Alcator C-Mod Weekly Highlights September 8, 2009 FY2009 weeks of research operations Target: 9 weeks Completed: 6.6 weeks Discharges: 879 Operations ----------- Plasma operations continued on Alcator C-Mod last week. Following a fresh boronization, four run days were scheduled and completed. Experiments supported research in the ICRF, Boundary, and ITER H-mode Scenarios topics. A total of 105 plasma discharges were produced with a reliability of 85%. Plasma operations are planned to continue this week. Operations Details ------------------ Tuesday's experiment continued MP#558 "Characterization of ICRF induced sheaths" by comparing results immediately following an overnight boronization with those obtained last Friday. Boronization of the machine clearly decreased the voltages measured by the emissive probe during the time when the D antenna was energized, compared to similar shots from Friday. The influence of the E and J antenna remained unchanged pre- and post boronization, with voltages of +10-30 V typically observed. No noticeable change on the Bdot probes, pre- and post boronozation, was observed and the bdot signals are generally not correlated with the voltages measured by the emissive probe. The magnetic field line mapping along the ICRF limiter tiles on the D antenna was accomplished via toroidal field scans from 5 to 6 T (at constant current). These Bt sweeps correspond to ~1.5 cm poloidal sweeps along the limiter tile faces and they show reproducible somewhat periodic signals on both the Bdot and emissive probes. We need to perform a more detailed mapping to determine if these signals correlate with magnetic field lines sweeping past limiter tile gaps. Wednesday's run was in support of MP#564, "Power requirements for high confinement H-modes and the role of radiated power spatial distribution". This experiment is aimed at providing data in support of a priority ITER task to determine power thresholds for access to various H-mode confinement regimes. Alberto Loarte (IO) particpated remotely from Cadarache. The run produced a useful set of data in discharges configured for both EDA and ELMy H-mode operation, with broad input power scans and some density variation. In the second part of this experiment, to be run at a later time, these results will be compared with impurity-seeded discharges with matched net power. Type I ELMing discharges were, as usual on C-Mod, obtained in an atypical equilibrium with weak shaping for the plasma crown and an X-point within 5cm of the inner divertor plate. Only about 250kW of ICRF power was required to trigger an H-mode in this configuration, confirming earlier suspicions of a relatively low power threshold (roughly 0.8 times the ITER scaling law). When the X-point radius was further decreased, the plasma began to graze (or strike) both the inner nose and shelf. In this novel configuration, the H-mode power threshold roughly doubled, and, prior to the L-H transition, an improved L-mode with H_98>1 was observed. This is a notable departure from the usual recipe for obtaining improved L-mode, which entails operating in unfavorable ion grad-B drift direction and with large amounts of input power. Thursday's experiment was devoted to MP#576 "Improve ITER-like discharge start-up". Chuck Kessel (PPPL) served as Session Leader for this experiment. The goal of this MP is to establish improved startup for ITER-like discharges by reliably diverting the plasma as early as possible. Reliable early x-point formation was obtained by crowbarring the OH and EF1 commutation circuits early (t=0.040sec) and relying on a combination of feedback and supply voltage pre-programming (feed-forward) to produce the desired shaping and current rise. Early diverting was obtained for ITER-like discharges going up to 1.3 MA, with divert time ranging from .080 to .120 sec. In addition to the ohmic startup, ICRF was injected just after x-point formation in several discharges, with H-mode transitions obtained between 0.15 and 0.2 sec. Further investigations of early heating in both L- and H-mode will be carried out under a future MP. On Friday we carried out the second day 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...". Scans of plasma density at 5.4T were completed for two values of current: 0.54MA and 1.1MA. Data were obtained from fixed and scanning probes, the IR camera, surface thermocouples and tile calorimeters, as well as turbulence measurements using Gas Puff Imaging (GPI). Problems with shape control previously noted at the lower current, associated with excessive time at zero current (crossover) by the OH1 "lockout dual" power supply, were successfully ameliorated by a newly implemented procedure run under the digital plasma control system (DPCS). Program Planning ---------------- A meeting of the Experimental Program Committee was held on Monday 8/31 to review proposals for experimental time. Three proposals were approved and two referred back to the authors for revision. ICRF Systems ------------ All antennas and transmitters were run successfully in support of physics operations. Issues with the reliability of the FMIT#3's FPA grid voltage regulator and FMIT#1's fast vacuum breaker, which caused minor delays during the week, are being investigated. Travel and Visitors -------------------- Michael Kaufman, a graduate student from the University of Wisconsin-Madison, visited MIT PSFC last week. He worked with Orso Meneghini and Syun'ichi Shiraiwa on the use of the COMSOL code for the analysis of the Lower Hybrid Interdigital-line antenna, which is operating in the MST reversed field pinch experiment. Chuck Kessel (PPPL) visited MIT on Thursday to serve as Session Leader for his experiment MP#576. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly