Alcator C-Mod Weekly Highlights June 28, 2010 FY2010 weeks of research operations Base Target: 13 weeks Base Completed: 9.7 weeks ARRA Target: 5 weeks ARRA Completed: 5.0 weeks Plasma Shots: 1630 Operations ----------- Following a final successful leak check of the vacuum vessel, the C-Mod magnets and structure were cooled down to LN2 temperature in preparation for operation. The new alternator cooling system was fully commissioned, and the alternator was brought up to full speed on Thursday. C-Mod operations resumed on Friday with full power tests of all supplies and magnet systems. A single plasma shot was attempted, resulting in a successful initiation but no sustained plasma current. Plasma operations for machine conditioning, diagnostic checkout, and commissioning of tokamak subsystems are scheduled to commence this week. Physics ------- During the 2009-10 experimental campaign concluded in March of this year, we operated C-Mod with specially prepared boron-coated molybdenum tiles located on the top outer divertor, and plasma and RF limiters Mo tiles. These tiles were vacuum-plasma sprayed with ~100um of boron, and the coating thickness of each tile was measured prior to installation. The purpose of this experiment was to investigate and further quantify the previously reported apparent erosion rate, up to 15-20nm/sec, of boron films deposited by boronization. We anticipated that the plasma performance and RF associated impurities would be improved, requiring less frequent boronization to achieve good plasma performance. Results of this initial ~6 month operating period with B-coated Mo tiles are summarized below. We found, consistent with previous results, that one boronization was required to reduce other metallic impurities like iron before high performance plasmas could be obtained. For ICRF heated H-modes, the core molybdenum levels have been significantly reduced and remained at low levels for increased injected RF energy. The core Mo level no longer scales with RF power in L-mode. Impurity seeding (typically nitrogen or neon) improved both the plasma and ICRF antenna performance. Surprisingly, impurity seeding did not result in increased core Mo levels and also suppressed antenna faults. Spectroscopic monitoring of the plasma limiter found that the impurity profile at the limiter was centered near the plasma mid-plane and the profile did not change shape with plasma current. We had expected that the boron would be significantly eroded, by up to 20-30 um, and that an erosion pattern would be evident post campaign. From post campaign inspection, the B coating was not significantly eroded except in locations where melting occurred or where it peeled. Coated tiles that peeled were scattered among the good coated tiles and no discernible pattern was found. Trace material analysis also found that the B surface was contaminated with Mo, tungsten (W), and stainless steel (likely iron and chromium). Improved performance with impurity seeding, the lack of erosion, and metallic contamination of the B coating suggest that the high boronization erosion rate is related to localized heat loads (that can lead to melting) rather than entirely a result of sputtering. Further analysis of results from the previous campaign is underway, and additional data will be obtained once C-Mod operation resumes. ICRF Systems ------------ New solid state IPA units for FMIT #1 and #2 have arrived in the U.S. and are in the process of clearing customs. We intend to install these units following completion of the FY10 experimental campaign. We have completed a full antenna model of the new four-strap antenna for RF analysis. This will allow verification of the proposed layout. We have received two additional ARRA funded 5" feed-through units from our vendor. Lower Hybrid System -------------------- The LH power system and the new launcher are being prepared for initial operation into C-Mod plasmas. High power calibration of the LH drive and monitor leg is proceeding. The reflected power signal diodes were recalibrated following installation of additional 3dB attenuators on the split waveguide channels on the jungle gym. Attenuation in the probe circuits has been set in preparation for probe calibration planned for the coming week. Diagnostic Neutral Beam Systems -------------------------------- Petr Deichuli and Grigory I Shulzhenko from the Budker Institute (Novosibirsk, RF) began a visit to C-Mod for the purpose of trouble-shooting the C-Mod Diagnostic Neutral Beam. These technical experts were part of the team tht built and delivered the DNB to MIT in 2005. A hard drive failure necessitated replacement of the DNB CAMAC control PC. Reinstallation of the necessary software and drivers was facilitated by the original programmer from the Budker Institute, in coordination with our visitors and PSFC personnel. The DNB was successfully returned to operation, and trouble-shooting of breakdowns in the arc circuit at high current settings is proceeding. Discussions of improvements to the cathode heater design are also in progress. Travel and Visitors ------------------- Jerry Hughes, Earl Marmar, John Rice, Yijun Lin, Orso Meneghini, and Greg Wallace participated in the 37th European Physical Society Conference on Plasma Physics in Dublin, Ireland last week. Copies of the C-Mod presentations will be available on-line at http://www.psfc.mit.edu/research/alcator/pubs/eps/eps2010/eps_2010.html Miklos Porkolab, Paul Bonoli and John Wright attended the Anomalous Absorption Conference 2010 (AAC2010), in Snowmass, Colorado, June 14-18th. This was at the invitation of the AAC 2010 organizing committee (LANL) on the occasion of the 40th anniversary of this conference, to encourage better communication and interaction among MFE and IFE/ICF scientists, including the subjects of diagnostics and large code development. Paul Bonoli presented a talk: "Integration issues for simulating wave-particle interactions in tokamak plasmas ". John Wright presented the poster: "Challenges in self consistent full wave simulations". Miklos Porkolab presented the poster: "Studies of waves and turbulence in tokamak plasmas with phase contrast imaging and comparisons with code predictions by synthetic diagnostics". In addition, during the session "Historical Talks/Comments", Miklos presented a brief talk regarding his recollection of the first two meetings in 1971 (PPPL) and 1972 (Boulder, Co). Hugh Bishop and Brian Steer from CPI visited the PSFC to discuss future needs and were given a tour of the facility. Senator Tom Carper (D-Delaware) visited the C-Mod facility Friday June 25 along with his wife and son. The visit was arranged by Dr. Reiner Beeuwkes who accompanied them. The visitors were given a tour of the C-Mod experiment and discussed the goals of the experiment. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly