Alcator C-Mod Weekly Highlights August 15, 2011 FY2011 weeks of research operations Target: 15 weeks Completed: 14.5 weeks Plasma Shots: 1706 Physics ------- We have begun data analysis and simulations of 1/1 snakes observed on the Soft X-Ray diagnostic in Ohmic C-Mod plasmas. Initial results from nonlinear MHD simulations of a 1/1 helical density perturbation centered radially at the q=1 surface, done with the M3D code have been obtained. The simulation shows that a large enough perturbation will trigger a sawtooth crash, compared to a case with a very small perturbation (peak density perturbation = 0.05 to 0.1 * peak n_o). The simulation used inputs taken from EDA H-mode data that were assembled as part of the FY11 Joint Research Target on pedestal physics. Despite having an edge pedestal, unlike the Ohmic discharges with snakes, the basic mechanism and connection with a 1/1 instability should still be valid. The simulations were also a first test with M3D of the edge stability of the JRT plasma with an experimentally observed QCM instability and a rigid, perfectly conducting boundary, at the plasma separatrix. Previous simulations used a free boundary and magnetics equilibrium reconstruction. These preliminary results show that the plasma was relatively stable to edge instabilities for the spectrum of toroidal harmonics up to n = 23 with a large central resistivity approximately 100 times actual. An experiment was performed on the DIII-D tokamak recently to obtain plasmas dimensionlessly matched to ELMy H-modes on C-Mod. The main goal of the experiment was to produce a companion data set to a recent series of ELMy H-modes on C-Mod, allowing the direct comparison of pedestal structure and edge relaxation mechanisms between the two devices. The atypical shaping employed in C-Mod to promote ELMs was successfully reproduced, and current, field, programmed density and input power were adjusted in order to obtain target values of collisionality, normalized gyroradius and plasma beta at the top of the pedestal. Using a mix of co-current and counter-current beams, ELMy H-modes were obtained, often with signatures suggesting the presence of quasi-coherent mode activity. A preliminary look shows that when DIII-D and C-Mod pedestal profiles are normalized by the appropriate size-scaling factors, the DIII-D density profile is wider with a more relaxed gradient, in contrast to prior results from pedestal identity experiments in the EDA H-mode regime. The pedestal data will be analyzed in detail to identify suitable match points to C-Mod, which will enable parallel tests of pedestal models as a part of the FY11 FES Joint Research Target on pedestal structure. One such model, the EPED model for pedestal height and width, shows promising agreement based on a preliminary analysis. Comparative analysis with the C-Mod ELMy data set is forthcoming. Operations ---------- Retroreflectors for the C-Mod Polarimeter have been removed from the inner wall and will be replaced with new ones for the next campaign. Work continued on installation of the new MSE inter-shot calibration system and the K-limiter bolometer box. Testing and calibration of the protective relays for the magnet power supplies was begun last week by one of our vendors. ICRF Systems ------------ Forty-five of forty-eight studs needed for the ARRA funded rotated antenna have been welded to the outer vessel wall. All antenna components have begun successfully copper plated and baked. One of four Faraday screen boxes for the antenna has been assembled and measurements indicate it is within specification. Other components are being assembled on the test stand. The antenna will be fully assembled and carefully inspected on the test stand before in-vessel installation is begun. Lower Hybrid System ------------------- Inconel mounting plates for the new LH limiter arrived from the vendor. The plates are fully machined and have been plasma sprayed with alumina on the surfaces facing the launcher to reduce electromagnetic forces. The molybdenum tiles, which had been previously cleaned and baked are ready for attachment to the mounting plates. Progress continues on the Transmitter Protection System (TPS) upgrade. The TPS SBIR vendor provided a detailed writeup of the TPS Collector Over Temperature System (COTS) calculations and bench testing. This document has been reviewed by the C-Mod LH team and follow-up questions have been passed to the vendor for clarification. COTS coolant temperature sensors were ordered. Digitizer reset box assembly and testing was completed. Filament power supply test results were reviewed and remaining issues resolved. The system is scheduled to be shipped by the vendor on Aug 17. Diagnostics ----------- Dimensions and surface accuracy of the parabolic and flat mirrors for the Correlation Electron Cyclotron Emission (CECE) diagnostic have been successfully verified. All in-vessel components for the CECE system are in-house and are being prepared for alignment and in-vessel installation. The new flange and window has been successfully leak-checked. Prototype IF channel electronics have been fabricated and tested against a calibrated noise source. The pre-campaign spatial and radiometric calibrations for the wide-view CXRS diagnostic were completed for the toroidal and poloidal views. The grating used for ion temperature, rotation, and boron density measurements, and the grating used for the fast ion measurements were calibrated. In addition, the poloidal optical system was further protected to reduce film buildup during boronization and plasma operation. The toroidal optical system was calibrated for polarization, an issue that is important for analysis of beam spectra. This in-situ measurement replaces the bench measurement of similar optics which was used during the last campaign. A calibration report was completed, and relevant summary data were entered in MDSPlus. Remote Participation, Travel, and Visitors ------------------------------------------ Professor Odd Erik Garcia has arrived at the PSFC to start a one year sabbatical. Dr. Garcia is from the University of Tromso, Tromso, Norway. He will collaborate with the C-Mod edge group during his stay here. Dr. Garcia has previously worked closely with experimentalists at the TCV Tokamak, where he studied edge transport phenomena. A PhD student supervised by Dr. Garcia will also join us from Norway later in the year. Martin Greenwald was in Livermore, CA, at LLNL for a meeting of the Physical and Life Sciences Directorate's External Review Board. Vladimir Fuchs, an expert in RF theory and modeling from the Czech Institute for Plasma Physics in Prague, visited the PSFC last Thursday, Aug 11th. He had discussions about LH modeling with several PSFC scientists. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly