Alcator C-Mod Weekly Highlights April 11, 2011 FY2011 weeks of research operations Target: 15 weeks Completed: 14.5 weeks Plasma Shots: 1706 Operations ----------- Plasma operations continued last week at Alcator C-Mod. Two extended research run days were scheduled and completed. The runs supported miniproposals in the LH and Boundary Physics areas and a microproposal from the Diagnostic Development effort. Helium majority plasmas were employed for most of these experiments. A total of 43 plasma discharges were produced with a reliability of 68%. Last week marked the final operations for the FY2011 Winter-Spring experimental campaign. The tokamak is now warming up in preparation for a major maintenance and upgrade vent. Publications -------- A paper on the I-Mode (Dennis Whyte, et al., "I-mode: an H-mode energy confinement regime with L-mode particle transport in Alcator C-Mod", Nucl. Fusion 50, 105005 (2010)) has been designated by the journal as a highlight paper for 2010, and was one of the 20 most frequently downloaded articles of 2010 http://iopscience.iop.org/0029-5515/page/Highlights%20of%202010 . Operation Details ----------------- The majority of Tuesday's run was devoted to MP659 to test LHCD in the single pass absorption regime using moderate density (nebar ~ 1e20m-3) He plasmas at high magnetic field (8 T) and plasma current (1.2 MA). Although density control in He plasmas was difficult and we did not obtain the high temperature discharge proposed in MP, we found a significant increase of hard x-ray (HXR) count rates and non-thermal ECE emission at nebar~1.2x1020 m-3. Further analysis to identify the relative contributions of the magnetic field, plasma current, and ion species to the improved non-thermal electron generation are underway. For the final hour of Tuesday's run, The MSE and DNB teams carried out a "microproposal" to measure the full-energy neutral DNB current. The DNB was injected at full energy (50 kV) in deuterium into an otherwise Ohmic deuterium majority plasma at 5.2 Tesla, 1.0 MA. The plasma density was ramped from 0.8 to 2e20/m^3 to provide a range of deposition profiles for the DNB. Two good shots were obtained with a total of 17 DNB pulses, and as expected the beam-target neutron emission rate was substantially above the thermonuclear background. The measured neutron emission rate will be compared to TRANSP calculations that assume classical beam deposition and thermalization. Wednesday's run was primarily devoted to MP#622 "Molybdenum nano-filament growth in the Alcator C-Mod outer divertor". The goal of this experiment was to test for the production of nano-filament growth on Mo tiles in the Alcator C-Mod divertor. This nano-filament growth has been observed on W and Mo targets in linear plasma devices that expose these targets at elevated surface temperatures (>1000 K) to helium plasmas [1-3]. This fuzz can grow to microns in thickness and, given its structure, raises issues for tungsten use in reactors with respect to heat load bearing capability, dust creation, fuel retention and arc enhancement. To date, nano-filament growth has only been observed in linear devices and never in tokamaks. Demonstration that this growth can take place in an all-metal tokamak would have significant implications for the prospective use of refractory metal surfaces in future long-pulse/steady-state devices with He ash production in the core. The experiment was carried out in helium majority 900kA L-mode discharges with nebar~2.4e20/m^3 using 3.25MW of ICRF heating. The outer strike point was maintained on the "nose" tile at the top of the vertical divertor plate. Electron temperatures in the range of 15-25eV and surface temperatures of 900 to 1100 K at the hottest location on the "ramped" tiles viewed by the IR camera were obtained. A total of 10-12 seconds with conditions suitable for fuzz growth were achieved over a series of 12 successful discharges. The divertor module with the ramped tiles will be removed during the upcoming vent and subjected to detailed inspection and analysis. During this run a micro-proposal was run in which a step 20 kA above the nominal 900 kA in plasma current followed by one 20 kA below was programmed and observed on the polarimeter. One discharge showed a clear signature of the step, but higher noise levels and drifts affected the remaining discharges. We continue the analysis of these data and are doing tests of the polarimeter optics to better understand these results. ICRF System ----------- We completed the campaign with two He plasma runs; ICRF operation was quite robust in these He discharges, as has been observed previously. Following completion of operations, the J-port antenna transmission network has been disassembled, in preparation for installation of the new ARRA-funded rotated antenna. The first two of eight side plates for the rotated antenna have been delivered and accepted. The second set of Faraday rods has been received and inspected. The rods have significantly fewer tool marks than the previous set, and will require much less cleanup time. They will be sent out for polishing this week. We have worked with an e-beam welding vendor concerning the welding required to fabricate the bellows tube assemblies, and have settled on a welding process. This work will begin after the tubes are plated. The long strip line machining, and surface finishing of the boron carbide resistors is progressing well in our machine shop. Lower Hybrid System -------------------- The LH system was employed during Tuesday's run to determine LH efficiency in near single pass absorption regimes. This experiment required operation of the LH system at higher than normal n|| values ( 2.5-3.1). Reliable operation at these n|| values was difficult to obtain due to high reflection coefficients on many columns. Reliability increased after adjusting trip levels and adding attenuators to several reflected power directional couplers. Work continues on prototype ferrite loaded waveguide stub tuners. Enclosures for the Transmitter Protection System (TPS) upgrade were installed on the east wall of the cell behind the klystron carts. Cabinets arrived for the 4th cart controls and 2nd launcher coupler protection system. Amplitude glitches in the prototype low power microwave attenuator and phase shifter were reduced to acceptable levels by isolating the active components from the measurement equipment with DC blocks. Diagnostic Neutral Beam System ------------------------------ Following the end of the formal Alcator C-Mod 2011 Spring experimental campaign, the Diagnostic Neutral Beam (DNB) was operated into its calorimeter using both hydrogen and deuterium as the working gas. The relative DNB performance was evaluated by comparing the temperature rise as measured by thermocouples in the calorimeter to the power extracted from the DNB acceleration grid (time integral of voltage x current). To examine perveance effects, the extracted ion current was varied from 4.5 to 8.2 amps at fixed acceleration voltage (50 kV). Preliminary analysis indicates that the neutralization efficiency for a deuterium beam is about 91% of the value computed from published cross sections, and about 75% of the computed value for a hydrogen beam. Travel and Visitors ------------------- John Rice attended the ITPA-TC group meeting at GA April 4-5 where he gave updates on Joint Experiments TC-9 and TC-19. Over 20 C-Mod researchers participated in the Joint US-EU Transport Task Force meeting held April 6-9 in San Diego. Presentations concerning C-Mod research included one plenary session talk, nine orals, and eight posters. C-Mod presentations are being made available at http://www.psfc.mit.edu/research/alcator/pubs/TTF/TTF_2011_abstracts.htm Diego del-Castillo-Negrete of ORNL visited MIT on Friday to present a PSFC Seminar on "Non-diffusive, non-local transport in plasmas". During his visit he had discussions with a number of C-Mod scientists. Steve Raftopoulos, a PPPL mechanical/test engineer, visited C-Mod last Tuesday. He met with members of the C-Mod engineering staff and the outer divertor upgrade team. Steve discussed upcoming in-vessel measurements and other focused outer divertor upgrade projects with Rui Vieira and Soren Harrison, and attended the weekly outer divertor upgrade meeting. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly