Alcator C-Mod Weekly Highlights Jan 12, 2004 Plasma operations resumed on Alcator C-Mod last week. Four run days were scheduled and three completed; Friday's run was cancelled due to a fault in one of the ICRF Transmitters. Progress continued on the Lower Hybrid project. Operations ---------- Plasma runs were carried out Tuesday-Thursday last week in support of experiments in the Transport and RF Physics programs. A total of 71 discharges were produced with a startup reliability of ~85%. A fresh boronization was carried out over Tuesday night. Tuesday's run completed MP#363 on LH Threshold physics in Single- and Double-null and Limited equilibria. The main thrust of this day's experiments was to determine threshold powers and corresponding plasma parameters in the Inner Wall limited configuration. This configuration proved to have an extremely high L-H power threshold. ICRF power up to 5.8MW was applied in a series of limited discharges. Of these, only one achieved H-mode; assuming 80 percent ICRF absorption and adding ohmic power, Ptot was 5.0 MW, as compared to 1.3 MW at threshold in LSN (ie 3.8 times higher); this is more than in either DN or USN configurations. In contrast, equilibria limited near the bottom of the inner wall, at the "nose" plate, transitioned to H-mode at essentially the same power as the lower null discharges. These results contribute to the level 1 JOULE milestone SC6-1b, "Compare energy confinement, H-mode thresholds, and divertor particle dynamics in Alcator C-Mod discharges". Prior to boronization on Tuesday night, a series of low density plasma discharges were run to "unload" the walls, to evaluate the effect on the boronization process. A total of 120psi of diborane was used (a "medium" level), followed by several hours of ECDC in helium. Subsequent recovery shots and RF antenna re-conditioning proceeded rapidly on Wednesday. The remainder of the run on Wednesday and the run on Thursday were devoted to continuation of MP#376 on ICRF Heating efficiency and performance. Variations in density, and midplane neutral pressure were performed on Wednesday, with higher plasma performance observed at the lower range of neutral pressure. A scan of the outer gap on Thursday indicated the dependence of heating on this parameter was insignificant, as had been previously observed. PEP mode shots were obtained using Li pellet injection, with similar stored energy to the gas-fueled shots. Finally, strongly radiating (near 100%) discharges were produced by puffing of Argon and Neon, to check efficiency of RF heating and calibration of spectroscopic and bolometric measurements. Physics -------- During recent LSN, DN, and USN discharges, chirping modes with a frequency between 200 and 400 kHz and k=1-2 cm^-1 have been observed by the Phase Contrast Imaging (PCI) diagnostic. The modes are most intense following a sawtooth crash and multiple modes appear during the sawtooth evolution. These modes appear to be suppressed by the sawtooth precursor. During monster sawteeth, multiple chirping modes are observed between 200-300 kHz. The identification of these modes is still under analysis. ICRF System ----------- On Wednesday, the FMIT#1 transmitter (D-port) experienced a fault of the final power amplifier (FPA) tube. This tube had ~2500 filament hours, which is beyond its estimated mean lifetime. A spare tube is being readied and will be installed this week. On Friday morning, a control system fault of the 13.8kV breaker for FMIT#4 led to damage of some components in the soft start and crowbar cabinets. Required repairs and impact on operations are being evaluated. Lower Hybrid System ------------------- Installation of the circulators has begun. Three of the twelve units have now been installed. Following measurements of the fitup between the forward and rear waveguide assemblies, the forward waveguide was shipped to a PPPL vendor to complete machining to assure proper gasket compression. Final assembly and tests are expected to begin at MIT by the end of the month. Travel and Visitors -------------------- Ron Bravenec (UT-FRC) spent the week at MIT to setup the BES diagnostic for beam width measurements and for poloidal wave number and phase velocity measurements of the QC mode. This involved moving fibers and acquiring new calibration data. Doug Loesser (PPPL) was at MIT 1/8 to review the Lower Hybrid Launcher assembly schedule and for final fitup measurements of the waveguide assemblies. Joe Frangipani and Dave Miller were also at MIT 1/8-9 to assist with the waveguide fit-up, measurements and transport back to the PPPL vendor. Dr. Maxim Umansky (LLNL) visited the PSFC for January 8 and 9. Dr. Umansky presented a seminar on the BOUT edge turbulence simulation code and had detailed discussions with C-Mod scientists, Miklos Porkolab, Peter Catto, Jim Terry, Brian LaBombard, Olaf Grulke, Martin Greenwald, and Bruce Lipschultz regarding comparison of future BOUT simulations of C-Mod with our extensive edge turbulence data. We are also planning C-Mod experiments that would benchmark certain crucial aspects of BOUT.