Alcator C-Mod Weekly Highlights Oct 7, 2002 Physics operations continued at Alcator C-Mod last week. Five run days were scheduled, and 4 1/2 were completed. Experiments were carried out in the transport, AT physics, divertor physics, and MHD areas. One run day was devoted to calibration shots for the MSE diagnostic. Progress also continued on the Lower Hybrid fabrication project. Plasma operations are scheduled to continue this week. Operations ---------- Operations were carried out on Monday through Friday last week. A total of 67 plasma discharges were obtained, with a startup reliability of over 75%. One run day (Friday) was devoted to non-plasma beam-into-gas shots with field, for MSE calibration (MP#259, 322). Monday's run was abbreviated when a capacitor in the OH2L commutation bank failed on shot #17. The capacitor was replaced and operation resumed on Tuesday. Monday's experiment, a triangularity scan in support of MP#304, "H-mode regimes", will be rescheduled. Tuesday's run included parts of two experiments involving Internal Transport Barriers (ITB). Additional information was obtained on the effect of additional ICRF heating on ITB's arising during ohmic H-modes. However, little further progress was made on determining the dependence on plasma current of the ITB location in discharges with off-axis ICRF. The low current (600kA) discharges featured multiple H-L transitions and RF power trips, which prevented the establishment of long-duration ITB's. The purpose of Wednesday's experiments was to assess the potential performance of a cryopump located in the upper divertor for density control in H-mode discharges. The goals were two-fold: (1) determine the range of neutral pressures in the upper chamber for a variety of secondary separatrix locations and (2) quantify the active deuterium inventory in the plasma, neutral gas, and walls. A series of ICRF-heated H-mode discharges (Prf=2MW, Ip=1MA, Bt-5.3T) were run with varying SSEP, the distance between primary and secondary separatrices. An improved SSEP observer/controller algorithm, based on one developed in 1999, was successfully implemented in the PCS plasma control system for this run. With the exception of very low target density, nebar<6e19/m3, the measured neutral pressures in the upper divertor always exceeded 1 mtorr when the secondary separatrix was within 3mm of the LCFS, as measured at the outboard midplane. For these conditions, the pressure in the upper chamber is similar to the pressure in a lower divertor at an open port. The gas inventory measurements clearly showed that the wall typically retains about 10 torr-liters of D2, which appears to be released over a long time scale after the shot. Shots with disruptions resulted in a prompt release of gas from the wall which was recorded as large torus pressures after the shot. During this run we were able to sustain H-modes in upper x-point geometry by changing from a lower to upper single null equilibrium after the L-H transition. Initial experiments with the C-Mod Active MHD Spectroscopy antenna were performed on Thursday. TAE resonances were observed on most shots showing a factor of two increase above the background at the driving frequency of 421kHz. During a series of density and toroidal field scans, the resonance was observed to be consistent with the expected scaling. Most shots were carried out at a drive power of 400W, but the resonance signature was also clearly observed down to about 20W. Results were initially obtained in inner-wall limited discharges. Two shots taken with SN diverted discharges also exhibited clear resonances at high density and field. Attempts were also made to sweep the frequency over the limited range where matching could be obtained, about 2% with the present matching circuit; resonances were observed, but further analysis is required to determine the resonance widths from these data. Initial estimates based on the TF ramps indicate a Q of about 40 or a damping rate of 2.5%, which is in the range observed in similar experiments on JET. Lower Hybrid MIE Project ------------------------ At MIT, work was carried out on the PLC software program and the operator interface graphics. Drawings for the LH Low power Microwave Rack AC/DC power distribution have been completed. Good progress was achieved at PPPL in the fabrication of the Lower Hybrid coupler. Three of the four coupler windows have been successfully brazed, and the ceramic to metal joints tested vacuum tight. The fourth window, plus a spare, will be brazed and tested this week. All ceramic windows needed for this project have now been delivered to PPPL. Modification of the interface flange is complete, and testing has begun on gold seal vacuum joints. The second forward stacked waveguide has been delivered; initial loss tests are promising. The vacuum bellows and flange assembly have been completed and are awaiting the next stage assembly. RFQ's for boron nitride tiles have been placed. RFQ's for rear standard components will be placed this week. DNB Systems ----------- The DNB operated well two days of this week. On the other days, operation was limited by a contactor which controls the charging of the caps and which would not remain energized. It was determined that the coil voltage applied to the contactor was marginal, and increasing this voltage appears to have eliminated the problem. We developed two additional beam voltage operating points. At 40 kV, the current was quite flat and the beam can support experiments. At 30 kV, the current peaked at 2A, the perveance value and then drooped. Some additional work on power supply settings for the arc and gas feed should allow reliable operation at this voltage. Some new CXRS spectra were aaquired and are being analyzed. One day was devoted to beam into gas measurements for MSE calibration. The beam width was confirmed to be approximatly 9 cm (full width at 1/e) during this experiment. Travel and Visitors ------------------- Amanda Hubbard participated in the meeting of the FESAC Fusion Energy Development Path Panel, at PPPL. D. Mossessian traveled to JET to participate in a joint experiment on dimensionless identity of C-Mod and JET tokamaks. The goal of the experiment is to investigate possible similarity of plasma characteristics (confinement, character of an H-mode, parameters of edge transport barrier etc.) in two different size tokamak plasmas with matched local dimensionless parameters. During this experiment the plasma was created on JET with the shape matching the shape of C-Mod plasmas and appropriately scaled values of plasma current, field, density and auxiliary heating. The H-mode was obtained and edge profiles and global plasma characteristics were measured. The results are being analysed. Martin Greenwald travelled to LBNL to attend a meeting of the ESnet steering committee. Topics discussed included the "Earth Simulator Rapid Response", planned network upgrades and new ESnet Services (PKI and ECS).