Alcator C-Mod Weekly Highlights Feb. 19, 2008 FY2008 weeks of research operations: Target: 15 weeks Completed: 4.9 weeks Operations ---------- Research operations continued at Alcator C-Mod last week with experiments supporting Miniproposals from the Lower Hybrid Physics, H-mode Integrated Scenarios, and Rotation groups. A total of 73 plasma discharges were produced with a startup reliability of 85%. An overnight boronization was performed prior to the plasma run on Wednesday. Instrumentation that was recently added to the TF power supply was successful in identifying the cause of intermittent current sharing imbalances in Converter #1. The problem was traced to a bad terminal board connection in one phase of the SCR gate drive circuit. The connection was successfully repaired. Plasma operations are planned to continue this week. Operation Details ----------------- A two hour experiment was carried out by the lower hybrid group on Monday to determine whether the replacement of rf gaskets in the external waveguides (Jungle Gym) was successful in increasing the power handling capability of the system. The tests indicated that the power handling was not improved, and that the weak point is probably in the adjustable mechanical phase shifters. Fabrication of new phase shifters is in progress, and these will be tested in upcoming experiments. Following a maintenance day on Tuesday and an overnight boronization, Wednesday's run was devoted to MP#525 "ELM control experiments using the A-coil", which is intended to investigate the use of low m/n non-axisymmetric resonant magnetic perturbations (RMP) for ELM mitigation. This experiment, carried out in the "JFT2-M" shape using n=1 perturbations, was unsuccessful in modifying the ELM behavior over a range 3.31) should have been present in the pedestal region, between the q=3 surface and the last closed flux surface. No locked modes or magnetic braking of the intrinsic rotation was observed due to the n=1 perturbations in these experiments, and the pedestal parameters were unchanged. Self-shielding of the perturbing field by the plasma response or modification of the edge shear due to pressure driven current in the pedestal gradient region, which was not included in the equilibrium reconstruction, may be responsible for the null result in this experiment. The runs on Thursday and Friday were devoted to exploring and documenting the recent observation of rotation modification during lower hybrid current drive. Experiments were conducted using a range of lower hybrid phases, including co- and counter-current drive phasing, and with a range of target plasma current and toroidal field. Preliminary results of these scans indicate the rotational change exhibits a strong correlation with launched phase, and is consistent with the effect being related to the amount of observed driven current, which is largest at 60 degree phasing. The magnitude of the rotation change did not seem to depend strongly on total plasma current. However, the time scale on which the rotation profile evolved increased with decreasing current. Both the magnitude and time scale of the rotation modification seem to depend on the magnetic field. At low toroidal field (4.5T) the change in toroidal rotation was both faster and larger (deltaV=-35km/s) than was observed at higher toroidal field (6.3T, -17km/s). Physics ------- Stewart Zweben (PPPL) carried out a preliminary analysis of the Bruce Scott's GEM SOL turbulence simulation runs for the B-field scan done in 2007. The results showed a ~ 25% increase in the radial correlation length between B=5.4 T and B=2.7 T. The autocorrelation times were roughly the same at these two fields. Significant electron temperature fluctuations were seen to be highly correlated with the density fluctuations. These results will be compared with the C-Mod GPI and probe turbulence measurements for these cases. ICRF Systems ------------- A water leak in the cooling line for the FMIT#4 transmitter on Monday necessitated disassembly of the cavity for the final power amplifier stage. The transmitter was successfully reassembled and returned to service on Wednesday. The voltage pattern in the present fast ferrite tuner (FFT) network was analyzed, revealing that the voltage in the FFT closest to the antenna has substantially higher voltage than the previous configuration. This tuner has a history of arcing at a lower voltage than the other FFT unit. Therefore we plan to exchange the tuners at first opportunity to improve the overall voltage and power performance of the FFT network. The intermittent computer issues associated with operation of the FFT have not recurred since the clock used for this system was replaced with a different unit. Further testing is required to determine if the original clock was defective or the signal was being corrupted by rf pickup. Lower Hybrid System -------------------- The remote drive system for the lower hybrid launcher, which permits positioning of the grill with respect to the local guard limiters without requiring a cell access, is now operational, and was employed during last week's experiments. Design work is proceeding on the four-way splitter to be employed in the next LH launcher. Diagnostics ----------- We have received a repaired 130 GHz mixer from the vendor. It has been re-integrated into the reflectometer system. All three 130 GHz mixers are now operational and the 130 GHz channel is functional. Travel and Visitors -------------------- The PSFC hosted an informal meeting of university fusion researchers on February 14-15, to share ideas for possible future collaborative work. Twenty visitors from twelve other universities attended, in addition to several MIT scientists. A few others participated remotely. Drs. Ray Fonck (U.S. DoE OFES), Evgeny Velikhov, and Sir Chris Llewellyn Smith (Director, UKAEA-Culham) visited the PSFC on Feb 15th, prior to their attending the AAAS meeting in Boston on Feb. 16th where they participated in the Symposium on 50 year History of International Collbaboration in Fusion Research. While at MIT, Dr. Velikhov, President of Kurchatov Institute in Russia, gave an Institute wide colloquium on Global Energy Challanges and the Role of Nuclear Power. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly