Alcator C-Mod Weekly Highlights August 22, 2011 FY2011 weeks of research operations Target: 15 weeks Completed: 14.5 weeks Plasma Shots: 1706 Physics ------- MIT Undergraduate Research Opportunities Program (UROP) student George Yu presented a summary of his summer research project entitled "Comparison of Zave with neoclassical model" at the C-Mod Science Meeting on Monday August 15. George worked with Matt Reinke and Anne White to investigate systematic differences between Zeff as measured with a continuum emission diagnostic and Zeff as calculated using the theoretical neoclassical conductivity and the measured surface voltage to calculate the plasma current. The calculated plasma current is then compared to the measured plasma current and Zeff is varied until the two agree. It was found that these methods agree well at high densities, for any plasma current, but for low density Ohmic plasmas, they agree better at high currents. The largest disagreement occurs systematically for low density, low current plasmas. Physics of Plasmas has chosen "Full wave effects on the lower hybrid wave spectrum and driven current profile in tokamak plasmas", by S. Shiraiwa et al, as a research highlight. In this paper current profile modifications by lower hybrid current drive (LHCD) are modeled using a fullwave/Fokker-Planck simulation code. An MHD stable LHCD discharge on Alcator C-Mod was analyzed, and the current profile from full wave simulations was found to show better agreement with the experiment than a ray-tracing code. A comparison of full wave and ray-tracing simulation shows that, although ray-tracing can reproduce the stochastic wave spectrum broadening, the full wave calculation predicts even wider spectrum broadening, and the wave spectrum fills all of the kinematically allowed domain. This result is the first demonstration of LHCD current profile modeling using a full wave simulation code in a multi-pass absorption regime, showing the clear impact of full wave effects on the LHCD driven current profile. The paper may be found at http://pop.aip.org/resource/1/phpaen/v18/i8/p080705_s1 Operations ---------- In-vessel work last week included removal of boron coatings from the D and E port antenna Faraday screen tiles, installation of the K port limiter, repair of the Z-Bolo shutter, and welding of studs for the second disruption gas jet clamp, impurity seeding tubes, and AXUV detectors. Six new polarimeter retroreflectors were installed, two flux loops were repaired, and windows at A, K, and F ports were cleaned. ICRF Systems ------------ Preparations for operation of the ICRF transmitters included work on the FPA grid regulator boards and refurbishment of the VAC-ION controller for the FMIT#4 FPA tube. Test assembly of the ARRA funded rotated antenna has begun. The large backplates, central components of the strip line, and two of the four current straps have been installed on the stand. Installation of two of the Faraday screen boxes is underway. Lower Hybrid Systems -------------------- A new prototype fast ferrite tuner with larger permanent magnets has been tested. The larger magnets reduce sensitivity to placement of the ferrite within the waveguide. Variations of ± 1/8 inch in ferrite placement are acceptable with the larger magnets. A meeting with the SBIR Phase II vendor for the TPS Collector Over Temperature system was held, and the control board delivery schedule was discussed. The LH model tree has been modified to accommodate the new TPS. Cables for the new elbow/RF switch box were sorted according to cart. Cart 1 elbow box cables were routed. All other cart cables were inspected, sorted, and prepared for installation.. Front panels for the TPS controller circuit boards arrived from the vendor. The new filament supply system was shipped last week. New Outer Divertor ------------------ Han Zhang, PPPL, has developed a parametric model for the outer divertor current shunts. The code is under test and will be loaded with experimentally determined contact resistances and detailed pin geometry. The design of the current shunt must allow for thermally induced changes in the divertor geometry. Design choices for the material and size of spherical bearings that provide much of the strength to support the outer divertor are being refined. The bearings must retain sufficient strength while operating at temperatures approaching 600 C. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly