Alcator C-Mod Weekly Highlights Dec 9, 2002 Alcator C-Mod is presently in a maintenance phase. In-vessel activities are proceeding. Work on ICRF and Lower Hybrid systems moved forward. Analysis of data from the last campaign continues. Operations ---------- In-vessel repair and upgrade activities are in progress. The active MHD spectroscopy antennas have been removed from the vessel and are being evaluated. These antennas were successfully used during the recent campaign to actively drive and study the damping of stable Alfvenic modes. Some damage was noted to BN protection tiles, and to ceramic-coated insulating supports. All BN tiles have been removed from the J-port (four-strap) ICRF antenna; in addition to the two tiles which were known to have broken free of this antenna during operation, several others were found to have sustained some cracking and wear. Re-designed tile hold-down hardware will be installed on this antenna before the next run campaign to increase their resistance to disruption forces. A shock testing table has been developed to qualify all tile designs at the high accelerations expected during disruptions. Tests will also be performed on the tile designs planned for the lower hybrid antenna. The MSE/BES optical assembly was inspected and two mirrors were removed for repair. All of the BN tiles have been removed from the D&E dipole antennas. Work will proceed with disassembly of these antennas, preparatory to a re-work of the antenna feeds to increase the RF voltage stand-off capabilities. The high-resistance ground path on the TF magnet has been localized using electrical measurements, and visual (borescope) inspection of the region is in progress. Physics ------- In the recent campaign, lithium pellet imaging experiments were performed using the PSI4 ultra-fast imaging CCD camera. The main goal of these experiments is to image the Li+ ion plumes, which are aligned with the local magnetic field, in order to measure the internal poloidal magnetic field profile. For these measurements, the images are typically acquired at a rate of 100 kHz, giving radial resolution of about 1 cm. However, for a small number of shots, the frame rate of the camera was increased to 500 kHz, and in these cases, vertical drifts of Li+ striations can be followed for 2 or 3 frames. This motion is attributed to E X B drift, and the measurements can be used to infer local radial electric field strength. For typical ohmic L-mode cases, near r/a=0.5, the drift velocity is of the order of 2 km/s, implying E_R~1e4 V/m, which is comparable to the local temperature gradient. There is also evidence that the drifts can change direction over a radial scale of approximately 1 to 2 cm, leading to the speculation that this might be indicative of zonal flows. Possible effects of the pellet ablation process, which is locally highly perturbative, also have to be considered. Milestone 80.0 , "Compare single-null, double-null and inner wall limited discharges", was completed during the most recent campaign. Experiments were performed to compare plasma operation in single-null, unbalanced double null, balanced double null, and inner wall limited discharges, addressing three topical areas: particle control, scrape-off layer physics, and access to H-mode confinement regimes. The results indicate that good plasma performance can be obtained in an unbalanced double-null configuration that is optimized both for particle removal by an upper cryopump and for heat flux handling by the lower divertor surfaces. Lower Hybrid MIE Project ------------------------ Assembly of the RF/Circulator support for Carts 1 & 2 is proceeding. Additional connectors for the semi-rigid coax cables were received. Measurements of the coaxes fabricated to date indicate that they have 0.1 dB of insertion loss, which is acceptable. The Raytheon circulators have been evaluated. All sixteen have insertion loss and isolation within specification and will be acceptable for use during early operation of the lower hybrid system. ICRF Systems ------------ The dummy load for long pulse tests of transmitters 3 and 4 is in place and sodium carbonate has been added to water. The network analyzer indicated that both load soda solutions are set correctly. The control system was checked for proper operation and flow was balanced to the two load circuits. The dummy load is now operational. Pulses several seconds long at 300 kW have been run. Preparations for initial tests of the HVPS crowbar unit are nearing completion. Travel and Visitors -------------------- Gerrit Kramer (PPPL) was at MIT 12/5-6, and successfully installed the three amplifiers in the new 130/140 GHz reflectometer system. A reflection from the machine inner wall was visible. Frequency drift due to temperature changes is still a concern. Gerrit also had useful discussions on fast particle physics with Joe Snipes of MIT. David Mikkelsen and Martha Redi at PPPL and Steve Scott at MIT participated in a three day video conference class for users of GYRO, a gyrokinetic turbulence simulation code. The class was presented by Jeff Candy at GA, and seven researchers attended at PPPL. During the class David completed the installation of GYRO on the petrel/fcc clusters, and multiprocessor batch jobs were successfully run.