Alcator C-Mod Weekly Highlights September 28, 2009 FY2009 weeks of research operations Target: 9 weeks Completed: 9.1 weeks Discharges: 1165 Operations ----------- Plasma operations continued on Alcator C-Mod last week. Four run days were scheduled. Only about half a day was completed on Thursday, due to the malfunction of a magnetics sensor (partial flux loop) used for plasma control as well as for equilibrium reconstructions. The control and analysis software were reconfigured to use the remaining sensors and operation resumed normally on Friday. Experiments last week supported research in the ITER H-mode Scenarios, Boundary, and MHD areas. A total of 94 plasma discharges were produced, with a reliablilty of 87%. C-Mod has now completed 9.1 weeks of research operation, satisfying our portion of the OFES Joint Facilities Operation milestone for FY09. This week will be a maintenance week at Alcator C-Mod. No plasma operation is planned. Operations Details ------------------ Tuesday's run was devoted to MP#564 "Power requirements for high confinement H-modes and the role of radiated power spatial distribution", submitted by Alberto Loarte of the ITER Organization. The goal of this run was to document impurity seeded Enhanced D-alpha (EDA) and ELMy H-mode discharges with net loss power through the separatrix (Input power - radiated power) matched to that of unseeded discharges obtained during the first run day devoted to this MP. Alberto participated in this experiment remotely from Cadarache, France. Data were obtained in EDA plasmas with both neon and argon seeding. Modest levels of neon puffing were found to result in improved H-factors relative to unseeded discharges, while reducing divertor power load and metallic impurities. ELMy discharges with argon seeding, and additional unseeded discharges in the ELMy configuration were also obtained. The dataset for neon-seeded ELMy discharges was not completed. Data analysis for this experiment is underway. Wednesday's run continued the series of experiments devoted to MP#570, "Boundary layer heat transport experiments in L-mode plasmas", which supports the FY10 OFES Joint Milestone "... measure the divertor heat flux profiles and plasma characteristics in the tokamak scrape-off layer in multiple devices to investigate the underlying thermal transport processes..." On Wednesday we completed an ohmic density scan at 2.7 T and 400kA, and began to assemble a dataset of ICRF-heated L-mode plasmas at 5.5T and 800kA. The low-field operation was facilitated by use of a low-field startup developed earlier in this campaign under MP#556 "Lower Bt operation, breakdown and discharge development". The outer strike point was swept along the divertor plate so that a fixed Langmuir probe at the divertor nose could record the full SOL profile, from private to common flux zones. Thermal and IR diagnostics were also employed, but because the heat flux in these low current ohmic plasmas is low, we rely primarily on the divertor probes for quantitative measurements in these discharges. The scanning probes were also employed to record upstream SOL profiles in these plasmas. At 5.4T and 800kA, we began a density and RF power scan, with the upper power limited by the L-H transition threshold. Data were collected at up to three power levels, at three densities, using the divertor probes, thermal sensors and IR camera, as well as the inner wall and A-port scanning probes. The runs on Thursday and Friday were dedicated to MP#567 "Spatial Distribution and Dynamics of Disruption Mitigation Radiation". The purposes of this experiment are to investigate what determines the length of time between when the gas jet first interacts with the plasma and when the prompt thermal quench (TQ) occurs, and to study the toroidal symmetry of the energy loss during these events, along with neutral and ion transport of the gas jet species. These studies contribute to a joint experiment with DIII-D organized through the USBPO to inform a design decision on the number of gas jets to be installed on ITER. Scans of the target q95 from 2.8 to 4.3 were obtained at two densities, nebar~1 and 2e20/m^3 in ohmic targets. The time to thermal quench was found to increase with q95, and was relatively insensitive to density. Using ICRF heating, we completed a scan of plasma energy, in L- and H-mode, between 55 and 180kJ. In this dataset the time to TQ was longest at the highest energy and shortest at the lowest, but the correlation was not consistent at intermediate values of Wmhd, and closer examination of the profiles will be required. After completing these scans using a standard He/Ar mixture for the gas jet, we investigated alterrnate mixes: D2/Ar and He/Kr. The deuterium-argon mix behaved similarly to He/Ar, while with the He/Kr mixture the gas arrived at the plasma later. A q95 scan with the He/Kr mixture had a similar relative dependence of the pre-TQ time to that observed with He/Ar. ICRF Systems ------------ All antennas and transmitters were run successfully in support of physics operations last week. Lower Hybrid System ------------------- All in-vacuum WR-187 waveguide is now in-house along with the vacuum flanges. E-beam welding tests of waveguide prototypes look very promising and will soon undergo high power testing. If the power tests go well, we plan to use e-beam welding to join both the vacuum flanges and the transformers to the WR-187 waveguide. This technique has already been successfully used to bond the 4-way splitters to the forward waveguide sections, and successfully tested on prototypes of the transformer bond to the splitters. The e-beam welding technique has several advantages over brazing, including repeatability, vacuum leak repair, and no need for post braze cleanup. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly