A booster pump has been installed and tested on the source cooling circuit to increase the incoming water pressure to approximately 2.5 atmospheres. Diagnostics ----------- Measurements of Faraday rotation in the MSE optics using an in-vessel wire-grid polarizer mounted in front of the plasma-facing lens indicate a rotation of ~0.14 degrees per Tesla, which is consistent with the rotation expected from the glass substrate in the wire grid polarizer itself. This suggests that the actual MSE optical system imposes negligible Faraday rotation on the incident light. Travel and Visitors ------------------- Paul Bonoli attended a Symposium on Integrated Modeling at Princeton Plasma Physics Laboratory on Jul 19-20, 2005. He gave an invited talk entitled "Status of RF Modeling in Integrated Simulations". Joshua Stillerman and Thomas Fredian attended the Fifth IAEA Technical Meeting on Control, Data Acquisition, and Remote Participation for Fusion Research in Budapest Hungary. Two papers were presented : Digital real time plasma control system for Alcator C-Mod (Stillerman) and Web based electronic logbook and experiment run database viewer for Alcator C-Mod (Fredian). In addition Stillerman was a member of the program advisory committee for the meeting. A half day MDSplus workshop was held immediately following the meeting chaired by Fredian. Martin Greenwald participated in the FESAC meeting held in Gaithersburg, MD on July 19. Miklos Porkolab and Earl Marmar also attended. Bob Harvey from CompX visited MIT for the week of July 18-22, 2005. He worked with Paul Bonoli, John Wright, and graduate students Andrea Schmidt, John Liptac, and Vincent Tang. Bob assisted in the implementation of the CQL3D-GENRAY code on the MIT MARSHALL cluster. He also worked on the implementation of synthetic diagnostics for C-Mod measurements of electron cyclotron emission, soft x-ray emission, and energetic particles. These synthetic diagnostics use the nonthermal electron and ion distribution functions computed by CQL3D and GENRAY. Dr. Olaf Grulke from IPP Greifswald is visiting C-Mod for the next two months as a continuation of his Humboldt scholarship. He spent last year at C-Mod, and will be continuing his studies on edge turbulence with Jim Terry and Stewart Zweben. Dr. Stewart Zweben (PPPL), was on site all week. He held discussions with Jim Terry, Olaf Grulke, Martin Greenwald, and Bruce Lipschultz, and is preparing a mini-proposal for studying SOL turbulence in limited, circular plasmas. Dennis Whyte (U. Wisconsin) visited the C-Mod facility for 2 days (July 20-21) to participate in an experiment on disruption mitigation as well as continue discussions of ongoing collaborations in other areas. Those discussions covered several topics, including D retention in Mo surfaces and continued experiments to understand the process; analysis of deposition of B and D on tile sides; and disruptive removal of D from the torus tiles. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly