Alcator C-Mod Weekly Highlights August 2, 1999 Plasma operations on C-Mod were interrupted last week to repair an internal gas feed line. Tuesday's run experienced a high incidence of current-rise disruptions, accompanied by unusual levels of iron influx as determined by spectroscopic diagnostics. The run was halted, and visual inspection revealed that a stainless steel gas line had broken and was sagging into the discharge region. On Wednesday the vacuum vessel was vented to helium and the broken line was removed, using special tooling introduced through a gate valve at K-port. Following a borescope inspection to verify that no other internal components were compromised, the machine was pumped down and Electron Cyclotron Discharge Cleaning (ECDC) was begun. Following one day of ECDC and low-temperature (55C) bake, plasma operation was attempted on Friday. However, vacuum conditions were not yet satisfactory, and only short, resistive plasmas were obtained. ECDC was resumed and continued through the weekend. Plasma operations are planned to resume on Tuesday this week. Once reliable discharges are obtained, the primary focus will be on ICRF conditioning. Physics and Analysis -------------------- Operation of the omegatron probe during recent tokamak discharges has shown that it is still possible to collect impurity spectra during ICRF heating, despite a general increase in the edge plasma temperature and density at the omegatron location. The M/Z species present in the spectra do not change before, during, and after ICRF. The absolute currents of resonant ions increase during ICRF. The fraction of current in the omegatron that is resonant appears to decrease during ICRF, but this may be due to decreased collection efficiency. Efforts to optimize the component biases to reduce non-resonant ion collection during ICRF heating are in progress. C-Mod abstracts for the upcoming APS-DPP meeting (4 Invited talks, 11 Contributed Orals, and 35 Contributed Posters) are now available on the Web at http://www.pfc.mit.edu/cmod/aps99/abstracts.html ICRF Systems ------------ All four transmitters were prepared for plasma operation with the caveats that FMIT#1 is limited to 0.6-0.8 MW (anode dissipation limits total output power), and FMIT#2 is limited to 1.2 MW for reliable operation at the present voltage tap setting; FMIT#3 and #4 are ready for further conditioning of the J-port antenna. In addition, several engineering issues were addressed. To limit the anode dissipation in FMIT#1, we set the FPA anode current overload to 70A. We repaired and upgraded the screen crowbars in #3 and #4; the units now have a limiting 1 ohm resistor in series with the screen. We began two additional projects: installing overload relays for FPA grid bias and driver screen current and creating as-built schematics of the breaker control wiring. A problem with FMIT#3 interfering with FMIT#4 was traced to the antilog converter that provides a voltage to the power feedback circuit that is linearly proportional to output power. Modifications were made and testing of the feedback circuit is underway. In addition, FMIT#3 and #4 power meters (the most accurate power measurement device available) are now read and processed in a normal shot cycle. FMIT#1 and #2 power meters are undergoing testing. To increase the output power in FMIT#2, the high voltage needs to be raised. The high voltage connection needs to be improved before this can be done reliably. We have begun the design process for this upgrade. Travel and Visitors ------------------- Bruce Lipschultz went to the APS DPP 'sorters' meeting where the program for the November meeting was organized (Thursday/Friday). Josh Stillerman visited EPFL in Lausanne, installing logbook/database software and discussing general MDSplus issues. Dr. Yuichi Takase is visiting from Tokyo University. He is primarily here to discuss ICRF modelling with Dr. Paul Bonoli.