Alcator C-Mod Weekly Highlights December 10, 2012 FY2013 weeks of research operations Target: tbd Completed: 0.53 weeks Plasma Shots: 61 Physics ------- This month's USBPO Topical Group Highlight was "Tungsten nano-tendril growth in the Acator C-Mod tokamak", by G.M. Wright, D. Brunner, M.J. Baldwin, R.P. Doerner, B. LaBombard, B. Lipschultz, J.L. Terry, and D.G. Whyte, can be found at http://burningplasma.org/enews113012.html This highlight reports on the the first observation of nano-tendrils ("fuzz") in a tokamak. Such structures are fragile and may impact both the integrity of the first wall, and the generation of tritiated tungsten dust, which could pose serious operational issues for fusion reactors. Experiments have been conducted on ASDEX Upgrade as part of the ITPA Joint Experiment TC-18 "Dimensionless identity experiment in I-mode". This work was designed to match dimensionless parameters in I-modes on C-Mod and AUG, and was a continuation of experiments begun during a visit by Amanda Hubbard to ASDEX Upgrade last July. On the most recent run day (Dec 4), long duration I-mode phases were obtained at the expected target Ip and Bt values, at 2 different densities within the range needed to match C-Mod I-modes, although the equilibrium shape was not identically matched. These experiments were led by Francois Ryter and followed remotely by Hubbard. A large dataset of C-Mod discharges in comparable conditions will now be investigated to find the closest dimensionless matches and compare pedestals and fluctuations. "Density sensitivity of intrinsic rotation profiles in ion cyclotron range of frequency-heated L-mode plasmas", by M.L. Reinke, J.E. Rice, A.E. White, M. Greenwald, N.T. Howard, P. Ennever, C. Gao, A.E. Hubbard and J.W. Hughes, has been published as a Plasma Physics and Controlled Fusion Brief Communication. Changes in the gradient of the rotation profile in ICRF heated L-Mode plasmas with small changes in electron density are discussed. The paper can be found at http://iopscience.iop.org/0741-3335/55/1/012001 Operations ---------- Routine maintenance work continued on the alternator and magnet power supplies. The backup diesel generators for the alternator oil lube system were tested by our vendor, and one of two battery chargers required replacement. Improvements to the TF cabinet bonding/grounding system are being made to reduce noise levels on some of the TF control signals. Advanced Outer Divertor (AOD) ----------------------------- Full-scale stainless steel sheets are being produced, and a fabrication fixture is being designed for divertor flex-shunt prototyping. A COMSOL model is also being developed to simulate electrical, thermal, and structural properties of the shunt. COMSOL modeling of the outer divertor continued last week as the fields generated by the simulation were verified, and work on mapping of the fields to a detailed model of the divertor was begun. This new model will be used for structural analysis of the divertor during disruptions. Securing threaded monoblock tungsten tiles onto the divertor support plates from the back using Inconel 625 bolts would be a simple technique compared to the cross-pins and hold down screws we have used in the past. We are in the process of qualifying a threaded tungsten tile and Inconel bolt configuration in terms of stress-strain and yield. 600C thermal cycle tests are being done on X-750 Inconel and tungsten springs to qualify them for use in the heater connectors. Diagnostic work for the AOD included the design of probe support bars and connectors, and tiles for the surface temperature probe and thermocouple assemblies. Work is proceeding on fabrication of components needed for high temperature load testing under vacuum of spherical bearings. ICRF Systems ------------ The J4 phase shifter has been tested to the 1 MW level by connecting the J3 and J4 coaxes at the antenna end (in the cell), This configuration allowed the FMIT4 transmitter to be connected to the dummy load with the phase shifter in-line. Power level measurements along the whole transmission line path were compared. We continue to prepare the transmitters for operation at 53 MHz. The dummy load was found to be well matched at 53 MHz, and the approximate cavity measurements, based on 50-70-78 MHz data, have been set in FMIT#4. Lower Hybrid Systems -------------------- The LH fast ferrite tuner (FFT) with rounded ferrite edges has been tested up to 170 kW for 0.035 s. This power level exceeds the design requirement of 150 kW. Arcing did occur at 177 kW, and the ferrite showed minor copper deposition as a result. The ferrite has been refurbished to its original condition so that testing can proceed. Further testing of the LH3 bi-junction 2-way splitter assembly showed that the unbalanced power splitting (up to 1 dB difference between output ports) was the result of mechanical deformation of the waveguide walls during the waveguide clamping and pressurization process. Even splitting was regained after intentionally deforming the waveguide walls in a controlled manner. The bi-junction section of the actual LH3 antenna will have no pressure differential across the waveguide walls (vacuum on both sides) and is not expected to deform. Diagnostics ----------- The MSE group has completed an evaluation of the performance of the Motional Stark Effect Intershot Calibration System (ISC) during the FY12 run campaign. The ISC system functioned very reliably throughout the campaign and has provided many insights into candidate-causes of MSE's observed shot-to-shot drift. The drift has been shown to vary smoothly in time and in space, with the minimum variability occurring at channels near MSE's optical axis; it correlates with measurements of circular polarization, consistent with thermal stress birefringence in MSE lenses; it is not caused by operation of tokamak power supplies; it is not caused by heating of the MSE plasma-facing lens, etc. However, the ISC appears to 'capture' only a fraction of the total variability that obtains when MSE measures pitch angles in plasmas, and for this reason it cannot, at present, be used to provide an absolute calibration for pitch-angle measurements. Fortunately, the ISC has demonstrated that the drift occurs on a time scale of minutes (much longer than a C-Mod plasma discharge) and previous comparisons with EFIT in plasmas with a current ramp demonstrated that MSE can accurately measure changes in pitch angle during a plasma. Therefore, for studies of current profile modifications during lower hybrid current drive experiments, MSE can be calibrated effectively against kinetic, sawtooth-radius-constrained EFIT in the pre-LH Ohmic portion of the plasma. Remote Participation, Travel, and Visitors ------------------------------------------ Dennis Whyte, Amanda Hubbard, Bob Granetz and Alex Tronchin-James participated via video conference in the DIIID Research Opportunities Forum last week. They presented proprosals for I-Mode and disruption physics research. Dennis presented a proposal by Anne White who was unable to attend. Miklos Porkolab, Dennis Whyte and Earl Marmar attended the 2012 Fusion Power Associates Meeting on Dec 5,6 in Washington, DC. Miklos and Dennis presented talks at the meeting entitled: "Continued Research on "Alcator C-Mod at MIT is Critical to Make Fusion a Viable Energy Source" and "Smaller and Sooner: How a new generation of superconductors can accelerate fusion's development " respectively. The presentations will be put on the FIRE website shortly. Stewart Prager, Mike Zarnstorff and Jon Menard visited the PSFC last week for discussions with C-Mod personnel on programmatic issues of mutual interest, including opportunities for increased collaborations between the C-Mod and NSTX-U teams. Menard also presented the PSFC seminar, entitled "Progress and Plans for NSTX Upgrade". _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly