Alcator C-Mod Weekly Highlights August 20, 2012 FY2012 weeks of research operations Target: 18 weeks Completed: 14.2 weeks Plasma Shots: 2019 Operations ----------- Plasma operations continued at Alcator C-Mod last week. Two and a half run days were scheduled and completed. A total of 79 plasma discharges were produced with a reliability of 89%. The runs supported research in the Pedestal Physics and H-mode Scenarios topical areas. Fresh boronizations were carried out over Sunday and Wednesday nights in preparation for experiments. C-Mod is now being configured for reversed field and current operation (with the ion grad B drift directed away from the vertical plate lower divertor). Plasma operations are planned to continue this week. Quarterly Review ---------------- On Monday, we held the C-Mod quarterly review for FY2012, Q3. The meeting was held by video-conference among MIT, DOE-OFES and U. Texas. Viewgraphs from the presentations can be found at http://www.psfc.mit.edu/research/alcator/pubs/Q_Reviews/Q-rev_Aug12/Q-rev_08-13-12.pdf. Attendees included Jim Van Dam, Mark Foster, and John Mandrekas (OFES), Bill Rowan (U. Texas, Austin), Steve Scott, Randy Wilson, and Bob Ellis (PPPL), and Earl Marmar, Jim Irby, Anne White, Steve Wukitch, Ron Parker, Greg Wallace, Geoff Olynyk, Miklos Porkolab, Amanda Hubbard and Dennis Whyte (MIT). Operation Details ----------------- Tuesday's run was devoted to MP#719, "Shoelace antenna operation in EDA H-mode plasmas". The goals of this experiment, which comprises part of the thesis research of an MIT graduate student, were to operate the "Shoelace" active MHD antenna into ohmic H-mode plasmas at low q95 and small outer gaps, for optimal coupling, and map out the plasma response using fast frequency sweeping and synchronous detection techniques. We successfully optimized the target plasma configuration with an outer gap of ~1mm and q95~3. A five point scan of q95 was performed. A new phase locked loop (PLL) circuit was used to successfully track the QCM, and drive the antenna with phasing relative to the QCM of 0, -90, and 180 degrees. Data were successfully obtained with the A-port scanning probe and Gas Puff Imaging (GPI). Mirnov coil data during dynamic gap scans is being analyzed to estimate k_perp. A mode driven by the Shoelace antenna was clearly observed on both the PCI and polarimetry diagnostics. These results show that the antenna is producing a density perturbation in the plasma, in addition to a magnetic perturbation. On Wednesday, we carried out MP#709, "Pedestal profile and fluctuation evolution in ELMy H-Mode". Fluctuation data were obtained in EDA and ELMy H-modes, using the modified "JFT2-M" equilibrium shape, with high lower triangularity and small upper triangularity. The first portion of the run concentrated on EDA H-modes. LHRF was injected during some of the EDA H-modes in order to analyze the attendant fluctuation suppression. The LH did not result in pump-out of the density pedestal under these conditions. In the second half of the run, the density and collisionality were reduced, and at lower ICRF power ELMy H-modes were obtained. Attempts to reduce the ELM frequency in order to optimize the profile reconstructions and better characterize the inter-ELM fluctuations were not successful, but sufficient data were obtained for analysis of the fluctuations and pedestal evolution. On Thursday, a half-day run was carried out in support of MP#716, "ITER demo discharge studies at 5.4T". The goal of this MP is to extend the C-Mod dataset of ITER-like discharges employing EDA H-modes to 5.4T, 1.3MA, kappa=1.75, q95~3.2. The target performance, as defined in IOS-1.1, is to achieve beta_N>1.7, which corresponds to Wmhd>300kJ and volume average pressure exceeding 2.2 atmospheres. To obtain these parameters with n/nG > 0.6 using the available ICRF power would require achieving H98~1. Thursday's experiment attempted to extend the results from MP#645a, which established the requirements for low-Z impurity seeding to optimize the core radiation and divertor heat loading in these high current EDA H-modes. The experiments aimed to increase the pulse length and further optimize the net power through the pedestal, and also to test the application of LHRF to help suppress fluctuations and reduce the pedestal collisionality, in order to increase the core confinement. However, we were unable to couple more than 250kW of LH into these discharges, because high density at the launcher resulted in arcing at higher powers. We did not have time to optimize the gaps to ameliorate this effect, and the LHRF did not improve confinement or affect the pedestal at the powers obtained. We did increase the nitrogen seeding level above the rates employed in the earlier experiment, and utilized the second divertor seeding pipe to improve the uniformity of the seeding. The outer divertor heat load was clearly reduced, based on IR camera, probe, and thermocouple measurements, and the level of core Mo radiation was also reduced. These results will provide additional data for the study of N2 seeding effects. However, the total core radiation remained at about 50% of Ploss, and initial analysis indicated that H98 was about 0.7. The highest beta_N values obtained were around 1.2, about the same as in the earlier experiments. An hour and a half on Thursday was devoted to completion of MP#715,"Do current ramps modify H-mode access?". L-H threshold data were obtained during positive and negative current ramps, using discharges similar to those on the previous run-day for this experiment. We were able to confirm and refine our measurements of the threshold power in cases with dIp/dt= +/-3MA/sec, and also with a half-speed ramp-up. We also documented the threshold with constant current. These data are being analyzed, and will contribute to the ITPA Joint Experiment PEP-33. ICRF Systems ------------ The ICRF was operated in support of mini-proposals last week. All systems successfully operated into high density H-modes that would have been outside the range of ICRF operation due to neutral pressure limits associated with the previously installed J-port antenna. We investigated minor damage to one of the delivered FFTs that appears to have occurred during shipping. We have requested replacement components from the vendor for the damaged gas and water connections. We have also modified the input flange to accept our coaxial lines and have water jet cut brackets required for installation. We expect to begin electrical measurements of the FFTs next week. Lower Hybrid System ------------------- The LH system was used during plasma operations on Wednesday and Thursday of last week. Preparations were made for reversed field operations following the completion of normal field operations on Thursday. Pre-cut phase shims were installed to switch the column-to-column phasing of the LH antenna from -90 deg to +90 deg. Klystron-to-klystron phasing is adjusted through software control of the drive phase for each klystron. Diagnostics ----------- First data was achieved on AGNOSTIC, a novel accelerator-based diagnostic to make in-situ PFC measurements of plasma-facing surfaces. A compact RFQ accelerator was installed on a C-Mod port and successfully injected a ~1 MeV deuteron beam to the inner wall. The deuteron produces high gain nuclear reactions with the surface isotopes and the resulting gamma emissions are detected and analyzed to identify the surface composition. Measurements throughout the week confirmed that AGNOSTIC was correctly identifying boron on the C-Mod surfaces, arising from previous boronizations. Beam optimization and numerical simulations of the results are underway. AGNOSTIC represents the first ever in-situ ion beam surface analysis in a tokamak and promises to be a powerful new tool to study plasma-surface interactions. Travel and Visitors -------------------- On Monday, Ted Strickland, former Governor of Ohio, visited the PSFC, where he toured the Alcator facility, and had detailed discussions about our research with Miklos Porkolab, Earl Marmar and Dennis Whyte. Last week, Michael Brookman, of UT Austin, began a planned two week visit to MIT. He is primarily working on the FRCECE diagnostic, and becoming familiar with our other ECE instruments, which are used in its calibration. Ahmed Diallo, PPPL, visited MIT last week. He served as Session Leader for the experiment on Wednesday, and worked with C-Mod scientists on pedestal physics issues. Zach Hartwig gave an invited talk at CAARI 2012 (International Conference on Accelerator Applications in Research and Industry) in Fort Worth, TX on the design and progress of AGNOSTIC, an accelerator-based in-situ plasma facing component surface diagnostic for Alcator C-Mod. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly