Alcator C-Mod Weekly Highlights August 17, 2009 FY2009 weeks of research operations Target: 9 weeks Completed: 3.9 weeks Discharges: 567 Operations ----------- Plasma operations continued on Alcator C-Mod last week. Four run days were scheduled. A total of 90 discharges were produced, with startup reliability of 85%. The first boronization of the FY09 experimental campaign was carried out on Monday night. The runs supported miniproposals from the Boundary, Transport, and ICRF physics topical science areas. The runs on Tuesday morning and Wednesday continued the evaluation of the boron-coated molybdenum tiles (MP#568) following boronization. Post-boronization plasma performance was not improved relative to previous experience with uncoated tiles. At high power (P_RF>3MW) we observed significant tungsten radiation from the core plasma, associated with increased disruptivity, when operating in standard lower single null (LSN) equilibria. Moving the strikepoint above the row of tungsten tiles was found to be successful in eliminating the excessive W influx and permitted operation at higher ICRF power. The second part of MP#561 "GPI Measurements of SOL turbulence and comparisons to the SOL width" was carried out on Thursday. Gas puff imaging turbulence data were obtained with D2 LSN discharges using D2 GPI puffs in both the midplane and x-region views. A current scan from 0.4 to 1.1MA at constant q95 was completed. Data were also obtained from the 2D array of views at the inboard midplane. It was also demonstrated that high density shots with low intrinsic D-alpha emission can be obtained using D2 GPI puffs into He background plasmas. Friday's run was devoted to MP#563 "Toroidal asymmetry of impurity seeding in support of ITER divertor gas injection requirements". We detected significant, systematic toroidal asymmetries in nitrogen as we puffed at the 5 different divertor capillary locations. Variations were observed in NV radiation as monitored on the K-port fibers, different AXUV diodes responses looking clockwise and counter-clockwise toroidally, and toroidal variations in neutral pressures as measured by in-vessel Penning gauges. A full dataset was obtained at nebar~9e19/m^3, Ip=1MA, Bt=5.4T. The shot-to-shot reproducibility was excellent. Clear variations in the signal levels emerged, dependent on puff location. We also made use of the capillary calibration shots that were taken on Monday to correct for variations in N2 puffing rates. Systematic toroidal variations in signal levels on order of ~30% were seen. However, the response to the puff location is complicated. Divertor N2 pressures seem to depend on whether the puff is at or 'upsteam' of the pressure gauges along field lines that strike the outer divertor. Similary, toroidal-viewing AXUV diode arrays appear to respond to an 'upstream' puff. However, NV near the x-point correlates with an 'upstream' puff, but only if one considers that the relevant field lines are the ones that map to the inner divertor. The D-port foil bolometer (newly installed for this campaign) also saw a sensitivity to puff location, with higher radiated power when the puff was nearer to D-port. These data are quite convincing that we are seeing real effects, but a full analysis is still required. Operation on Friday had to be suspended early due to a ground fault in the toroidal field system that was localized to an external instrumentation chassis. The fault detection circuitry performed nominally, and subsequent measurements confirmed the integrity of the magnet system. Some damage was sustained to the instrumentation, and repairs are underway. Plasma operations are planned to continue at Alcator C-Mod this week. Program Planning ---------------- A meeting of the Experimental Program Committee was held on Monday 8/10. The status of the campaign and major auxiliary and diagnostics systems were reviewed. A decision to carry out the first boronization of the campaign, scheduled for Monday evening, was reconfirmed. Four experimental proposals were approved and one sent back for revision. ICRF Systems ------------ All four ICRF transmitters and three antennas were successfully employed in support of experiments on Tuesday and Wednesday. A high voltage fault on FMIT#1 was successfully identified and repaired. We are troubleshooting RF pickup in the poloidal field power supplies (EF1), and have identified the source as being associated with FMIT#3; further investigation is planned for the next maintenance day. Lower Hybrid System ------------------- Thirteen of the sixteen 4-way splitters needed for the new lower hybrid launcher are leak-tight and ready for e-beam welding to the transformers. Two more splitters are at the vendor awaiting a final e-beam welding operation. The last 4-way splitter required a replacement waveguide/window assembly that is being chemically cleaned and will be ready for e-beam welding soon. Tests of the induction brazing of WR187 waveguide to the transformers have been performed successfully. Travel and Visitors -------------------- Ian Hutchinson and Amanda Hubbard were invited lecturers at last week's Plasma Physics Summer School organized by the International Center for Theoretical Physics in Trieste, Italy. During the past week Yuichi Takase from the University of Tokyo continued his August 3-14, 2009 visit to the PSFC and the Alcator C-Mod Group. He continued working with John Wright and Paul Bonoli on lower hybrid full-wave simulations for the TST2 ST device using the TORIC-LH field solver. The LH full-wave simulations concentrated on investigating the sensitivity of wave coupling and absorption to changes in the plasma current and plasma density. They also conducted high harmonic fast wave (HHFW) simulations in TST2 using TORIC. In this work they found that recent improvements in TORIC in the treatment of the second order electron finite Larmor radius (FLR) current resulted in the elimination of spurious mode generation at high poloidal mode resolution. This mode had previously been thought to be due to a HHFW to slow wave mode conversion. Yuichi's graduate student, Hidetoshi Kakuda, also visited last week. He worked with Syun'ichi Shiraiwa and Orso Meneghini on the use of the COMSOL code for analysis of Lower Hybrid results. Yijun Lin was at JET last week to participate in the ICRF mode conversion flow drive experiment (done on Wednesday). The experiment was successful, with clear effects observed from mode conversion. Preliminary analysis shows that, unlike that on C-Mod, the rotation was driven in the counter-current direction. The data are being further analyzed. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly