Alcator C-Mod Weekly Highlights August 31, 2009 FY2009 weeks of research operations Target: 9 weeks Completed: 5.6 weeks Discharges: 773 Operations ----------- Plasma operations continued on Alcator C-Mod last week. Following a fresh boronization, completed on Saturday, August 22, five run days were scheduled and completed. Experiments supported research in the ICRF, Boundary, and Transport topics. A total of 146 plasma discharges were produced with a reliability of 97%. Plasma operations are planned to continue this week. Operations Details ------------------ Runs on Monday and Thursday supported MP#568, "Plasma performance evaluation with boron-coated molybdenum tiles", and were designed to evaluate the persistence of the beneficial effects of boronization, compared to previous experiments with all-metal plasma facing components. Preliminary analysis suggests that the boron coating on tiles in the outboard limiters and outer "shelf" has substantially reduced, but not completely eliminated the Mo sources. Contributions to radiated power from other metallic impurities are being evaluated. The experiment on Tuesday was the first in a series of runs called for in MP#570 "Boundary layer heat transport experiments in L-mode plasmas", which supports the FY10 OFES Joint Milestone "... measure the divertor heat flux profiles and plasma characteristics in the tokamak scrape-off layer in multiple devices to investigate the underlying thermal transport processes..." Most of the goals set out for the first experimental session were accomplished, including a density scan in LSN ohmic L-mode plasmas at 0.8MA, 5.4 T, and initial points on a density scan at 0.54MA, 5.4 T. Data were obtained from fixed and scanning probes, the IR camera, surface thermocouples and tile calorimeters, as well as turbulence measurements using Gas Puff Imaging (GPI). On Wednesday morning we attempted to finish the high density (2e20/m^3) portion of the ohmic L-mode part of MP#563 "Toroidal asymmetry of impurity seeding in support of ITER divertor gas injection requirements". However, difficulty in setting an acceptable level for the divertor nitrogen puff, together with high disruptivity apparently associated with molybdenum injections, precluded completion of the planned sequence within the available time, and the experiment was changed in favor of the second scheduled program, in support of MP#560 "High density fueling and density limits". Density limits in helium were investigated in this half-day experiment. The density limit discharge obtained on August 20 was re-established and the equilibrium changed to inner-wall limited. Te profile and turbulence data were collected, including GPI with the two fast cameras and diode arrays. ICRF was successfully coupled to these high density helium plasmas, up to 75-80% of the Greenwald limit. Diverted equilibria with ICRF heating were also investigated. In these cases fueling was less efficient, and a strong "pump-out" effect was observed at the time of RF turn-on, followed by a slow recovery in density back to the level reached in the ohmic phases. At high power the edge temperature (at the 95% flux surface) exceeded 200eV, and brief H-modes were obtained in some discharges with target density over 70% of the limit. Friday's run was devoted to MP#558, "Characterization of ICRF induced sheaths using emissive, Katsumata, and bdot probes". The purpose of this run was to determine the governing parameters that lead to high rectified DC voltages in the presence of ICRF. Plasma potentials over 200V were observed on the emissive probes when the D-port antenna (which is magnetically connected to the probe location) was energized. The potential was highest in L-mode discharges and reduced during H-mode. Scans of plasma current and toroidal field, as well as comparison of different antennas, confirmed a maximum in the sheath potential corresponding to flux tube mapping to a region near the top of the D-antenna for this probe location. Signals on the RF B-dot probes showed no clear correlations with the measured potentials, indicating the sheath is not a result of local RF fields. Detailed analysis of these results is underway. Program Planning ---------------- A meeting of the Experimental Program Committee was held on Monday August 24. Three experimental proposals were reviewed and approved (MPs 569-571 see http://www.psfc.mit.edu/research/alcator/program/cmod_runs.php?miniproposals for details.) There was discussion and agreement on the schedule for runs and maintenance over the next 3 weeks. ICRF Systems ------------ All four transmitters were employed in experiments on Monday, Wednesday, Thursday, and Friday. Short interruptions were required for replacement of fuses in the FMIT#3 screen voltage supply and FMIT#1 charging circuit, resulting in minimal operational delays. FMIT#3 also had an issue with the FPA bias supply late in Friday's run, and should be repaired on Monday. Investigation of the source of interference from the FMIT#3 transmitter (J-port antenna) on the EF1 poloidal field coil supply continued. Lower Hybrid System ------------------- All sixteen 4-way splitters have now been completed and are leak-tight. All sixteen transformers have been brazed to the stainless steel extensions that will allow them to be e-beam welded to the 4-way splitters. Two steps remain to finish the high power front end components of the launcher. The transformers will first be brazed to the WR-187 waveguide, and the transformers will then be e-beam welded to the 4-way splitters. Travel and Visitors -------------------- David Mikkelsen (PPPL) spent the week at MIT working with the C-Mod group on GYRO turbulence simulations of C-Mod plasmas with a special focus on momentum transport. Plans for particle transport experiments were also discussed. Stewart Zweben (PPPL) was at MIT to help get GPI data for two experiments, "Boundary layer heat transport experiments in L-mode plasmas" by LaBombard et al, and "High density fueling and density limits in He plasmas" by Greenwald et al. Good GPI data were obtained using both the midplane and X-region view at 250,000 frames/sec for both experiments. In particular, the edge turbulence images obtained near and at the density limit were the best obtained to date on C-Mod. Jim Terry spent Aug 19-Aug 21 in Washington DC as part of the "Committee of Visitors" charged by FESAC and OFES to assess processes used by OFES to select and monitor their funded programs, as well as the effect of those processes on the resulting portfolios. Jim was chairman of one of the sub-committees within that committee. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly