Alcator C-Mod Weekly Highlights March 19, 2012 FY2012 weeks of research operations Target: 17 Completed: 5.6 Plasma Shots: 753 Research -------- "Poloidal variation of high-Z impurity density due to hydrogen minority ion cyclotron resonance heating on Alcator C-Mod" by M.L. Reinke, I.H. Hutchinson, J.E. Rice, N.T. Howard, A. Bader, S. Wukitch, Y. Lin, D.C. Pace, A. Hubbard, J.W. Hughes and Y. Podpaly has just been published in Plasma Physics and Controlled Fusion: http://iopscience.iop.org/0741-3335/54/4/045004 This new paper discusses the strong, steady-state variations of molybdenum density within a flux surface that are routinely observed in Alcator C-Mod in plasmas using hydrogen minority ion cyclotron resonant heating. In/out asymmetries, up to a factor of 2, occur with either inboard or outboard accumulation depending on the major radius of the minority resonance layer. These poloidal variations can be attributed to the impurity's high charge and large mass in the neoclassical parallel force balance. The large mass enhances the centrifugal force, causing outboard accumulation while the high charge enhances ion-impurity friction and makes impurities sensitive to small poloidal variations in the plasma potential. Quantitative comparisons between existing parallel high-Z impurity transport theories and experimental results for r/a < 0.7 show good agreement when the resonance layer is on the high-field side of the tokamak but disagree substantially for low-field side heating. Ion-impurity friction is insufficient to explain the experimental results, and the accumulation of impurity density on the inboard side of flux surface is shown to be driven by a poloidal potential variation due to magnetic trapping of non-thermal, cyclotron heated minority ions. Parallel impurity transport theory is extended to account for cyclotron effects and shown to agree with experimentally measured impurity density asymmetries. Operations ---------- Alcator C-Mod is presently up-to-air for planned maintenance and upgrades. Work in-vessel concentrated on refurbishment of the ARRA funded field aligned ICRF antenna, and the repair of the polarimeter shutter. Calibrations of in-vessel diagnostics continued. ICRF Systems ------------ Allan Binus was at our vendor's site last week to begin acceptance tests of the first pair of ARRA-funded Fast Ferrite Tuners (FFT). The test of the FFTs magnet system went well. The vendor has also made changes to the water cooling system that simplify its design without affecting performance. Lower Hybrid Systems -------------------- The integrated LH2 limiter was removed from the LH launcher in preparation for the re-installation of the fixed LH protection limiter. The integrated limiter was found to be in good condition. It exhibited a boron coating. A new calibration technique for the LH control system upgrade has been tested. The new technique involves calibrating the drive and monitor legs of the control system simultaneously, thus removing a source of error from the calibration process. Drive levels must be adjusted to avoid saturating the amplifiers at minimum attenuation. Diagnostics ----------- A spatial and angular calibration of the Motional Stark Effect (MSE) diagnostic was completed last week to cover the preceding run campaign. The magnification of the MSE optical system had changed slightly, causing a change in the MSE edge channel positions by ~2 mm. This effect was caused by movement in components outside the vacuum vessel. It will be corrected this week. The poloidal optics for the UT-IFS Core CXRS system was removed, refurbished, realigned, reinstalled, and spatially calibrated last week. The polarimeter shutter has been removed from the vessel to repair a helium leak that appeared during the last run period. The leak was localized to the bellows used to close the shutter. A new bellows assembly has been sent out for welding. Remote Participation, Travel, and Visitors ------------------------------------------ Earl Marmar, Miklos Porkolab, and Bruce Lipschultz attended the DoE FES Budget meeting in Gaithersburg, Maryland last week. Earl presented a talk on C-Mod highlights and plans. Miklos discussed the current funding status of the Plasma Science and Fusion Center. Robert Granetz attended a special US BPO meeting on disruption mitigation at General Atomics, in San Diego, CA on Monday and Tuesday of last week. The US has recently become responsible for providing the disruption mitigation system (DMS) on ITER. The first step is to specify the number and location of required port plugs, which requires knowledge of the asymmetry of radiated power to the first wall, and how it changes with multiple injection locations. C-Mod has recently started experiments to address this issue. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly