Alcator C-Mod Weekly Highlights February 7, 2011 FY2011 weeks of research operations Target: 15 weeks Completed: 11.1 weeks Plasma Shots: 1323 Operations ----------- Plasma operations continued at Alcator C-Mod last week. Three run days were completed; operations on Wednesday were canceled due to weather, and the planned experiment was re-scheduled to this week. The experiments supported miniproposals in the transport and Advanced Scenarios tasks. A total of 74 plasma discharges were produced with a reliability of 80%. An overnight boronization was carried out prior to the run on Tuesday. Plasma operations are planned to continue this week. Run Planning ------------ A meeting of the Experimental Program Committee was held on Monday 1/31. Plans for the remaining weeks of the 2011 campaign were discussed, especially the timing for changes in ICRF frequency and toroidal field direction. Seven mini-proposals were reviewed and approved. Operation Details ----------------- Tuesday's run supported MP#644, "Edge profiles and fluctuations in EDA and ELM-free H-Modes", which contributes to the 2011 OFES Joint Research Target to develop predictive models for pedestal structure. The primary goal of this experiment is to assemble an H-mode data set for testing models for the pedestal structure, edge relaxation mechanisms and transport. In addition to edge profile data, we acquired an extensive set of measurements of the radial electric field and edge fluctuation data associated with the pedestal in EDA and ELM-free H-mode discharges. In the experiment, we fixed the magnetic field at 5.4T and varied parameters that significantly affect both pedestal structure and the character of edge fluctuations. Plasma current was varied between 0.5MA and 1.2MA, spanning the boundary between steady EDA H-modes with a strong quasi-coherent mode (QCM) in the edge and more quiescent ELM-free H-modes. Additionally, at a fixed current of 0.9MA, L-mode target density was scanned in the range of 1.1 to 1.9x10^20 m^-3, also to change the quality of the QCM. Finally we scanned upper triangularity from less than 0.3 to nearly 0.9 with other shape parameters fixed. On Thursday we began the experimental sequence in support of MP#562, "Assessing feasibility of producing the hybrid scenario on Alcator C-Mod". In this experiment we attempted to broaden the current profile using LHCD, while maintaining q0 near 1, and then producing H-modes using ICRF heating. The key questions to be addressed were whether the j(r) modification could be maintained during ICRF and H-mode and whether the H-mode behavior changed, either transiently or continuously, as a result of the j(r) differences. Tuesday's experiment was concentrated on a 5.4T, 450kA target plasma with a relatively slow current ramp (beginning of flattop at 0.35sec), employing LHCD during the current rise and continuing into the flattop. Much of the day was spent in establishing reliable target plasmas with the desired parameters. Steady H-modes with nebar~1.5e20/m^3 were obtained in targets without LHCD, and LHCD was successfully used to modify the current profile, as evidenced qualitatively by sawtooth suppression, in L-mode discharges without ICRH. We then successfully combined the LHCD and ICRH to produce H-mode discharges during the flattop. Optimization of the H-mode behavior was not completed, although at least one example of transiently high performance (H98~1) was obtained. The non-linear decrease of non-thermal electron production and LHCD efficiency was apparent. In all cases we observed renewed sawtooth activity after the application of ICRH, though not immediately. Analysis of MSE and polarimeter data to obtain quantitative changes in the j(r) profile are underway. Significant progress was made on the MP, but we have not yet obtained a clear conclusion. Additional runtime will be required to build on these results. On Friday we carried out MP#599, "Up/Down poloidal impurity density variation", which comprises part of the thesis research of an MIT graduate student. The purpose of this experiment is to scan parameters thought to be responsible for causing poloidal variation of impurity density on a flux surface. According to theory, the primary drivers are the ion collisionality (v_ii ~ n_i/T_i^3/2) and toroidal and poloidal magnetic fields (~Bt/Bp^2). Data were obtained for ohmic plasmas covering a wide range of current (0.6 < Ip < 1.4 MA), toroidal field (2.7 < B < 7.5T), and density (0.4 < nebar < 2.3e20/m^3). These data are being analyzed. ICRF Systems ------------ ICRF operations at 78,80, and 80.5 MHz continued in support of C-Mod experiments last week. Initial tests of modulated RF power at 50-100kHz as a means of exciting edge modes to modify particle transport were carried out during boronization recovery discharges on Tuesday. Progress continued on fabrication of the ARRA-funded rotated 4-strap ICRF antenna. Our vendor has received the rough cut material for the top/side/bottom plates, and we plan to visit them shortly to review fabrication drawings and production details. The side tile design is completed and the rough cutting of the TZM can begin this week. The fabrication drawings for the back plates have been reviewed. The boron carbide resistors were tested electrically and found to be within specifications over the specified temperature range. We have successfully verified our ability to re-plate stripped components, if necessary, with tests on a stainless steel strap and inconel drop. Straps #1 and #4 are to be prepared (glass bead blasted and polished) for brazing this week, with brazing expected to start 2/14. Lower Hybrid System -------------------- The LH system was used to support the C-Mod run on Thursday. The system performed nominally. Low power ( < 5 W) reflection coefficient measurement electronics have been assembled and installed on the LH launcher. This system will allow for comparison with high power (1 kW to 1 MW) reflection coefficient measurements to determine if non-linear effects play a role in determining reflection coefficients at high power. Initial data were collected with the low power system on Friday. Travel and Visitors ------------------- Dennis Whyte attended the NRC committee meeting on Inertial Fusion Energy in San Ramon, CA. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly