Alcator C-Mod Weekly Highlights August 14, 2000 The 120 C vessel bake ended on Tuesday of last week after one week at temperature. Leakchecks were completed Wednesday morning, and ECDC was started by the afternoon. The vessel is cleaning up well, and we plan to complete power tests and begin plasma operation by the end of this week. The Alternator and RF systems are also being brought back online. Engineering ----------- Pre-operation tests and calibrations of the magnet power systems were completed successfully last week. They are now ready for operation. We have also begun testing and bringing up alternator systems. All klystrons for the Lower Hybrid Project have now been tested. We have fourteen tubes qualified for operation, giving us two spare tubes. Two out of sixteen tubes showed a similar type of filament fault. These tubes will not be needed for the initial LH 3 MW operation scenario, but will have to be repaired for 4 MW operation. All four FMIT transmitters are delivering more than 1.8 MW of power into a dummy load, and are ready for plasma operation. The transmission lines are reassembled and ready to be pumped down and purged as soon as new manifolds are installed. Vacuum conditioning of the antennas will begin later this week. Work also continued on the new glow discharge system. The glow discharge switch control chassis wiring was completed. The control chassis and HV chassis were bench tested. A second glow paddle was completed and installed on our vacuum test stand and the system pumped down. The glow system was installed on the vacuum test stand and both probes were tested successfully, one at a time, using the one available control board. Glow discharge currents of over 4 amps, at 600 volts were produced and controlled. This test included operation of the PLC and Paragon control systems. The second control board is now complete and will be tested early this week. Physics ------- New data from June 2000 runs was added to the H-mode Threshold database, including low density limit H-modes and hydrogen H-modes for isotope scaling. An analysis of the new data showed that the lowest density in which H-modes are achieved on C-Mod is slightly lower than before, now reaching as low as 5 x 10^19 m^-3 whereas previous data found H-modes only as low as about 6 x 10^19 m^-3. Note that the H-modes are much more difficult to achieve at these low densities requiring about 2.5 times the standard 1999 H-mode threshold scaling power (P (MW) = 0.054 nebar^0.49 BT^0.85 S^0.85, with nebar in 10^20 m^-3, BT in T, and S in m^2), which was a best fit to 10 tokamaks in the International H-mode Threshold Database for deuterium plasmas. Note that these are also very marginal H-modes which drop easily out of H-mode and never achieve good confinement. An analysis of the H-mode run in hydrogen plasmas showed that the threshold was indeed higher in hydrogen than in deuterium, as expected from the 1/M scaling found comparing H, D, and T plasmas in JET. These Ohmic H-modes required a threshold power about 1.7 times higher than the 1999 threshold scaling above. Note that the ratio H/(H+D) ~ 0.6, so we did not achieve 100% hydrogen plasmas, which would be expected to require twice the deuterium threshold power based on the JET results. Within errors of about 10 - 15%, these results are in agreement with the JET scaling for hydrogenic plasmas. Travel and Visits ----------------- David Mikkelsen spent the week at C-Mod continuing his work on transport modeling. Stewart Zweben was at C-Mod 8/8-10 working on the external connections and hookup for the new edge turbulence imaging diagnostic. Gerd Schilling spent the week at PPPL on collaboration administrative work and discussions on diagnostic and rf topics. All of the collaborators on the heterodyne ECE diagnostic met at C-mod this week to discuss progress of and plans for the instrument. These included Perry Philips, Romik Chattergee and Alan Lynn of U. Texas, Christopher Watts of Auburn, John Heard, Clarion Univ., Yongkyoon In of Univ. Idaho, and Amanda Hubbard of MIT. The diagnostic is now operational, and several ideas emerged to improve its poloidal resolution, which is important for fluctuation measurements.