Alcator C-Mod Weekly Highlights August 13, 2007 FY2007 weeks of research operations: Target: 15 weeks Completed: 12.25 weeks Operations ---------- Research operations continued at Alcator C-Mod last week, producing a total of 19 plasma discharges with a reliability of 60%. One research run day was completed on Tuesday, carrying out an experiment by the Integrated Scenarios H-mode Baseline thrust. At the end of the first pulse on Wednesday we observed a ground-fault on the TF magnet. Instrumentation indicated a maximum ground current of 80 amperes for less than 1 msec, appearing during toroidal field rampdown about two seconds after the end of a normal full length plasma pulse. All engineering systems responded correctly, terminating the magnet pulse safely and flagging the appropriate alarm condition. Plasma operations were then suspended, and a series of measurements to identify the source and nature of the problem have begun. At this time, we have isolated a single intermittent short (less than 1 Ohm) to ground localized to one turn of the TF magnet. A single ground-fault on the magnet poses no direct hazard, and no damage resulted from this event. However, once a ground path exists, the appearance of a second ground-fault at a different location during a pulse could potentially lead to significant damage. Further testing is underway, and we are considering various options to diagnose and remedy this problem. Our local power utility has completed work required to remove the high impedence (reactance) component from the 13.8kV AC line that supplies prime power to several C-Mod auxiliary heating systems. The final transfer back to our permanent substation connection was carried out on Saturday. One overnight boronization was performed before the run on Tuesday. No plasma operations are planned at C-Mod this week. Operation Details ----------------- Tuesday's run supported MP#503 "Scaling of the L-H transition at low density". This experiment addresses an issue of critical importance to ITER. The power required to access H-mode, which over most of the operating range decreases with decreasing density, has been found to increase non-linearly below some critical density that varies from machine to machine. On C-Mod at 5.4T, this critical density is about 8e19/m^3, which is higher than the planned L-mode target density in ITER at the same field. Other facilities, operating at lower field, have observed much lower critical densities. The present experiment is part of an ITPA Joint Experiment (CDB-10) among DIII-D, JET, ASDEX-Upgrade and C-Mod to determine the scaling of the critical density with plasma parameters to better predict the critical density and minimum threshold power required for ITER. During last week's run, the threshold power was determined at densities below 1e20/m^3 for three values of plasma current, 0.6, 0.9, and 1.2MA. There appears to be no significant current dependence to the critical density or to the threshold power at a given density in these experiments. In particular, the hypothesis that the critical density could be represented as a constant fraction of the Greenwald density (proportional to current) is not supported by this experiment. ICRF System ----------- The J-port antenna remained configured with straps 1 and 2 powered from FMIT#3 and straps 3 and 4 from FMIT#4 in (0,pi,pi,0) phasing. This configuration performed reliably, providing up to 2MW from J-port (up to 3.5MW total ICRF power) in a ramped waveform for the H-mode threshold experiment on Tuesday. Long Pulse Diagnostic Neutral Beam System ----------------------------------------- The DNB performed nominally during plasma operation last week. In addition, following Tuesday's run additional beam-into-gas pulses (with no magnetic field) were taken in support of further debugging of the MSE diagnostic. Line spectra were recorded with and without the linear polarizer normally used as part of the MSE polarization analysis. Travel and Visitors -------------------- Martin Greenwald was at PPPL Aug 7-9, to chair a meeting of the FESAC planning panel. On Tuesday, the panel heard presentations from the community including three from members of the Alcator team covering first wall issues, RF heating and current drive and steady state physics. Professor Yuichi Takase of the University of Tokyo continued his visit to C-Mod last week. He is working with the lower hybrid group. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly