Alcator C-Mod Weekly Highlights Sept 16, 2002 Plasma Operations continued at Alcator C-Mod last week. Four run days were scheduled and three were completed. Progress continued on the Lower Hybrid fabrication and on the Diagnostic Neutral Beam System. Operations will continue this week. Operations ---------- Plasma runs were carried out from Tuesday through Thursday. A total of 37 discharges were produced in support of experiments from the Transport and RF Groups. Tuesday's run began with a series of recovery shots, as this was the first plasma operation since last week's vent. The first few shots were short and disruptive, as expected, but full length discharges were established after only a few attempts. Although the startup reliability was under 50%, the current was successfully increased to 1MA, and the initial sequence of MP#314, "Evaluate BT sweep method of LTe determinations" , was begun. The aim of this MP is to establish the reliability and uncertainty of measuring Lte with ECE using Bt sweeps, in ohmic plasmas; and to measure q dependence of LTe, using an Ip scan. As these discharges all had fairly high radiation and modest but clear and steady decreases in Te and stored energy throughout the flat top, they were not ideal targets. However they were adequate for scoping studies. The experiments began with slow Btor variations, from 4.6 T to 5.8 T. During this sweep, the various GPC (grating polychromator) channels in the gradient region overlaid well, demonstrating that Te(R) is fairly insensitive to B (one of the main assumptions of the method). Smaller, more rapid sweeps of 0.2 T on subsequent shots were carried out to determine the practical limitations in slew rate imposed by the alternator and power systems. Considering the non-steady nature of the discharge, the parameter scans called for in the MP were deferred until a future run. The runs on Wednesday and Thursday were devoted to ICRF antenna reconditioning and mode conversion experiments. Since the H/D ratio following the vent was relatively high, efficient H minority heating was not possible. However, for these high hydrogen concentrations, direct electron heating due to mode conversion at the ion-ion hybrid layer can take place. This effect was observed both at f~80 MHz (D- and E-port antennas) and f=7O MHz (J-port antenna). While the single-pass absorption associated with this technique is low, the damping is in principal highly localized. Different toroidal fields were run to study the localization of the heating. Variations in the H/D ratio during the discharges were also observed to affect the damping location. Using the cold plasma dispersion relation, and the measured H/D ratio, the heating was found to be consistent with mode conversion heating. For J-Port, the nominal field required was 5.15 T, while D and E-Port required 5.9 T. At the end of the run on Thursday, a phase-to-phase fault occurred in the Air Switch which connects the C-Mod 13.8 kV alternator to the main AC breakers in the Power Room. The fault occurred as voltage was being ramped up, before the beginning of the tokamak pulse. Fast-acting high-current protection fuses between the alternator and the air switch performed as designed to limit the current and prevent any significant damage. The cause of the arc was subsequently determined to be incomplete engagement/locking of the switch contacts due to a loss of air pressure. On Friday, the switch was cleaned up, repaired, and tested, and is now ready to go back into service. The experiment planned for Friday has been rescheduled to next week. Since friday was used as a maintenance day, we plan to run 5 days next week. ICRF Systems ------------ All ICRF systems operated last week, with D and E-PORT reaching 1.6 MW each into L-mode plasmas and J-Port reaching 2 MW. Diagnostic Neutral Beam Systems ------------------------------- After replacement of a faulty roughing pump, the DNB was evacuated on Monday evening and obtained base pressures of 1e-8 Torr by Tuesday morning. After 20 conditioning shots, the beam ran well into Alcator plasmas throughout the run week. The cryo pump LN2 fills are now done using the new transfer line in manual mode. PLC logic and operator interface screens are complete and are being tested and debugged. Line insulation is about half complete, and should be finished early this week. Lower Hybrid MIE Project ------------------------ The electronics and control cabinet for transmitter #2 are now online. In conjunction with the transmitter #2 work, the associated water-cooling system instrumentation was also powered up successfully. The Allen-Bradley data highway plus (DH+) network was expanded to include TX #2. There are now four stations on the DH+ network: TX #3, TX #2, HVPS, and Operator Interface Station. The screen display for the Lower Hybrid Operator Interface Station (LOIS), was modified to display correctly the data from TX #2. Work is proceeding on fabrication of semi-rigid cables and associated connectors to be used in the low power microwave system. Diagnostics ----------- A recalibration of the integrator circuit used in the magnetics measurement of the vacuum toroidal field seems to resolve discrepancies reported between the position of features in the ECE Te profiles, and the corresponding locations derived from the EFIT equilibria. The magnitude of the change in calibration relative to the previous calibration, carried out during the 2000 campaign, is ~2%. Travel and Visitors ------------------- Miklos Porkolab attended the FESAC meeting in Gaithersburg, MD, on Sept. 12,13. David Mikkelsen presented a talk entitled "Nonlinear Simulations of Drift-Wave Turbulence in Alcator C-Mod H-mode Plasmas" at the EU-US Transport Task Force Workshop in Cordoba.