Alcator C-Mod Weekly Highlights July 16, 2007 FY2007 weeks of research operations: Target: 15 weeks Completed: 10.1 weeks Operations ---------- Research plasma operations resumed at Alcator C-Mod last week, following a scheduled one-week maintenance interval. Three and a quarter days of research operations were completed. A total of 82 plasma discharges were produced with a reliability of 84%. The runs supported three miniproposals, from the Transport and AT Integrated Scenarios areas. Technical difficulties involving the power systems, vessel heater system, and the digital plasma control system computer resulted in reduction of useful experimental time by about two hours on each of the first three run days (Tuesday through Thursday). On Friday all tokamak systems performed nominally. Plasma operation is planned to continue next week with scheduled experiments in the reverse field and current orientation. Operation Details ----------------- The run on Tuesday was devoted to MP#493 "Ohmic L-H Threshold Experiments", from the Transport Group. The goal of these experiments is to investigate in detail the plasma conditions near the last-closed flux surface associated with ohmic L-H transitions. The scanning probes were among the key diagnostic systems for this experiment, with scans programmed to bring the probes close to or across the last closed flux surface (LCFS) to measure temperature, density and flow profiles as well as characterize turbulence. Some of the deep scans into the H-mode phase resulted in disruptions, which in turn caused damage to the exposed probe heads, reducing the amount of useful data that could be obtained. Datasets were obtained in the pre-transition L-mode phase, and these data are being analyzed. Repairs to the accessible probes and associated power supplies are underway. MP492 "Lower Ip Long Pulse L-mode and H-mode Advanced Scenarios" was run on 7/11 and 7/12, targeting slow Ip rampups to 600 and 450 kA plasmas. The ICRF and LH heating/CD were injected with various timing during the ramps to identify the impact on the safety factor evolution. The ICRF and LH systems worked reliably, delivering up to 2 MW and 0.5 MW, respectively. Heating as early as 200 ms was obtained, with divert times as early at 150 ms, and the effects of the heating on the surface voltage, li, and sawtooth onset were clearly identified. Both L-mode and H-mode phases were produced. Work continued throughout the runs days on Ip ramp optimization and plasma to LH launcher positioning. Chuck Kessel (PPPL) served as Session Leader for these experiments. Friday's run was dedicated to MP#472 " Continued Studies of Turbulence in Low Density Ohmic Plasmas". The goal of this experiment, which comprises part of the thesis research of an MIT graduate student, is to study the turbulence characteristics of ohmic plasmas in the electron-transport dominated neo-Alcator regime at low densities and in the saturated ohmic confinement regime at higher density. The Phase COntrast Imaging (PCI) diagnostic was employed in the medium-k configuration with localization to determine the propagation direction of the turbulence. Density scans from 0.3