Alcator C-Mod Weekly Highlights June 4, 2007 FY2007 weeks of research operations: tentative target: 15 weeks Completed: 5.82 weeks Operations ---------- Four run days were scheduled and successfully completed last week with a plasma startup reliability of 78%. On Tuesday the DNB was run into gas with fields as part of the MSE calibration activity. On Wednesday plasma striations during lithium pellet injection were studied with a stereo-scopic fast camera. On Thursday pedestal scaling and ion physics were studied using the upgraded CXRS diagnostics, and on Friday experiments with the new cryopump were continued. On Wednesday evening our second boronization of the campaign was completed successfully, again using the addition of a vertical field to better localize the boron deposition. We believe this technique has extended the lifetime of the boronization, and reduced the deposition on outboard optical components. Work last week, and over the weekend, included installation of the switches, breakers, relays, instrumentation, and control software required to operate the EF4 boronization supply remotely. This work is required for the between shot boronization experiments planned for later this week. Operation Details ------------------ On Tuesday work was carried out to further calibrate the MSE diagnostic. The first half of the experiment involving beam injection into a gas-filled torus with magnetic fields was completed successfully. The second half of the MP calling for plasma operation was not completed; small amounts of room air were inadvertently admitted into the vessel during the first part of the run, making plasma startup unreliable. Analysis of the calibration data is ongoing. On Wednesday initial work was completed on stereo imaging of lithium pellet ablation striations. During the first half of the day, data was taken with pellet injections into 5 LSN plasmas and 1 USN plasma in H-mode. Reliable USL plasmas with good ICRF were also produced later in the day. As had previously been observed, the striations moved exclusively in the negative ion diamagnetic direction during the LSN H-mode shots; there also seemed to be a preference for the striations to move in the negative ion diamagnetic direction during the USL L-mode shots with ICRF, but not all striations. Also, the presence of the ICRF heating, and resulting higher electron temperatures and pressures, significantly shortened the lifetime of the striations from 8-10 usec without ICRF heating to 2-4 usec with ICRF; this is the time required to ionize from Li^+ (which is the observed ion state) to Li^(2+). The two major goals of Thursday's run were (1) to measure the response of the ion temperature pedestal width to changes in collisionality using edge CXRS measurements, and to determine whether the ion temperature pedestal differs from the electron pedestal in low collisionality regimes; and (2) to measure the response of the edge velocity profiles to changes in stored energy and magnetic topology. The run was successful in terms of obtaining the plasma conditions called out for in the run plan, and good spectra were obtained using the CXRS diagnostics, but limitations in the available ICRF power prevented access to H-mode in USN geometry. However, a significant span in H-mode stored energy was obtained (40kJ < 170kJ), which should provide a range of points for the correlation of rotation with stored energy and/or pedestal pressure. Analysis of the data is ongoing. The cryopump systems performed well on Friday, cooling to LHe temperature on any shot that we desired pumping. Cool-down from room temperature was fast (~20 minutes) and well controlled, resulting in the helium and nitrogen exhaust temperature transients staying close specification. The cryopump consumed only about 60 liters of liquid helium during the full day of operation, indicating very efficient operation. A wide range of plasma geometries, and both L- and H-mode plasmas with ICRF were investigated during this very successful run day. ICRF Systems ------------- The J-Port antenna will be switched to a two-strap configuration this week in a effort to illuminate the observed degradation in the neutral gas pressure limit relative to that observed in previous campaigns. Lower Hybrid System -------------------- Last week was spent in preparation for operation of the lower hybrid system later this week. Calibrations for 120 degree phasing were confirmed, and work continued on the improved optical arc detection system. Long Pulse Diagnostic Neutral Beam System ------------------------------------------ The DNB operated successfully in support of mini-proposals on three run days last week at near maximum parameters of 50 kV and 6 A. Travel and Visitors -------------------- Martin Greenwald travelled to the University of Maryland to chair a meeting of the FESAC planning panel. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly