Alcator C-Mod Weekly Highlights June 21, 1999 Last week continued the scheduled maintenance interval on Alcator C-Mod. No plasma operation was scheduled. This week is also a maintenance week. Physics and Analysis -------------------- Statistical analysis of fast magnetic fluctuations arising from edge turbulence shows that they often propagate at a well-defined speed that can be extracted automatically. This speed is now incorporated into the routine data-analysis as a monitor of a critical edge-barrier parameter. H-modes have perpendicular edge propagation speeds up to 40 km/s, with enhanced-D-alpha modes perhaps slightly faster than ELM-free. In L-modes the velocity is roughly four times smaller, but still in the electron diamagnetic direction. Temporal cross correlation analysis between the magnitude of high frequency electron density fluctuations at the plasma edge (measured by reflectometry) and the x-ray emissivity pedestal width show that there is a strong cross correlation between bursts of reflectometer fluctuations and temporary increases in the x-ray pedestal width. There is no measurable timing difference between the broadening of the x-ray pedestal and the onset of the reflectometer fluctuations. Any timing difference must be smaller than 100 microseconds. These bursts show up in the H alpha signals as well but the cross correlation analysis does not yield a clear result. The collapse of the x-ray emissivity pedestal has been studied during an H-L transition. Simultaneous measurements at the outboard midplane and the top show that the collapse starts at the pedestal location in both views and then propagates inward at a velocity of approximately 150 m/s. The collapse happens at the same time at the two locations (to within 24 microseconds). The x-ray pedestal width at the outboard midplane is a strong inverse function of plasma current in EDA H-mode, and is significantly wider in EDA H-Mode than in ELMfree H-mode. However, the top x-ray width does not seem to vary at all, but remains between 1.5 and 3 mm regardless of plasma current or type of H-mode. In some low current EDA H-mode cases the pedestal is more than twice as wide at the outboard midplane. This could indicate that the instability causing the increased particle transport in EDA is mostly localized to the outboard midplane. ICRF System ------------ We continued testing FMIT#3 and #4 into dummy load. FMIT#3 was operated up to 1.5 MW reliably into dummy load. The limit is set by the FPA anode current. FMIT#4 was operated to 1.8 MW reliably into dummy load. The power limit is set by the available DC power, which has been reduced from the nominal value to gain operating experience under conservative conditions. Compared with #4, #3 has higher FPA anode current. The difference stems largely from the different tubes in each transmitter, 2242 and 2274 in #3 and #4 respectively. To obtain these results, each transmitter underwent tuning adjustments and testing of the RF control system. Vacuum conditioning of the J-port antenna using FMIT#3 and #4 will begin this week. The screen crowbar and bias regulator units have been installed in FMIT# 1 and #2. This installation completes all new hardware installations in the #1 and #2 transmitters, and brings them into compliance with the standards set for transmitters #3 and #4. Testing of the #1 crowbar unit and testing of the RF control system for units #1 and #2 has also begun. Diagnostic Neutral Beam ------------------------ The digital control board for the high voltage cage has been completed, the board design sent out for production, and procurement of parts begun. Layout of the screen control board is currently being completed. Travel and Visitors ------------------- Several C-Mod physicists attended the EPS Conference on Controlled Fusion and Plasma Physics, in Maastricht, the Netherlands last week. Amanda Hubbard presented a talk on "Pedestals and Confinement in Alcator C-mod H-modes." Poster presentations were given by Joe Snipes, Rejean Boivin and Spencer Pitcher, and on behalf of Jim Terry (who didn't attend). Miklos Porkolab also attended the EPS Conference in Maastricht. In his capacity as APS DPP Chair, he also attended their Board meeting and Program Committee Meeting for the 2000 EPS meeting, to be held in Budapest.