Alcator C-Mod Weekly Highlights October 22, 2012 FY2013 weeks of research operations Target: tbd Completed: 0.53 weeks Plasma Shots: 61 Physics ------- Dennis Whyte and Brian LaBombard presented "Heat flux footprint puzzles to resolve in scaling to ITER" to the ITPA Divertor/SOL meeting held in San Diego October 15-18. It was noted that present experiments consistently project to a separatrix pressure that is no greater than 10% of the H-mode pedestal pressure. This, combined with plasma physics constraints of SOL pressure balance and sheath physics at the divertor surface, leads to a simple and robust projection for the integral heat flux footprint width in ITER -- no less than 10 mm for divertor electron temperatures less than 100 eV. This result is in contradiction to the ~1 mm width that is presently being projected to ITER based on a multi-machine regression analysis of divertor strike-point data obtained in low-recycling, low divertor radiation regimes. While a high divertor electron temperature in ITER (approaching 600 eV) might resolve this puzzle (albeit not an acceptable divertor condition), it was pointed out that C-Mod already operates with ~1 mm heat widths with power densities similar to ITER yet does not have divertor temperatures approaching separatrix values. Additional physics that might resolve this puzzle was discussed, including heat flux 'spreading' due to divertor dissipation physics (radiation, transport) which may ultimately dominate the scaling of the heat flux footprint width, and/or a changeover in the physics that controls the heat flux widths under the conditions of high divertor/SOL collisionality. On the latter point, data was presented from an extensive study of C-Mod L-mode plasmas, showing a changeover in the heat flux width scaling that depends on collisionality -- going from an inverse plasma current scaling to a scaling that is independent of plasma current. In these cases, SOL pressure profiles were found consistent with the idea that a critical poloidal beta gradient (collisionality-dependent) sets the SOL pressure profile, implying that a major radius scaling to the heat flux widths should be observed in collisional divertor regimes. Lively discussions among ITPA participants are ongoing as this is an important topic that needs to be revolved. Operations ---------- Routine maintenance of engineering systems continued last week including work on the alternator, ICRF transmitters, and the Lower Hybrid System. Alternator ---------- Preventive maintenance work scheduled for this shut down period continued. The alternator seal oil drain pump was removed and a new pump was installed. Preventive maintenance was performed on the alternator 13.8KV outdoor sub-station. Maintenance and inspection continues on the alternator emergency lube oil and seal oil DC pump starters. ICRF Systems ------------ Inspection of the FMIT#3 and #4 transmission line continued as we search for an explanation of the faulting observed during plasma operation. No arcing or other indications were found in the phase shifters or stub tuners. The gas seal on the FMIT#3 stub tuner has been repaired. Inspection of the new FFT system also continued. FFT#2 was found to be in good condition, and its water system was found to be leak-tight. FFT#1 is being removed from its housing as we investigate a significant water leak that stopped operation of that unit at the end of the last run campaign. General maintenance of the transmitters has also been started. The de-ionized water system has been inspected and filters were cleaned. Lower Hybrid System ------------------- Fast thermocouples (FTCs) are now installed at the coolant outlet on all klystrons. Pressure testing of the klystron coolant loops showed no leaks with the FTCs installed. Wiring of the FTCs into the TPS COTS box is complete. The FTCs and associated transmitter boxes have been tested and read ~25C as expected. Calibration of the collector heat capacity for each klystron will begin once the high voltage yard work is finished. Work continues on LH FFT development and the 4th klystron cart. Interlock keys are now installed on the side panels of cart 4. An external magnet coil was set up to test the effect of stray magnetic fields on performance of the double stub FFT design. Remote Participation, Travel, and Visitors ------------------------------------------ Federico Felici, Eindhaven Universitiy of Technology, visited MIT last week and gave the PSFC seminar on "Physics-model-based real-time reconstruction and control of tokamak plasma profiles" on Friday. He met with many C-Mod scientists including Anne White, Steve Wolfe, Greg Wallace, Bob Mumgaard, Ian Hutchinson, and Steve Scott. Attending the ITPA meetings in San Diego last week were Bruce Lipschultz (Div/Sol), Amanda Hubbard (IOS), Bob Granetz (MHD), Yunxing Ma (PED), John Rice (T&C), Syun'ichi Shiraiwa (IOS), Matt Reinke (T&C, IOS), and Istvan Pusztai (T&C). Presentations given included: "Assessment of lower hybrid current drive at high density for extrapolation to ITER advanced scenarios" by Amanda Hubbard. "Critical E-field Study in Alcator C-Mod", "Update on Disruption Mitigation Experiments Using Two Gas Jets on Alcator C-Mod", and "Update on MDC-16: Runaway electron generation, confinement, and loss", by Bob Granetz. "Rotation gradient studies in auxiliary heated plasmas: The next step in cross-machine intrinsic rotation scaling", by John Rice. "Experiments to study the impact of poloidal asymmetries on radial transport of high-Z impurities in Alcator C-Mod", "Effects and control of low-Z seeding in Alcator C-Mod high confinement plasmas", and "H-mode and discharge termination in Ne and N2 seeded Alcator C-Mod plasmas", by Matt Reinke. "Poloidally varying equilibrium potentials and their effect on impurity peaking", by Istvan Pusztai. "Heat flux footprint puzzles to resolve in scaling to ITER", by Dennis Whyte and Brian LaBombard via teleconference. Dennis Whyte gave two presentations at the IAEA DEMO Workshop held at UCLA October 15-18: "Tungsten and Steel as Plasma-Facing Components in DEMO" and "Smaller and Sooner: How new superconductor technology can accelerate fusion's development". _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly