Alcator C-Mod Weekly Highlights September 6, 2011 FY2011 weeks of research operations Target: 15 weeks Completed: 14.5 weeks Plasma Shots: 1706 Physics ------- Predictive capability for H-mode pedestals is the focus of the FY11 Joint Research Target. The EPED series of models uniquely predicts the pedestal structure in ELMing H-modes based on scalar inputs. A set of 24 ELMy C-Mod discharges were analyzed and compared to EPED predictions. ELMing periods were selected for consistent density and ELMs - while pedestal models mostly correspond to the peaks of the ELM cycle, ELMs on C-Mod are sufficiently rapid that it is difficult to gather pedestal data timed to the ELMs. Instead, data from the selected periods were collated into "ensemble-averaged" pedestals. A subset of discharges were also prepared with ELM-synced data. Pedestal pressure predictions were found to be consistently higher than the measured values, with greater discrepancy at higher pedestal pressures. The ELM-synced data broadly fit expected trends, and corrected the high-pressure discrepancy. Alternately, the correlation could be improved with an alteration to the diamagnetic stabilization model used in EPED. It was difficult to distinguish a trend in the width scalings, as the pedestal width was robust across the parameter range. However, the ELM-synced widths were consistent with previously observed scalings. The Continuous-Wavelet-Transform (CWT) has been used to post-process full-wave fields of Lower Hybrid waves which have been generated using the LHEAF code. Compared to the Fourier transform, the CWT has the appealing property of yielding information on the spatial location of spectral modes. Using a complex-Morlet CWT, the full-wave field pattern is decomposed into its spectral components parallel to the static magnetic field, n||. The eventual goal of this analysis is to provide a tool that can be used to assess the range of validity of ray-tracing calculations. The importance of full-wave effects on the transformation of the n|| wave spectrum, as well as on the LH power absorption and driven currents, can also be studied. In particular, attention has been given to LHEAF simulations of Alcator C-Mod LHCD discharges at high densities, in a regime where an anomalous decrease of Lower Hybrid Current Drive (LHCD) efficiency has been reported. LHEAF simulations using a realistic 2D SOL model were found to reproduce qualitatively well the experimental drop of HXR emission that is observed for increasing plasma density. This result has been attributed to the combined effect of an n|| upshift and radial diffusion of fast electrons. CWT analysis attributes the origin of such full-wave-induced spectral broadening to diffraction effects taking place close to caustic surfaces and after reflection off the low density cutoff layer or the vacuum vessel walls. Also, in LHEAF the antenna-plasma coupling was considered in an exact way and a broad poloidal spectrum was observed. This effect is attributed to Fresnel diffraction due to the finite poloidal extent of the waveguides. The design, construction and initial results from the LHCD launcher on Alcator C-Mod are presented in a paper by S. Shiraiwa, et al. in Nuclear Fusion. The LHCD launcher (LH2) is based on a novel splitter concept which evenly distributes the microwave power in four ways in the poloidal direction. This design allows for simplification of the feeding structure while keeping the flexibility to vary the peak launched toroidal index of refraction, Ntoroidal, from -3.8 to 3.8. An integrated model predicts good plasma coupling over a wide range of edge densities, while poloidal variations of the edge density are found to affect the evenness of power splitting in the poloidal direction. The measured transmission loss is about 30% lower than the previous launcher, and a clean Ntoroidal spectrum has been confirmed. Power handling capability exceeding an empirical weak conditioning limit and reliable operation up to 1.1 MW net LHCD power have been achieved. A survey of antenna-plasma coupling shows the existence of a mm vacuum gap in front of the launcher. Fully non-inductive, reversed shear plasma operation has been demonstrated and sustained for multiple current diffusion times. The current drive efficiency of these plasmas is 0.2-0.25 X 10^20 A/W/m^2, which is inline with the expected efficiency on the International Thermonuclear Experimental Reactor (ITER). The paper may be found at http://iopscience.iop.org/0029-5515/51/10/103024/pdf/0029-5515_51_10_103024.pdf Operations ---------- Routine maintenance on alternator, magnet power supplies, and other engineering systems continued last week. In-vessel work was primarily involved with documentation of outer wall radii where the new ICRF antenna will be installed. ICRF Systems ------------ Invessel fit-up tests of the ARRA-funded rotated antenna showed that the antenna position was outside of the allowed tolerances, due to non-uniformities in the outer wall of the vacuum vessel. The antenna mounting plates have been removed for modifications based on detailed precision mapping of the wall taken with a combination of our computerized Coordinate Measuring Machine (CMM) arm and conventional mechanical measurements. FMIT#4 has been converted to a driver only configuration in support of thesis work devoted to understanding rf breakdown in antenna structures. The other three transmitters are being readied for plasma operation. Lower Hybrid Systems -------------------- Increasing the drive level to one of the LH vector modulators improved its performance. We are working to make this change to all the vector modulators. Diagnostics ----------- Work needed to provide three polarimeter chords for the next run campaign has progressed to the point of obtaining local oscillator signals and a strong signal from one plasma chord. Installation of optics needed for the other two chords is ongoing. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly