Alcator C-Mod Weekly Highlights August 10, 2009 FY2009 weeks of research operations Target: 9 weeks Completed: 3.0 weeks Discharges: 477 Operations ----------- Plasma operations continued on Alcator C-Mod last week. Four run days were scheduled and completed. A total of 101 discharges were produced, with startup reliability of 80%. The runs continued the ICRF commissioning process, and also supported miniproposals from the Boundary, Transport, MHD, and ICRF physics topical science areas. The runs on Tuesday morning and Wednesday continued conditioning of the ICRF systems, bringing the total power coupled into the plasma up to 5.5MW using all three antennas. The total ICRF energy coupled to the plasma during the current campaign has now reached 192MJ. Wednesday's run also produced data in support of MP#558,"Characterization of RF sheaths during ICRF", including results from pre-boronization H-modes. The B-dot probe signals during H-mode were observed to be significantly lower than in L-mode. Wednesday's run also provided data for the pre-boronization evaluation of L- and H-mode performance and radiation characteristics with the new boron-coated limiter and outer shelf lower divertor tiles at high RF power. On Tuesday afternoon we carried out part 1 of MP#561, "GPI measurements of SOL turbulence and comparison to the SOL width". This was a run to test the Gas Puff Imaging (GPI) systems under plasma measurement conditions, in particular to test the emission "contrast" between before- and during-puff conditions at the outboard midplane view and the "Xpt" view. Good images were obtained of the edge turbulence in both the outer midplane and X-region GPI views. The contrast ratio with/without the GPI gas puff seems to be high enough to proceed with parameter scans using the normal D2 gas puffs in a LSN configuration. The measured contrast for the x-point view was significantly improved compared to the results from previous campaigns. The experiment on Thursday morning supported MP#560, "High density fueling and density limits", which is designed to look at the interplay of fueling, transport and the density limit. The first part of the experiment investigates the evolution of the temperature profile, on closed field lines in the region 0.8 < r/a < 1, as the density limit is approached. The NINJA capillary system was used to fuel from the vicinity of the inner divertor "nose". Densities from 65% up to 94% of the empirical limit were obtained, and the profiles are being analyzed. On Thursday afternoon we ran part 1.B of MP#563 "Toroidal asymmetry of impurity seeding in support of ITER divertor gas injection requirements", proposed by Richard Pitts of the ITER Organisation. This portion of the experiment calls for an ohmic L-mode target plasma with N2 puffing from each of the five divertor floor capillaries, located at B, D, F, H, and K-ports. Two sequences of shots were taken with nominally identical target plasmas, as a check against shot-to-shot variations and evolution in wall conditions. We observed clear correlations between the divertor pressure rise at a fixed location and the toroidal puff location. Additional off-line testing is required to assess possible differences in the capillary gas flow rates before quantitative conclusions can be made. Spectroscopic data were obtained for low ionization states NII and NIII, and these data are being analyzed for any dependence on puff location. There was no correlation between puff location and the NV radiation. Observed variations in the NV radiation were associated with the degree of detachment of the outer divertor, and were sensitive to the temperature at the strike point, with the NV emission in the private flux zone reducing with strike point Te and the emission above the x-point decreasing, while the dominant source near the X-point remained steady. Friday's run was devoted to the plasma portion of MP#555 "PF error field evaluation and compensation". The goal was to evaluate possible changes to the error field compensation algorithm suggested by analysis of the single coil non-axisymmetric field evaluations carried out last month. Those tests were motivated by the possibility that the error-fields may have changed due to the disassembly and reassembly of the tokamak in 2008-9, and some differences were indicated by the analysis. Plasma experiments were carried out using our present algorithm, developed in 2003, and two alternate schemes, based on the recent tests. It was determined that for standard 1MA, 5.4T lower single null equilibria the original compensation algorithm performs better, based on the minimum accessible density without locked modes, than either of the alternate configurations. 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 August 3. Six experimental proposals were reviewed and approved (MPs 559-564 see http://www.psfc.mit.edu/research/alcator/program/cmod_runs.php?miniproposals&sort=id&dir=desc for details.) Three of the proposals were directly from the ITER team. There was a discussion of the schedule for the first boronization of the campaign with a decision to defer this for a week. ICRF Systems ------------ Conditioning of the D- E- and J-port antennas into plasma continued, with over 5MW successfully delivered. Troubleshooting of control issues on the FMIT#3 and 4 systems was carried out, and several components were replaced. Lower Hybrid System ------------------- We continue to fabricate 4-way splitter assemblies for the lower hybrid launcher. We now have ten of the sixteen couplers ready to be welded to the transformer/waveguide assemblies. Braze tests of stainless steel waveguide sections needed to mate the transformers to the 4-way splitter have been successfully completed, as have tests of the e-beam welding process of the waveguide sections to the 4-way coupler. The Inconel 625 side plates that contain the dummy waveguides are back from the vendor, and the stainless steel box that contains the port bellows has been welded and is leak tight. The first set of samples needed to verify the induction heating process needed for the transformer to WR187 waveguide braze has been prepared. The layout of the waveguide network (Jungle Gym) at the back of the launcher has been completed. DNB System ---------- Troubleshooting of the DNB arc power supply continued. During the course of testing the cathode heater failed (high resistance). The cathode assembly was been removed and repaired, and was re-installed on Friday evening. Travel and Visitors -------------------- Yuichi Takase from the University of Tokyo is visiting the PSFC and the Alcator C-Mod Group from August 3-14, 2009. He is working (in part) with John Wright and Paul Bonoli on lower hybrid full-wave simulations for the TST2 device using the TORIC-LH field solver. They will also be conducting high harmonic fast wave simulations in TST2 using TORIC. Last week Stewart Zweben (PPPL) was at C-Mod for two days to run the first part of MP #561 with Jim Terry, "GPI measurements of SOL turbulence and comparisons to the SOL width." _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly