Alcator C-Mod Weekly Highlights October 4, 2010 FY2010 weeks of research operations Base Target: 13 weeks Base Completed: 16.0 weeks ARRA Target: 5 weeks ARRA Completed: 5.0 weeks Plasma Shots: 2599 Physics ------- Fluctuations associated with the weakly coherent (WC) mode during I-Mode operation have been observed on edge FRCECE radiometer channels. Determining if electron temperature fluctuations are associated with the WC mode is very important for constraining models that describe the underlying instability; since, for example, the assumption of an adiabatic electron response in models for the WC mode [Coppi, PSFC Seminar, September 24th] results in the prediction of no electron temperature fluctuations. The fluctuations in radiation temperature seen in the radiometer signal are at the same frequency as those seen in the density fluctuations from edge reflectometer channels, and are localized in the same narrow radial region in the plasma edge. Density and magnetic fluctuations, both of which are known to be associated with the WC mode [Whyte et al. NF 2010], can potentially contribute to the measured radiation temperature fluctuations, making it difficult to determine what, if any, electron temperature fluctuations are present. The same difficulty is encountered when analyzing the Quasi-Coherent (QC) mode seen in Enhanced D-Alpha (EDA) H-modes [Phillips et al. APS 2006]. For the I-Mode cases, analysis is ongoing to determine the fluctuation level and the radial localization of the fluctuations in the radiation temperature associated with the WC mode. Modeling is also being performed to estimate possible contributions from density and magnetic fluctuations to the measured radiation temperature fluctuations. Specifically, an improved model for the sensitivity of the radiation temperature fluctuations to density fluctuations that includes the effects of wall reflectivity will be used to replace the simple model (which did not include the effects of wall reflectivity) that was presented at the science meeting. One of three chords of the C-Mod Polarimeter is operational. Initial Faraday rotation angle measurements agree to within 10% with the rotation calculated from a simulation using EFIT results and the Thomson scattering density. The noise level of the polarimeter is currently dominated by the effects of the magnetic field from the tokamak on the FIR lasers. A magnetic shielding structure has been designed to reduce this effect during plasma discharges. Modeling indicates that current density profile changes expected during high power LHCD in low density plasmas should lead to about a two degree change in Faraday rotation angles, which should be well within the measurement capability of the polarimeter system. Three chords should be available by the end of the next run period. These data will be used to provide constraints to EFIT reconstructions. Operations ---------- Inspection of the power transformers and bus work for the magnet power systems proceeded last week. The alternator will be brought up to full speed late this week in preparation for resumption of plasma operations next week. Cryogenic systems have been inspected and will also come back on-line late in the week. ICRF Systems ------------ FMIT#2 has been power tested into the dummy load up to 2 MW. This system is ready for plasma operation after installation of a new solid state 10 kW power amplifier and an upgrade of the low power RF controls. Alan Binus and Andy Pfeiffer discussed the FMIT#2 control ladder logic in detail with regard to implementation of a fast opening high voltage switch to replace the crowbar cabinet with our vendor. Implementation of this upgrade is planned for mid CY2011. The possibility of replacing the fast vacuum breaker and overall power coordination were also discussed. A prototype current strap for the new 4-strap antenna has been cut from stainless steel to check the manufacturing process and time requirements. A full radius will be cut on the strap edges after which it will be plated. These steps qualify the cut radius and plating process of the strap. The results from the ICRF breakdown experiment suggest a region in the transition from coax to ridged waveguide is problematic. We plan to make a series of changes to this transition to eliminate breakdown, Optical arc detection will also be implemented to monitor the high voltage in the transition region. Lower Hybrid System ------------------- Last week was spent primarily calibrating the klystron drive and monitor systems to allow increased lower hybrid power during the next run period. Pat MacGibbon was on-site at our vendor's location last week to witness the acceptance tests of two new klystons. The tests went well, and arrangements are being made to ship the klystrons to MIT. These tubes are the fourth and fifth of a total of seven procured under ARRA funding. Travel and Visitors ------------------- Pat MacGibbon traveled to Palo Alto, CA, last week. Three experts on edge turbulence were on-site this week. Dr. Matteo Agostini from RXF-mod began his one-month visit. Prof. OLaf Grulke from IPP Greifswald and Stewart Zweben, PPPL, were here for the week. During the week numerous small meetings with C-Mod staff were held in which the interpretation of existing turbulence measurements and the future emphases for C-Mod turbulence research were discussed. Dr. Agostini is visiting for the third time in the last three years and is examining C-Mod's Gas-Puff-Imaging data. During this stay he will also finalize and submit a manuscript on C-Mod edge turbulence. Dr. Grulke is in the process of writing up C-Mod work on the parallel dynamics of SOL blobs/filaments. He has been working with C-Mod since 2003. Dr. Zweben, a long time member of the C-Mod edge turbulence team, timed his on-site visit to coincide with those of Drs. Agostini and Grulke. Dr. Novimir Antoniuk Pablant, PPPL, visited last week. He spend two days helping with work on the HiReX-Sr spectrometer and learning about its operation. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly