Alcator C-Mod Weekly Highlights December 21, 1998 Engineering: Alcator C-Mod is now under vacuum with pumpdown having occurred late Monday night. We now expect to go through several days of leak checking followed by approximately two weeks of baking and ECDC cleanup. To reach pumpdown a great deal of work had to be done last week both inside and outside the vessel. Of particular importance was the completion of the new PPPL J-port 4-strap antenna installation. A picture of the new antenna can be seen on the WEB at http://www.psfc.mit.edu/cmod/eng_srv/invessel/1998/j_ant_a.jpg More generally, invessel survey pictures can be seen at http://www.psfc.mit.edu/cgi/cmod_images?name=invessel/1998/ Pictures of the new PPPL MSE collection optics, new K-port midplane limiter and bolometer box, Omegatron and RF probes, X-ray detectors, and many other invessel components can be seen at this location. Other invessel work included installation of the new divertor bypass flappers, installation and alignment of the TTCI retro-reflector mount and mirror/shutter assembly, and many position measurements and optical calibrations. The diborane gas manifold was re-installed as was the glow discharge system. An extensive documentation campaign was mounted to take 35mm and digital camera pictures, and a video of all the invessel components both new and old. The digital camera pictures will be added to those already available at the above mentioned WEB site. By Saturday both halves of the igloo cover had been installed along with all large neutron cross section equipment in preparation for the neutron calibration that was completed on Sunday. The east side igloo cover has been removed for better access during heater installation and leak checking. The omegatron electronics and TCI beamline and mirror mount are being re-installed on the west cover. The diborane supply line has been installed and the new feedback system is being tested through to the PLC. The diborane gas lines have been leak checked and certified ready for operation. New TF bus work has been fabricated and installed in the power room. Several TF electrical joints have been removed and replaced with one continuous bus. The new bus has been fabricated with somewhat wider spacing to reduce Lorentz loads in the bus tunnel region. New one inch hardware has been installed and torqued to above 200 ft-lbs resulting in a joint force of more than 12,000 lbs. The magnet bus system is now complete and intial inspections have been done. Bringing the bus instrumentation back online is now the primary focus of the Power Systems Group. All Ross relays contacts that isolate the bus from the instrumentation during a plasma shot have been cleaned and polished. Magnet resistance measurements are back online and being logged to the cluster. TF magnet thermocouple reading are also back online and being logged. Bus instrumentation fuses have been checked and all fuse connections cleaned and reconnected. The complete bus system has undergone the first of several torque checks. Development of the DNB and its diagnostics for C-Mod continued. Norton Bretz (PPPL) and Ron Bravenec (UT-FRC) visited to complete installation, alignment, and documentation of the MSE/BES optics. Testing of the fiber transmitters and receivers for the accelerator supply and for the Mod/Reg started. Work on electronics for the Mod/Reg continued. Physics: Data from loop probes on the inner wall have been analyzed in order to understand the changes in fast wave propagation caused by sawteeth. The loop probes are opposite one of the fast wave antennas. During fundamental minority heating in D(H) at 5.4 T, the signals from the loop probes often show an abrupt change at the time of a sawtooth crash. During shots for which the hydrogen concentration is low, the signals increase at the time of the sawtooth crash, but during shots when the hydrogen concentration is high, the signals decrease at the time of the crash. The low-hydrogen-concentration behavior can be understood in the context of a single-pass analytic model by invoking the well-known redistribution of hot minority ions by the sawtooth crash, which results in a decrease in the minority temperature at the resonance layer and hence an increase in transmission. However, it is not clear that the high-hydrogen concentration behavior can be due to this cause. An alternative explanation is that the hydrogen fraction at the resonance layer decreases at the sawtooth crash. This effect would also cause an increase in transmission at low Hydrogen fraction. At high hydrogen fraction, the transmission is dominated by the wave polarization, which becomes worse (if absorption is desired) as the hydrogen fraction increases, so that a decrease in hydrogen fraction would cause a decrease in transmission, as is observed. These two competing explanations are being subjected to further analysis. Travel and Visits: A large number of scientist and Engineers from PPPL continued work last week on a variety of projects. Norton Bretz worked to finish installation of the MSE diagnostic. Les Gereg and Joe Frangipani completed work on the J-port antenna installation. Randy Wilson and Gerd Schilling were also here helping with the J-port antenna installation. Gary Taylor was also here working on the GPC2 ECE system. The GPC2 detector has been cooled down and the electronics is being checked out. Miklos Porkolab attended the VLT (Virtual Laboratory for Technology) PAC meeting at UCSD, San Diego, on Dec. 10,11.