Alcator C-Mod Weekly Highlights December 3rd, 2012 FY2013 weeks of research operations Target: tbd Completed: 0.53 weeks Plasma Shots: 61 Physics ------- A C-Mod research highlight on studies of I-mode has been posted on a DOE Office of Science website, and can be found at http://science.energy.gov/fes/highlights/2012/fes-2012-10-d/ I-Mode is an attractive tokamak operational regime, combining the high energy confinement and edge thermal barrier of H-mode, with the low particle confinement of L-mode, avoiding impurity accumulation and the need for ELMs to expel particles; ELM divertor heat fluxes are an issue of great concern for ITER. Alcator C-Mod experiments have confirmed and extended prior results which found particularly robust operation in this configuration, maintaining I-mode up to the highest ICRF heating powers on C-Mod, expanding the ranges of magnetic field, and obtaining detailed information on the core and edge profiles and turbulence which should help us understand better its physical mechanisms. Initial assessments of the potential application of I-mode to ITER were positive, indicating that I-mode may be accessible on ITER with the planned heating power, at average density of about 5 X 10^19/m^3, and that Q=10 could be achievable at about 30% higher density. An open issue was whether such a controlled density increase was achievable while maintaining the I-mode. This motivated recent C-Mod experiments to assess density dependences and implement active density control. Results were extremely positive. Gas fueling was added to an I-mode phase, increasing average density from the initial 1.5 X 10^20/m^3, to a final value of 2 X 10^20/m^3. Plasma pressure remained nearly constant, with energy confinement following the ITER H-mode scaling, while I-mode turbulence features and edge temperature pedestal are clearly maintained. With further increases in power, from external sources, or from alphas in a burning plasma, it could well be possible to extend the I-mode operating space to even higher densities and performance. Additional experiments, both on C-Mod, and in coordination with larger, lower field tokamaks, are urgently required to increase our confidence in the extrapolations to burning plasma conditions on ITER. "X-ray imaging crystal spectroscopy for use in plasma transport research," by M.L. Reinke, et al., has been published in Review of Scientific Instruments. It can be found at http://rsi.aip.org/resource/1/rsinak/v83/i11/p113504_s1 This paper describes advancements in the spectral analysis and error propagation techniques associated with x-ray imaging crystal spectroscopy (XICS) that have enabled this diagnostic to be used to accurately constrain particle, momentum, and heat transport studies in a tokamak for the first time. Doppler tomography techniques have been extended to include propagation of statistical uncertainty due to photon noise, the effect of non-uniform instrumental broadening as well as flux surface variations in impurity density. These methods have been deployed as a suite of modeling and analysis tools, written in interactive data language (IDL) and designed for general use on tokamaks. Its application to the Alcator C-Mod XICS is discussed, along with novel spectral and spatial calibration techniques. Example ion temperature and radial electric field profiles from recent I-mode plasmas are shown, and the impact of poloidally asymmetric impurity density and natural line broadening is discussed in the context of the planned ITER x-ray crystal spectrometer. Operations ---------- Alcator C-Mod is now up-to-air, and an initial photographic survey of the state of the vessel has been completed. Some in-vessel diagnostic calibrations have been done, and preparations for others are underway. For example, the post-campaign laser alignment of HIREXSR will allow accurate processing of the new "high-Te" configuration which was used during the reversed field campaign. Calibration of the SOL reflectometers at C and H ports has also begun. Advanced Outer Divertor ----------------------- Testing programs are being developed for prototypes of multiple systems, including: heaters; spherical bearings; current shunts designed to carry halo currents; and mono-block tungsten tiles. The heater system will provide the power needed to bring the divertor up to its 600 C design temperature. The spherical bearings are needed to restrain the divertor during disruptions while also allowing 6 to 7 mm of radial movement as the divertor comes up to temperature. Designs for the heater connectors and the load cell test chamber for the spherical bearings are being finalized. Drawings for the tungsten tiles are also being generated and vendors for supplying the tungsten contacted. Tungsten and Inconel springs for the heater connectors are being tested and procured. Pre-disruption comparisons of fields generated by a COMSOL model and EFIT reconstructions indicate good agreement. A full ring divertor model with the latest geometry has been generated in COMSOL and will be used for structural analysis of disruption loads on divertor components. Two designs for the divertor current shunts are being considered. An Inconel pin design is being pursued at PPPL. A flexible laminated shunt design is being pursued at MIT. An electromagnetic/thermal/structural coupled analysis of the laminated shunt is being developed. Design of an integrated set of divertor diagnostics is proceeding. Tiles are being designed to accommodate surface temperature probes, thermocouples, and Langmuir probes. A set of diagnostic views, based on a mirror-based endoscope, will carry IR and visible images of the divertor out of the machine where they can be slit by wavelength regions. These will be used to monitor the temperature (IR) and impurity source rates (visible) at this surfaces. ICRF Systems ------------ The FMIT#4 Phase Shifter has been re-assembled following repair of an arc tracked push pull rod. The range of phase shift for various frequencies has been measured. The transmission lines for FMIT#3 and #4 have been connected to allow testing of power measurements into dummy load. We continue to work on FMIT 3 and FMIT 4 tuning and characterization. Initial check out of sweep generation equipment is encouraging: sufficient sweeps have been produced and recovered by the data system with widths as short as 3 milliseconds. Both the driver crystal detector and the regular power demodulator for the fpa are operating well. The FFT damaged in shipment was shipped back to the vendor last week. Preparations are being made for installation of the new FFTs at the D and E antennas. Initial observations of the field aligned antenna revealed a single tile (K side upper) was over heated and sustained significant melt damage. The tile also appears to be de-laminating. Lower Hybrid Systems -------------------- Directional couplers and detector diodes for the LH3 bi-junction test setup were calibrated. Analysis of high power testing of the bi-junction showed uneven power splitting between the two outputs of the splitter, while previous testing at low power showed near perfectly balanced power splitting. Mechanical deformation of the splitter dimensions at 10 psi inside the waveguide (low power tests were at ambient pressure) is the suspected cause of the discrepancy. Future tests will examine performance of the splitter as a function of pressure. Low power testing of the full rounded ferrite for the lh fft has been completed, and it was found to be superior to the slab ferrite. The electromagnetic modeling fails to reproduce the improvement and is likely due to missing physics in the ferrite model. An engineering drawing has been prepared detailing the shape of the ferrite for discussions with our ferrite vendor. A short paper, "Lower Hybrid Current Drive Experiments on Alcator C-Mod", was submitted for inclusion in the December USBPO eNewsletter. Remote Participation, Travel, and Visitors ------------------------------------------ Chang-Le Liu, EAST tokamak, Hefei Institutes of Physical Science, will be visiting MIT for two weeks to work with Bill Beck, Rui Vieira, Lihua Zhou, and others to begin an electro-magnetic/structural analysis of the EAST tokamak divertor during disruptions. This work follows naturally from previous work done for the C-Mod Advanced Outer Divertor. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly