Alcator C-Mod Weekly Highlights May 19, 1997 Tokamak plasma operations resumed on Alcator C-Mod last week. Three runs were scheduled and carried out as part of the start-up process. The purpose of these runs, covered by Mini-Proposal #170A, is to re-establish reliable operation, provide time for diagnostic re-alignment and calibration, checkout the cryopump and the new gas puffing system, and condition the 40 MHz ICRF system, prior to the resumption of physics runs. Power tests were completed on Tuesday. All of the PF supplies were operated into the coils successfully under PLC and Hybrid Control. The toroidal field was tested up to 5.4 tesla. Following a final sequence of integrated power and gas system tests on Wednesday morning, we began attempts to produce full tokamak plasmas. Programming was taken from a shot run in August, 1996, during our last experimental campaign. Breakdown was produced on the third attempt, and a successful current rise to 400kA on the subsequent try. As expected, the initial plasma was very dirty and resistive, and terminated in a disruption. The fact that a plasma was obtained with only very minor adjustment from settings established last year indicates that control and power systems are indeed behaving nominally, and no significant perturbations were introduced during the disassembly/reassembly process. As was the case at the start of previous campaigns, each tokamak plasma evolves and redeposits impurities which can prevent successful startup of subsequent discharges. As a result, only eight such plasmas were obtained last week. The best had a 600kA flattop current, diverted, and disrupted at about 400 msec due to vertical instability. Based on past experience, after a good null and current rise have been established, it is still necessary to have a number of cleanup runs before good plasmas are reliably obtained. We are presently in that stage of operation. During TF tests, small pressure rises were noted on the vessel gas gauges. According to the residual gas analyzer, these pressure bursts are not due to air or nitrogen leaks, but consist of fluorocarbons; the composition appears to be consistent with Teflon. At this point we cannot identify any deleterious effect of the small bursts on plasma operation, but are continuing to monitor the phenomenon. During non-run hours, we are continuing ECDC (electron cyclotron discharge cleaning) to condition the machine. The ECDC plasmas are also being used for checkout and calibration of several diagnostics, including spectroscopy and probe systems. The glow discharge cleaning system has been checked out, but not yet used. Diagnostics are continuing to come on-line. The TCI interferometer was ready for the first shot and is being used for density control as usual. Both the Michelson and Grating Electron Cyclotron Emission electron temperature diagnostics are operating, although alignment and calibration of these instruments is not complete. HIREX high resolution xray spectroscopy is operational, and obtained ion temperatures from Doppler broadening of (intrinsic) chlorine lines. Various visible and UV spectroscopy diagnostics are being brought into operation. Progress continues on the 40 MHz ICRF system. Transmitter testing is being carried out in the evenings, following tokamak operation. The dummy load transformer and switch have been reassembled. Cabling for transmitter control has been completed. Initial power tests of FMIT #3 into the dummy load have been started. The gas system for pressurizing the coax is being installed and tested. Cabling for the RF instrumentation (directional couplers and current probes) has been run, and the RF rack has been moved onto the diagnostic stand. Setup for conditioning is progressing. The decoupler loop assembly and 9" coaxial lines have been completed and the decoupler stub reset. The vacuum tune was completed. New features continue to be added to the C-Mod Web pages. A live view of the control room is now provided during tokamak operation at http://www.pfc.mit.edu/cmod/control_room.html This page, as well as the live view of the experimental cell and the view from inside the machine, which provides a nice image of the discharge cleaning plasma during ECDC, are all accessable from the "Facility On-line" section of the C-Mod links page at http://www.pfc.mit.edu/cmod/cmod_links.html Yuichi Takase was at Princeton this week for the NSTX Physics Advisory Committee Meeting. He also discussed the upcoming 40MHz physics experiments with our PPPL collaborators. G. Cima from UTx was at MIT working on his new ECE heterodyne diagnostic. Ricky Maqueda from LANL was on site preparing the IR camera and periscope view of the divertor which the camera uses. In addition, he setup the fast framing camera (1000 frames/s) for use with visible views. Ben Welch from the Univ. of Md. was here preparing the high resolution spectrograph. It is now fully operational, and was taking data on Thursday and Friday. Dr. Keith Burrell of DIII-D presented the M.I.T. Physics Department Colloquium this week, and also held discussions with C-Mod staff on a number of topics. The UT-FRC reciprocating Langmuir probe arrived from UT and was assembled for final testing before installation. This probe will be used for measurements of edge turbulence. The data acquistion equipment and control room workstations for this experiment and for the other FRC experiments planned for the Spring Campaign were installed by Don Patterson of UT-FRC with the assistance of PFC staff. Norton Bretz of PPPL presented his plans for MSE using the DNB and then discussed them with members of the PFC and UT-FRC staff. Bill Rowan, UT-FRC, visited PPPL to discuss instrumentation for the DNB poloidal rotation diagnostic.