Alcator C-Mod Weekly Highlights August 30, 2004 FY 2004 weeks of operation planned: 18 weeks, completed: 18.9 weeks. We are currently up-to-air with the primary goal of installing the lower hybrid launcher. In addition, a large number of in-vessel activities are ongoing in support of ICRF and diagnostic systems. Maintenance and upgrades to diagnostic and power systems are also in progress. Physics ------- Ion beam surface analysis was carried out by Dennis Whyte, UW-Madison, on a selection of molybdenum (Mo) armor tiles removed from C-Mod during the summer vent in order to examine impurity sources and hydrogenic fueling from the wall. The tiles were from the inner lower divertor, inner wall, and upper divertor. The plasma-facing surface composition is dominated by boron (B), the boron being a result of frequently applied boronization depositions, with the Mo/B fraction only ~ 1-2% in the first ~ 1 micron below the surface, indicating that the inner wall surface should be a weak source of Mo in the plasma. A surprising result is the lack of stratification in the Mo/B ratio, which had been expected due to the intermittent application of the B films. A smooth transition was measured from nearly pure B at the surface to nearly pure Mo about 3-10 microns below the surface, suggesting a diffusive-like mixing of the two elements, which may be related to the grain orientation of the Mo. Thick boron deposits were found on regions of the inner divertor tile that preferentially receive the local ion flux. This discovery indicates that the inner divertor is a region of net boron deposition during plasma operation, in a very similar way to deposition patterns found in tokamaks with carbon PFCs like DIII-D and JET. Preliminary analysis showed that the Hydrogen/Deuterium ratio was ~ 5-10 in the near surface after exposure to air, with the near surface of the boron appearing nearly saturated with hydrogen (H to B ratio ~ 0.25). The D / B ratio is ~ 2% and extends throughout the B/Mo surface layers, such that significant D was measured even after wiping the visible B layer from the tile surface. This result may have implications for predicting tritium retention in mixed material burning plasma devices like ITER. Summer student Istvan Cziegler from Eotvos Lorand University has continued his analyses of data from the 2D 250kHz/300 frame camera. Previously it was shown that the Quasi-Coherent Mode could be detected using the camera along with "gas-puff imaging". This analysis did not exploit any k _poloidal information, even though in principle it is present in the 2D camera data. When that information is used, the camera data yields a clear QCM with k _poloidal ~1/cm near the outboard mid-plane (as expected from previous BES measurements), and a propagation direction for the QCM that is in the electron diamagnetic direction (also as observed previously by other diagnostics). The new analysis shows the radial location of the QCM in same location as the previous analysis, but with much better contrast than in case in which k_poloidal information was not used. Indeed, there appears to be a radial structure to the QCM amplitude as well as a phase that depends on radius. This technique now resolves the mode with ~1 pixel (~1.2 mm) radial resolution. Martha Redi, PPPL, has submitted a manuscript for publication on microstability analysis of the C-Mod ITB, "Microturbulent drift mode stability before internal transport barrier formation in the Alcator C-Mod radio frequency heated H-mode". Operations ---------- Now that most inner wall work has been completed, we are in the process of removing outer divertor modules. Moly tiles at four toroidal locations in the outer divertor will be replaced with tungsten brush tiles. Twenty of the brush tiles, each made up of an inconel baseplate and 108 tungsten rods, are now in-house. The brush tiles will be cleaned, inspected, and then sent out to be machined to match specific outer limiter surfaces. The moly tiles will be sent to Dennis Whyte, UW-Madison, for analysis. We continued maintenance of the MIT alternator control cabinet, vapor extractor, and oil filtration system. The river water cooling system upgrade for the alternator was finished last week with installation of the water pipe insulation. Power Systems ------------- We are are preparing to bake out the reconditioned 4160V power transformer needed for the long pulse DNB. The transformer primary has been wired up to a 208V, 3-phase, 100A temporary service through a set of 3-phase power variacs. The secondary connections will be instrumented with a set of high current shunts for monitoring purposes. This process removes moisture from the transformer oil. Lower Hybrid System ------------------- The first 24-window PPPL coupler was successfully brazed over the weekend. The coupler is leak-tight, and the use of alumina plates on the outside of the coupler has eliminated crazing of the alumina on the outer-most windows. The braze joints on the coupler look very good, and the braze clean up required is minimal. The coupler is being prepared for high power testing. The ability of the alumina plates to reduce stresses in the outer-most windows was verified early last week with the successful braze of a 3-window prototype. Vector modulators needed to control the klystron amplitude and phase have been returned to MIT from the vendor. Programming errors in these components have been fixed and the modulators now perform to specification. All of the new klystron solenoid power supplies have been installed and tested. The transmitters are now available for continued high-power testing. The waveguide needed to connect the klystrons to the launcher is being prepared for installation in the cell. ICRF System ----------- A CST simulation of the new 4-strap antenna design, without a Faraday screen, has been completed. We have seen for the first time details of the rf current distribution in this complex system. Results from the simulation will be discussed at a design review of the new antenna scheduled for this Tuesday, 8/31. Diagnostics ----------- Ron Bravenec, U. Texas, has completed the initial design for a new BES fiber optic ferrule and is arranging manufacture of the ferrule and acquisition of additional fibers. The goal is to have a close packed array of channels for improved observations in the plasma edge and conventional fiber bundles for other BES observations. The additional fibers will be provided by PPPL from older, existing stock. Ron will polish the fibers using the PPPL fiber polishing facility. A new poloidal optical system for CXRS measurements from F-Top was designed and bench tested by Bill Rowan, UT-FRC. The optical system is based on a high quality commercial video lens. It will have 15 channels. The optics will focus in the plasma mid-plane with a spot size of approximately 1 cm. The center-to-center spacing of channels will be 1.5 cm. The tests were conducted with discrete fibers, and these will be replaced by a 15 channel fiber optic dissector for the actual installation. Transmission measurements indicate that the system will be useful above 370 nm and thus cover the CXRS spectra as well as the beam emission spectra. Preparations are being made to polish and then gold coat the new inner wall retro-reflectors. These components are part of the prototype polarimeter/interferometer diagnostic. The new TCI CO2 laser has been bench tested successfully and has been moved into the C-Mod cell for tests in the TCI system. Travel and Visits ----------------- Bob Granetz and Dexter Beals met with Dr. Anatoly S. Medvedko from the Budker Institute, Novosibirsk, to discuss the new long pulse diagnostic neutral beam system. Details of the DNB power system were discussed and specifications were given to Dr. Medvedko to be reviewed by engineers at Budker. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly