Alcator C-MOD Weekly Highlights Nov. 20, 1995 The Alcator C-MOD fall operating campaign is now underway. The first operating week was primarily devoted to diagnostic calibration and alignment and RF conditioning. The initial run on Tuesday was dedicated to re-establishing routine plasma operation after the summer shutdown. Full-length, 800kA plasmas were produced. Startup used pulsed gas only (no steady fill), and this technique will now become the standard startup scenario. A new gas valve driver based on modulating the duty cycle of the voltage applied to the main piezo-electric valve was used successfully for density feedback; this driver gives a more linear response to demand than the direct voltage drive employed previously. Diagnostic calibration and alignment were carried out during Wednesday's run. The main systems being worked on were the perpendicular neutral particle analyzer and the Hirex high resolution x-ray spectroscopy. A reproducible series of shots at 800kA and relatively low density (nebar~1.1e20/m3) were used to cross-calibrate channels of the NPA and verify the HIREX alignment. Other diagnostics also took advantage of the constant shots to check alignment and calibrations. A vacuum incident occurred on Thursday morning after completion of routine discharge cleaning (ECDC), resulting in higher than normal background water levels as observed on the RGA. After several unsuccessful attempts to produce useable plasmas, the scheduled run was abandoned and bakeout and ECDC were resumed. On Friday, operation resumed, with the primary activities being RF conditioning and checkout of the Lithium pellet injector and the Zeeman polarization q-profile diagnostic. This run was a combination of the runs originally scheduled for Thursday and Friday. RF conditioning was quite successful, with up to 1.4 MW coupled through the D-port antenna by the end of the day. The Li pellet injector also performed well, and a new automatic frame grabbing system was successfully used to capture the "cigar" angle, which provides an alternative measure of the field angle. Use of the neutral Li line at 6708A for Zeeman polarimetry was explored. As expected, this line is much brighter than the 5485 Li+ line, but due to the higher density in the part of the ablation cloud in which the neutral Li is found, the lines are also significantly more broadened, so that the polarization is degraded. The signal to noise is therefore not improved by using this line. Preliminary analysis indicates that the improvements in baffling of the outer divertor installed during the summer break have in fact reduced the neutral pressure in the main chamber for a given plasma density by about a factor of two to three. Divertor to midplane compression ratios up to 200 were observed. A check of the absolute calibration of the midplane and divertor neutral pressure diagnostics is being performed this week to confirm and better quantify these results. Operation last week was typical of the early stage of a campaign in that startup reproducibility was less than optimal and disruption recovery took several shots. The situation was somewhat improved by the end of the week as the tokamak cleaned up with repeated discharges. There will be no C-MOD runs this week, due to the Thanksgiving holiday. Further bake and discharge cleaning are being carried out, in addition to regular maintenance activities. Operation is scheduled to resume next week. Dr. Bill Rowan from the University of Texas (FRC) is visiting the PFC to further develop plans for collaborations on C-MOD. Dr. Alan Glasser of LANL has successfully ported his MHD stability code DCON to the C-MOD cluster, where it is running on an AlphaStation under OpenVMS.