Alcator C-Mod Weekly Highlights June 25, 2001 Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and 3 1/2 were completed. Run time was devoted to ICRF conditioning, cross-calibration of diagnostics, density limit studies, and imaging of edge turbulence. A recurrence of the arcing in the phase shifter for ICRF transmitter #4 resulted in the loss of a half day. Plasma operations are scheduled to continue this week. Physics and Operations ---------------------- A half-day run was devoted to study of the density limit. The goal was to collect data on the changes in turbulence and transport which have been linked to the density limit. In the previous experiments it was observed that the regime of large coherent fluctuations which normally exists in the far scrape-off, moves inward to the separatrix and intrudes into the closed field line regions as the density limit is reached. In this experiment, we inserted the probe more deeply into these plasmas at high densities in order to follow the evolution of the profiles and fluctuations, to assess how far into the main plasma this phenomenon reaches. With the probe inserted about 50 msec before the density-limit disruption, we observed a break in profiles, turbulence and autocorrelation function about 1cm inside the LCFS. The temperature profile was seen to shrink from the plasma edge as the limit was approached even with Prad << Pinput. Several shots were identified for further analysis. A half-day was dedicated to a cross-calibration of the Thomson scattering density response based on cut-off of the ECE signal in high density, reduced field discharges. Corrected calibration factors were obtained, and are being applied. Two experiments involving the fast CCD cameras were carried out in the half-run on Friday. The original camera was used in conjunction with the fast scanning probes to study edge turbulence (MP#281). On the same shots, the new million frame/sec PSI camera was used to image the ablation cloud from lithium pellets. ICRF Systems ------------ The J-port antenna was tested into vacuum and plasma after repair of the phase shifter. It was vacuum conditioned to 32 kV and then was operated into plasma. J-port reached 2.5 MW without significant RF-plasma edge interaction at the antenna corners. However, some interaction was observed on a couple of discharges on the K-side of the septum at the midplane. Comparing the faults with the optical monitor, we conclude that the system reliability was limited by arcs in the transmission line, at the phase shifter, Into plasmas, the voltage reached ~22-24 kV during H-mode which is ~50% higher than during the last campaign. Given the continuing problems with the phase shifter, we have removed the teflon support from the phase shifter (arcing is going across the teflon surface). Repair of the phase shifter controller was completed on Saturday. The phase-shifter will be tested during vacuum conditioning on Monday, and is expected to be available for plasma operation on Tuesday. E-port was operated upto 1.5 MW into plasmas quite reliably. D-port however was less reliable. It was tested into vacuum and arcing was detected at relatively low voltage. It is awaiting inspection and repair. Diagnostic Neutral Beam Systems ------------------------------- The DNB did not operate into plasma this week. A failure in the arc supply was traced to overheating of a supply. Repair consisted of replacing a failed fan and replacing the air filters. Following repair of the arc, a second problem appeared with the HV timing. The testing that we were able to do each day before the runs, between shots, and on the bench eliminated a number of possibilities for the problem. The timing of the suppressor is now working normally. Trouble shooting of the HV controls continues. BES filter functions as a function of filter rotation angle were completed and included in the computational model for the BES signal. Inner Divertor Fabrication Project ---------------------------------- Parts continued to arrive for the new Probe Box assembly. Mock-up building is moving forward, and a new Stainless Steel Wall was installed. Stud welding procedures for shooting long studs and welding studs-on-studs were developed and documented; testing indicates that 5,000 Lbs tensile strength can be obtained consistently. The vendor has finalized the manufacturing process for the Rear Girdle plate and is proceeding with production. Our other vendor has completed all twelve inconel C-plates; final inspection, including 100% dimensional check is proceeding. Lower Hybrid MIE Project ------------------------ The modifications and wiring for the klystron carts are basically complete. Some additional modifications may be necessary later to facility the circulator mounting. Work continues on the equipment rack wiring and PC board assembly. Travel and Visitors ------------------- Stewart Zwebwen (PPPL) spent the week at C-Mod. He brought and helped set up the Princeton Scientific Instruments PSI-3 million frame/sec camera, got new Gas Puff Imaging data during the density limit experiment, and worked on data analysis. He also served as co-session leader for Friday's experiment on edge turbulence imaging. Gerd Schilling (PPPL) was at MIT 6/19-22, working on ICRF antenna conditioning and attempting to help with the diagnostic neutral beam (which was down this week with an electrical problem). Yi-Jung Yang, one of this years' recipients of the National Undergraduate Fellowship in Plasma Physics, began her summer internship at C-Mod this week. She will be here for a total of nine weeks, and will be working with Brian LaBombard and Sanjay Gangadhara on plume image analysis. J. Snipes, I. Hutchinson, D. Mossessian, C. S. Pitcher, and J. Rice attended the EPS meeting in Madeira, Portugal from 18 - 22 June 2001. Four papers on C-Mod results were presented.