Alcator C-Mod Weekly Highlights July 11, 2011 FY2011 weeks of research operations Target: 15 weeks Completed: 14.5 weeks Plasma Shots: 1706 Physics ------- High resolution SEM images of a Langmuir probe tip confirm that fully developed nano-tendril structures have been grown in the lower divertor of C-Mod. Energy-Dispersive X-Ray Spectroscopy has also confirmed that these nano-tendrils are essentially pure tungsten. The nano-tendrils range in thickness from 50-100 nm, which is slightly thicker than tendril tips seen on similar surfaces grown in linear plasma devices (20-30 nm thick). The coverage of the surface by the nano-tendrils is nearly complete, with the only "bare" regions being in areas of sudden change in "elevation" on the surface. This result indicates growth conditions can change on the length scale of micrometers, which is comparable to the Debye length in these plasmas. Higher resolution images of the nearby molybdenum tiles and calorimeters showed that there were no nano-tendrils or pre-cursors grown on the tile surfaces. Low secondary electron emission from the calorimeter surfaces may indicate an impurity coating (boron) which may have inhibited growth. Other potential reasons for lack of nano-tendril growth on these tile surfaces (sputtering, surface temperature, growth time, etc) need to be investigated in more controlled experiments in linear plasma devices. However, the clear evidence of nano-tendrils on the Langmuir probe surface is proof that these structures can be grown in the transient and unique environment of a tokamak divertor. Timeslice identification and preliminary analysis of I-mode discharges from the last campaign has been completed, particularly focusing on transitions between L, I and H-mode. As a result, we have now identified over 200 I-mode timeslices and over 50 transitions from L to I-mode. New observations of fluctuations in I-mode transitions show further evidence that the Weakly Coherent Mode plays an important role in density control in I-mode. Operations ---------- In-vessel work last week included the routing of the Fast Response Thermocouple cabling out through the vertical port at K. The SLA stud welding fixture for new ARRA funded rotated antenna is in house and test fit-ups in-vessel have begun. The stud welding template for the QCM antenna was successfully tested in-vessel. ICRF Systems ------------ Rui Vieira, Lihua Zhou, and Steve Wukitch visited our vendor responsible for the plating of components for the new ARRA funded rotated antenna last week. They discussed details of the plating process being used and went over details of the masking procedure. New tanks have been fabricated by the vendor for the largest components, and how these components will be supported was also discussed. Several components received earlier in the month did not meet specifications and are being re-plated. Work continued on preventative maintenance of, and upgrades to, the FMIT transmitters. Lower Hybrid System ------------------- A representative from the vendor supplying most of the components for the lower hybrid Transmitter Protection System (TPS) visited MIT last week. The approximately 90% of the first TPS system that has been assembled was discussed. Loop back tests of the TPS Ethernet boxes were completed successfully. Diagnostics ----------- Ken Liao, Bill Rowan, and Igor Bespamyatnov, UT-Austin, are developing new beam diagnostics to improve beam operation and to monitor beam characteristics in support of beam-based plasma diagnostics. A design for a profile monitor has been completed to measure beam composition as a function of beam radius before the beam enters the plasma Remote Participation, Travel, and Visitors ------------------------------------------ On July 7, Earl Marmar participated via remote video-conference in a review of staffing plans for the Wendelstein 7-X stellarator currently under construction at IPP Greifswald in Germany. The super-conducting advanced stellarator is scheduled to start operation in 2014. This meeting covered staffing and organizational plans for scientists, engineers and technicians, and included reviewers from Germany, France, U.K., Switzerland, Japan and the U.S. Bruce Lipschultz traveled to Strasbourg, France for the Nuclear Fusion Board of Editors meeting on June 30th and then stayed for the remainder of the EPS conference. John Rice attended the Workshop on Electric Fields, Turbulence and Self-Organization in Magnetized Plasmas (EFTSOMP) in Strasbourg, France July 4-5 and presented an invited talk on "Rotation Reversals and Ohmic Energy Confinement Saturation". _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly