Alcator C-Mod Weekly Highlights July 26, 2010 FY2010 weeks of research operations Base Target: 13 weeks Base Completed: 10.9 weeks ARRA Target: 5 weeks ARRA Completed: 5.0 weeks Plasma Shots: 1942 Operations ----------- Plasma operations resumed at Alcator C-Mod last week. Four run days were scheduled and completed. A total of 93 plasma discharges were produced with a startup reliability of 80%. Runs on Tuesday and Friday supported Diagnostic development and calibration, and ICRF conditioning; experiments on Wednesday and Thursday contributed to research in Boundary and Lower Hybrid Physics. Diagnostics continue to be brought on-line. The HIREX_SR High Resolution Spatially Resolving X-Ray Spectrometer is installed and aligned. The Laser Blow-off Impurity Injector is operational. Plasma operations are planned to continue this week. Operations Details ------------------ Tuesday's run was devoted to a combination of MP#534 "Foil/AXUV Bolometer Calibration in Helium" with piggybacking for MP#355b "ICRF Conditioning". This run was carried out with Helium majority species to reduce charge exchange contributions to the foil bolometer signals. An outer gap scan was performed for spatial calibration of the HIREX_SR and Bolometry systems. Conditioning shots were run in Upper Single Null and Inner-Wall Limiter configurations. Wednesday's run continued the experiments under MP#570 "Boundary layer heat transport experiments in L-mode plasmas", which contributes to the 2010 SOL Heat Flux Joint Research Target. Heat flux profile data were collected for 1.1MA ohmic conditions in the low- and mid-density range. In the sheath-limited regime, the heat flux profiles inferred from the surface thermocouples and Langmuir probes indicated a narrower e-folding width (2 mm) compared to 0.8 MA (2.5 mm) and 0.54 MA (exceeding 3 mm) in the same regimes. Additional data at higher density still need to be obtained. Thursday's run was dedicated to MP#608 "Lower Hybrid Wave Coupling Study on LH2 Using Microwave Probe Diagnostic". Phasing at the waveguide mouth as determined by the probes was verified to be consistent with the control demands. Powering selected subsets of the 10 klystrons resulted in patterns of reflection coefficients which were consistent with expectations. Energizing a single column qualitatively confirmed results from simulations, which predicted that the reflected power should be low on the directly fed column and higher on the adjacent ones. A power scan revealed that oscillations in the reflectivity observed at 400kW were not present when the power is reduced to 160kW. An experiment was carried out using a modified "JFT-2M" equilibrium, which is not well-matched to the launcher shape. When this equilibrium is displaced vertically, forcing the density profiles to differ across each row of the coupler, the RF probes confirm that in the case of asymmetric loads the power is redistributed and coupled preferentially through the rows of the four-way splitter with higher loading. Fine tuning of the equilibrium to be more conformal to the launcher did not improve the coupling or the power splitting. Some inconsistencies between the measurements from the probes and the intermediate couplers remain to be understood. Conditioning of the ICRF antennas (MP#355b) continued on Friday. Lower single null deuterium majority target plasmas were employed for this work. Sweeping of the outer strike point was employed to reduce the local energy flux to the divertor. Brief ELM-free H-modes were observed early in the run, when the hydrogen fraction was relatively low, but H/(H+D) increased as the run progressed and RF power increased. The elevated hydrogen fraction and corresponding low single pass absorption continued to restrict simultaneous operation of the D- and E-port antennas, each of which operated independently above the 1.3MW level. The J-port four-strap antenna continues to condition, and achieved extended periods at 1.1MW by the end of the day. ICRF Systems ------------ Antenna conditioning continued last week and the first H-modes of the current run period were observed. The H/H+D was about 10% before the RF on the first discharge, but the H concentration increased with injected RF energy. The E antenna continued to perform well and D antenna significantly improved on Friday. J antenna has had some matching issues that appear to be improving and the overall conditioning is improving slowly. Lower Hybrid System -------------------- The lower hybrid system performed satisfactorily in support of experiments on Thursday last week. Preparations were made for replacement of the A2V3 circulator, which has been limiting the power available from this klystron. A replacement will be installed before the next scheduled LH experiments. Installation of the LH scrape off layer (SOL) reflectometer external waveguide run is nearing completion. The tapers and associated bends on the LH launcher and the straight run from the microwave electronics to the catwalk have been installed. The support for the remaining section has been installed and we will continue with installation this week. Travel and Visitors ------------------- Steve Wukitch participated in a Fusion Simulation Project-Wave Particle Interaction conference call last week. He also participatd in the sorters' meeting for the APS Division of Plasma Physics Meeting at APS Headquarters in Greenbelt, MD. Dennis Whyte and Joe Minervini attended an OFES meeting in Washington, DC discussing Fusion Nuclear Science Pathways. Ian Hutchinson and Anne White traveled to Cambridge, U.K., July 19-23 to attend a Workshop on Kinetic-Scale Turbulence in Laboratory and Space Plasmas at the Isaac Newton Institute of Mathematical Sciences. This Workshop was the first week of a month long meeting on Gyrokinetics in Laboratory and Astrophysical Plasmas. Ian presented an invited talk entitled "Spontaneous tokamak rotation: observations turbulent momentum transport has to explain" and Anne presented an invited talk on "Measurements of electron temperature fluctuations on DIII-D". The program focused on gyrokinetics for fusion plasmas as well as applications to space and astrophysical plasmas. More information can be found at http://www.newton.ac.uk/programmes/GYP/gypw01p.html Randy Wilson (PPPL) visited MIT last week to participate in the C-Mod Lower Hybrid experiments. Rich Hawryluk visited MIT on Wednesday, July 14, and together with Randy Wilson, held discussions with Earl Marmar on the PPPL collaboration. David Brower (UCLA) visited MIT last week to work on the C-Mod Polarimeter diagnostic. With Will Bergerson and Peng Xu, he worked on assessing the FIR polarimeter phase signal in a test optical configuration. They determined noise contributions from electrical pickup, stray magnetic fields and mechanical vibrations on both the laser source and detection system, and worked to identify and initiate mitigation techniques. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly