Alcator C-Mod Weekly Highlights April 25th, 2011 FY2011 weeks of research operations Target: 15 weeks Completed: 14.5 weeks Plasma Shots: 1706 Physics ------- In a recent experiment, we created very high performance I-mode plasmas with the help of ICRF mode conversion flow drive. By launching 50 MHz RF power to plasmas with He3 minority, we were able to drive the plasma rotation up to 100 km/s, and then by adding 80 MHz RF power, I-mode was triggered. With the large toroidal rotation, these I-mode plasmas have confinement quality up to 1.4 times that of nominal H-mode confinement (HITER98_y2 ~ 1.4). An MHD mode appears after large sawtooth crashes in this high performance period. From its magnetic signature, spatial location (near q=3/2 surface), and correlation with sawtooth crash, the MHD mode is possibly a neo-classical-tearing mode (NTM). The onset of the MHD mode coincides with the slow down of the rotation, thus it is possibly braking the rotation. The MHD mode does not appear to significantly lower plasma performance. Operations ---------- Work continued in-vessel last week. Numerous diagnostic calibrations were performed and the J-Port antenna was removed. Routine maintenance of the C-Mod power systems proceeded. ICRF Systems ------------ Back plates have been shipped from our vendor for the new ARRA funded rotated antenna. The first set of Faraday rods coated with TiCN blistered during a high temperature bake test and are to be stripped and re-coated. A second batch of 78 Faraday rods have been polished and require two holes to be drilled in each prior to TICN coating. Machining of the back center plate for diagnostic access and Z plate for a shorting bridge have been completed. The long strip line has been EDM'ed and awaits final machining. Trial plating for the bellows tube has been evaluated, and the masking technique needs to be improved due to the length of time in the plating bath to build up 0.003" of copper. A second trial is underway. The old J antenna has been removed and some arc damage was found between the back of the antenna and the vacuum vessel wall. Further inspection of the antenna components is planned for this week. The FMIT#1 and #2 dummy load has been disassembled and pressurized to investigate a water leak. Lower Hybrid System ------------------- Work continued on the design of a waveguide stub fast-ferrite-tuner (FFT) based on garnet. We received new thicker garnet samples for testing to determine if the thicker samples have higher arc immunity than the thinner stacked material. Various configurations of the phase shifter and attenuator assembly, now called the "LH Modulator Box", were tested. We were able to do a raster calibration in less than 0.5 seconds as required for high power calibrations with the klystron in the control loop. Results look promising for the 0.5 s calibration shots, although some anomalies in the calibration data were seen, probably due to aliasing in the network analyzer measurement. Preparations are under way for calibrations using two consecutive half-second shots to see if it helps the accuracy. The filament power supply vendor has provided more information on the systems. We provided the cart models to them that indicate the spacing among components and they have provided us with a model of the proposed transformer design for the new system. We have substituted this into the cart model along with the proposed power supplies to check for HV insulation. The air insulation spacing looks acceptable, but we will have to move some components around in the carts to accommodate the change in size of the power supplies. We hope to make a final decision on the new transformer design and power supply enclosures early next week after more details are provided. Diagnostics ----------- Last Friday the MSE team conducted a preliminary design review of an in-vessel, between-shots calibration system for the Motional Stark Effect diagnostic. This system is designed to provide a three-point (three different polarization angles) calibration of MSE within a few seconds of an actual plasma shot, to compensate for the system’s tendency to ‘drift’ between shots. We plan to fabricate and install this system during the current C-Mod vent. Early last week the Motional Stark Effect team loosened the 2” wide steel strap that secures the MSE optical canister to the C-Mod vacuum vessel, pumped down to the 10^-5 scale, heated the vessel to 35 Celsius, and measured the temperature response of the optical system with a set of thermocouples. Preliminary data analysis indicates that the thermal response was nearly identical to that achieved during the FY09 campaign, when the system was operating as designed. The temperature gradients in the MSE optics were an order of magnitude smaller than observed in bakes during the FY11 C-Mod campaign. This test tends to confirm our hypothesis that the tightened support strap created a thermal short to the vessel wall. We will continue to analyze the data from the test and proceed with designing hardware changes needed to regain good thermal isolation. Travel and Visitors ------------------- Joshua Stillerman served as a member of the NIF Controls and Information Systems data system review panel at LLNL on April 13 and 14. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly