Alcator C-Mod Weekly Highlights July 12, 2010 FY2010 weeks of research operations Base Target: 13 weeks Base Completed: 10.1 weeks ARRA Target: 5 weeks ARRA Completed: 5.0 weeks Plasma Shots: 1759 Operations ----------- Plasma operations resumed at Alcator C-Mod last week. Three run days were scheduled and 2.25 were completed. A total of 57 plasma discharges were produced with a startup reliability of 85%. Computer delays, vacuum issues and a power system repair reduced the experimental time. The runs on Wednesday and Thursday supported startup and conditioning activities under Miniproposals from the Lower Hybrid Physics and Diagnostic Development tasks. Friday's experiment supported the FY2010 SOL Heat Flux Joint Facility Research target. The C-Mod upper divertor cryopump system was brought back into operation, and was employed for particle control during experiments on Wednesday and Friday. Plasma operations are planned to continue this week. Operations Details ------------------ Wednesday's run was dedicated to conditioning of the new LH2 lower hybrid launcher (MP#605). Early LH pulses exhibited a sudden jump in forward power on one klystron, resulting in nuisance trips. The problem was remedied by replacing a faulty circuit in the LH control and protection system. The launcher's coupling to the plasma was then measured as the outer gap was scanned and plasmas were produced with and without the cryopump. The best coupling results (Gamma^2 ~ 22%) were obtained with 1.5cm outer gap and nebar~1e20/m^3. Thursday's run was dedicated to MP#607, "Optimize the Gate Timing for the Thomson Scattering Data Acquisition System". A series of identical 1MA ohmic target shots were produced as the gating of the integrators for the Thomson Scattering data acquisition system was varied to determine the best setting for the different spectrometer systems. The optimum gate was determined to be 110ns, which was found to be appropriate for 90% of the detectors. These results are expected to improve the accuracy of the TS measurements. The target shots for this run were also used for calibration of other diagnostic systems, including the HIREX_SR spatially-resolved X-ray spectrometer. Friday's run was dedicated to the study of heat flux footprints in ohmic L-mode discharges (MP#570), with emphasis on commissioning two new diagnostic systems on the instrumented "ramped tile" of the J-port divertor module: an embedded Langmuir probe array and an array of surface heat flux sensors. These studies are in support of the FY2010 joint facilities research milestone. Discharges were operated with a sweep of the outer divertor strike point, allowing these new diagnostics to map-out the heat flux footprints. Data from the new Langmuir probe array, which is located at the top of the ramped tiles, appears to be free from field line shadowing effects that are evident in data from the old probe array (F-port divertor module). These data will therefore allow a good test of pressure and heat flux mapping of plasma conditions upstream at the outer midplane location, an important goal of MP#570. Initial assessment of parallel heat fluxes inferred from surface thermocouple sensors also look very good, with heat flux profiles essentially overlaying with those deduced from the Langmuir probes under low density, sheath-limited conditions. However, some discrepancies appear when the divertor enters into a high-recycling regime; Langmuir probes appear to overestimate the heat fluxes. Further experiments are planned to explore these dependencies as well as the scalings of the heat flux footprint as plasma current, toroidal field and edge collisionality are varied. ICRF Systems ------------ We continue to condition the antennas in preparation for plasma operation. D and E antennas have operated to 1 MW each independently, after considerable vacuum conditioning, J antenna is operating up to 30 kV into plasma. Further vacuum and plasma conditioning is expected to enable J antenna to reach its normal operational limit of 40 kV. The new solid state IPA sources were inspected. Primary activity has been to determine the logistics of providing the requisite electrical service for these new IPAs. Sample materials for resistive grounds for the new antenna have been received and are undergoing vacuum compatibility tests. Sample connectors for the voltage and current probes for the new antenna have been received and we plan to do vacuum compatibility tests on these this week as well. We are pursuing three solutions for Faraday screen attachment and one based on the old J antenna design appears to be viable. RF simulations of the proposed antenna were completed last week and a few details need to be addressed. Lower Hybrid System -------------------- Evaluation of the LH Visual Arc Detector system is being carried out during C-Mod operations. This new detector system images the lower hybrid launcher grill onto a large area detector and looks for rapidly changing signals. Work continued on the LH probe calibrations. The 16 probes corresponding to launcher waveguide columns driven by single klystrons have been calibrated. Calibration of the remaining probes, used on the "split" klystron columns, is in process. The Lower Hybrid System was successfully employed during C-Mod experiments on Thursday. DNB Systems ------------ Two experts from the Budker Institute (Petr Deichuli and Grigory Shulzhenko) have been visiting C-Mod to help refurbish our diagnostic neutral beam system. They finished their stay on Thursday, and have returned to Novosibirsk. At the time they left, the beam was operating at 45 kV and delivering 7 amps from the source, for 100 msec pulses. C-Mod staff are continuing to resolve remaining issues of noise and ground faults. Travel and Visitors ------------------- Randy Wilson (PPPL) was at MIT last week to work with the Lower Hybrid system. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly