Alcator C-Mod Weekly Highlights March 29, 2010 FY2010 weeks of research operations Base Target: 13 weeks Base Completed: 9.7 weeks ARRA Target: 5 weeks ARRA Completed: 5.0 weeks Plasma Shots: 1630 Operations ----------- Plasma operations continued at Alcator C-Mod last week. Five run days were scheduled and completed. Experiments supported research in the ICRF Physics, and Transport topical science areas. A total of 135 plasma discharges were produced with a reliability of 89%. Fresh boronizations were performed on Sunday, Monday, and Tuesday nights. The diborane bottle has now been removed from the cell. Last week's runs conclude research operations for the first FY10 campaign. C-Mod will now enter a planned maintenance period, including a manned access vent to refurbish internal hardware and install new diagnostic capabilities. FY10 plasma operations are planned to resume this summer. Operation Details ----------------- Operations on Monday, Tuesday, and Wednesday were devoted to research on Internal Transport Barriers (ITB) which are initiated by off-axis ICRF heating. The experiments made use of the J-port antenna (FMIT#3 and 4) operating at 70MHz and the D- and E-port antennas at 80.5 and 80MHz, respectively. Three miniproposals were supported: MP#512 "Fluctuations at Foot of Low Density ITBs Using Frequency Scanned Reflectometry", MP#521a "ITB formation - Ti and rotation profile effects", and MP#602 "Core Fluctuations and Transport in Two-Frequency ICRF Heated ITB Plasmas with Jogs and Modulated ICRF"; the first MP comprises part of the thesis research of an MIT graduate student. ITB discharges were produced using off-axis heating on both the high and low field side. A scan of resonance location (toroidal field) on Tuesday provided data on Ti and rotation profiles associated with formation and development of the ITB. Modulation of on-axis power (80MHz) during sustained ITB discharges was used to study the arresting of central peaking of the density and associated core transport. The run on Thursday was devoted to MP#599 "Up/Down Poloidal Impurity Density Variation". This experiment comprises part of the thesis research of an MIT graduate student. The purpose of this MP is to study parallel impurity transport over a wide range of parameter space (Ip, Bt, nebar) in Ohmic and ICRH L-mode plasmas. The key parameter under study is the sine (up/down) term in the distribution of impurity density on a flux surface. Key diagnostics included the HIREX_SR high resolution x-ray crystal spectrometer observing H- and He-like argon and the tomographic soft xray (XTOMO) arrays. Data were obtained at Bt=5.4, 4.0, and 2.7 T, and currents Ip=1.0, 0.8, and 0.6MA. Density was scanned during the discharges. The up-down asymmetry was observed in both HIREX_SR and XTOMO raw data, with qualitative changes as plasma parameters were scanned. The data are being analyzed, and the results will be compared with theoretical predictions of parallel impurity transport. Friday's run was dedicated to MP#603a "Investigation of RF sheaths using Surface Science Station", which comprises part of the thesis research of an MIT graduate student. The first part of the day's experiment was used to assess the evolution of W radiation at different ICRF power levels with the strike point of a LSN discharge fixed on the row containing broken tungsten lamella tiles. The experiment was designed to distinguish the dominant mechanism of tungsten influx, either molten droplets responding to JxB forces or local evaporation of molten W near the source surface. Data obtained with a range of RF power are being analyzed. The main part of the experiment was designed to measure RF-generated sheaths using emissive and ion-sensitive (ISP) probes, mounted on the Surface Science Station (S3). The probes were scanned into the SOL plasma on a shot-to-shot basis. A threshold-like appearance was observed in RF sheath potentials observed with the emissive probe at radial locations mapped to the J-port antenna RF limiter. The RF potentials increased dramatically, from ~20V to greater than 150V, as the line-averaged density increased above ~1.5e20/m^3, with constant RF power ~1.2MW. The sheath voltage increase as a function of density was less steep at higher RF power, Prf~1.7MW. The threshold-like behavior was not present when the emissive probe was scanned to radii that do not map to the antenna limiter. These observations are qualitatively consistent with theoretical proposals of Myra and D'Ippolito for propagation of RF voltage along field lines. The ISP probes were also operational, and I-V characteristics were obtained which can be analyzed to yield local denaity and potential estimates. The ISP mapping needs to be correlated with the emissive probe mapping in order to compare their estimates of the plasma potentials. ICRF Systems ------------ The RF antenna recovery from boronization last week was superior to previous experience; full power was achieved within a few discharges, compared to the usual 10 discharges typically observed in the past. The reason for the improvement is still under investigation. The FMIT#2 (E-port) driver system was repaired and returned to service on Tuesday. A control issue with the E-port antenna system (FMIT#2) resulted in some delays during last week's operation. The problem was traced to an intermittent fault at the input to the fault protection system. We now have a total of 4 feedthroughs in house for the new rotated four-strap antenna and are in the process of manufacturing components to allow high power testing of these feedthrus. ARRA Activities ---------------- We have received confirmation of the fast ferrite tuner (FFT) order from the vendor, and have identified power amplifiers to power the magnets. We expect to place an order for the power supplies within the next week, using ARRA funds, once specifications of the magnets in the FFT have been confirmed. Work continued on additional specifications for the 4th lower hybrid klystron cart. Purchase orders are being generated, and components that have been received are being tested. Lower Hybrid System -------------------- The new Lower Hybrid launcher was successfully leak checked on the vacuum test stand, following replacement of a leaking viton gasket identified following the previous leak check. The launcher system will now be baked in preparation for installation. Preparations are being made to resume calibration activities of the lower hybrid system as soon as the launcher is returned to the experimental cell. Travel and Visitors ------------------- John Rice was at Culham, UK to attend the ITPA Transport and Confinement group meeting Mar. 22-25. Earl Marmar travelled to Germantown to participate in the March 24 US-Japan Executive Secretary's Meeting. He made presentations on C-Mod status and plans, and on proposed collaborations. Martin Greenwald was at PPPL to make presentations to the Fusion Simulation PAC. Earl Marmar attended as a member of the PAC, which met on March 25-26. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly