Alcator C-Mod Weekly Highlights 21-Nov-2005 FY2006 weeks of research operations: Planned: 12 weeks (tentative) Completed: 1.75 weeks Operations ---------- Research operations continued at Alcator C-Mod last week, with three run days scheduled and completed. A total of 50 plasma discharges were produced with a startup reliability of 70%. Experiments were carried out in support of research in the RF Science area and for diagnostic calibration. One of the two turbomolecular pumps on the main C-Mod pumping station failed prior to the start of operations last week. A spare pump has been installed and will be ready for operation this week, following bakeout over the weekend. Plasma operations were not significantly impacted. Plasma operations are planned to continue next week. Physics ------- Experiments on Wednesday, Thursday, and Friday were dedicated to MP#438, "Evaluation of J-port antenna without Faraday Screen". The J-port antenna was successfully conditioned up to 3MW operation; the D- and E-port antennas were also conditioned to produce the same power level when operated together. Comparisons of heating efficiency, loading, and impurity generation were carried out in both lower- and upper-single null discharges. For lower null discharges the H-mode threshold was found to be quite similar for J-port and the combined D&E antennas. A scan of the outer gap from 1.5cm to 0.5cm resulted in no measureable change in antenna loading for the screenless J-port antenna. In upper single null discharges, the J-port antenna exhibited lower heating effectiveness, by about 20%, than D+E antennas at net powers of 2, 2.5, and 3MW. This difference in heating effectiveness is much more apparent with upper null target plasmas than for lower null. The 2pi bolometer consistently showed ~25-30% more radiated power with the J-port antenna. Spectroscopy indicated that Cu impurity radiation correlated with operation of the J-port antenna, and an interaction between an upper and lower antenna strap was clearly observed on video images. The neutral pressure limit for operation of the J-port antenna was determined to be ~0.4mTorr, essentially unchanged from the limit observed with the Faraday screen in place. Operation of all three antennas simultaneously successfully coupled up to 6MW of power to the plasma, resulting in an H-mode transition in an upper single null discharge (unfavorable grad-B drift direction). In addition to the ICRF experiment, half of Thursday's operation was devoted to MP#296, "Confirmation of Thomson scattering density calibration using electron cyclotron emission cutoffs". High density (nebar>4e20/m^3) plasmas were run with a toroidal field Bt=4.6T to observe cutoff of the 2nd harmonic ECE emission in order to provide an independent calibration density for the Thomson scattering diagnostic. Calibration of the core TS channels was successfully completed. Cutoffs corresponding to the edge TS channels were less clear, owing to the relatively low edge temperatures in these ohmic L-mode discharges. Additional data, suitable for calibration of the edge TS channels, is anticipated during H-mode operation. Lower Hybrid System ------------------- Work continues on the modifications to the klystron body current monitoring system. Fabrication and testing of the new circuit boards and fiber optic receivers is proceeding. Magnetic shielding boxes for the sensors have been completed, assembled and bench tested and are now ready for installation. Calibrations of the monitor and drive legs are proceeding, with in-cell work being carried out during off-hours. The monitor leg calibrations are now finished. The new 5W calibration amplifier was characterized. The output levels were found to be repeatable to 0.1dB or better. The new amplifier has none of the acoustic modulation observed with the previous 2W unit, and has a shorter warm-up time. Calibration of the forward and reverse diode detector legs of the TPS (transmitter protection system) was also completed. All four stainless steel couplers have been given another leakcheck in preparation for installation on the launcher. Only one of the 96 windows has a detectable leak, and this leak is too small to affect the C-Mod vacuum. A teleconference between MIT and PPPL was held last Thursday to discuss any further tests needed before we proceed with the installation and also to set an installation schedule. ICRF System ------------ Repair of the FMIT#3 driver unit was completed on Tuesday, and all four transmiters operated successfully during C-Mod runs last week. Travel and Visitors ------------------- Manfred Bitter and Ken Hill (PPPL) visited C-Mod on hursday, Nov. 17 for discussions with John Rice, Earl Marmar, Rui Vieira, Alex Ince-Cushman, and Jim Irby relating to a proposed spatially imaging X-ray spectrometer system. Options are being considered for testing a new high count rate detector on C-Mod, and locations for the spectrometer are being discussed. Stewart Zweben (PPPL) was at C-Mod last week, working with Jim Terry on planning for future experiments in our continuing study of edge/SOL turbulence. Specifically, we are considering the possibility of a tangential view of the X-point region using in-vessel fiber-bundle-based optics and gas-puff-imaging. This view would be coupled the the ultra-fast framing camera. He was also here to discuss nearer-term experimental mini-proposals involving edge turbulence and to discuss the possible effects of fast-ion losses on boronized surfaces in C-Mod.