Alcator C-Mod Weekly Highlights December 14, 2009 FY2010 weeks of research operations Base Target: tbd Base Completed: 1.1 weeks ARRA Target: 5 weeks ARRA Completed: 5.0 weeks Plasma Shots: 727 Operations ----------- Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and completed. A total of 112 plasma discharges were produced with a startup reliability of about 90%. The runs supported experiments from the Transport Physics, ICRF Physics, and Boundary topical areas. Plasma operations are plannned to continue this week. Operation Details ----------------- Tuesday's experiment was dedicated to MP#572 "Detailed scan of low collisionality H-Modes". The objective of this miniproposal, which comprises part of the thesis research of an MIT graduate student, is to expand the database of the low collisionality H-mode regime, and to characterize the fluctuations observed in this regime. Key diagnostics included correlation reflectometry and the O-mode fixed frequency reflectometry. A feature of particular interest was the peaking of the H-mode density profile observed at low collisionality. The database of well-documented low collisionality peaked H-modes was successfully extended to q95>7. Data are being analyzed. On Wednesday we continued experiments on ICRF sawtooth pacing (MP#588), using H-minority with the J-antenna operating at 78MHz. The toroidal field was varied during the shot to move the resonance across the q=1 surface. Clear changes in the sawtooth period were observed between +90 and -90 degree phasing (co- and counter-current drive, respectively). The experiment was complicated by L-H transitions, despite the use of upper single null equilibria. Thursday's run was dedicated to MP#589, which is designed to address the question "Does the quasi-coherent mode limit pedestal gradients?". Rich Groebner (GA) served as co-Session Leader for this experiment, which parallels research carried out on DIII-D for the kinetic ballooning mode. This experiment explored time-dependent aspects of H-mode pedestal formation, in particular the evolution of edge profiles and the establishment of the quasi-coherent mode (QCM). In contrast to typical pedestal studies, which tend to focus on steady H-mode phases, this experiment concentrated mainly on the initial 50--100ms following the L-H transition. To maximize data collection, several discharges were run at each target condition, with 3--4 H-modes triggered in each discharge. At each of three target currents (0.65, 0.8 and 1.0MA), a thorough data set of pedestal profiles from Thomson scattering was collected, as were fluctuation measurements from PCI, reflectometry, gas puff imaging, and fast magnetics. By assembling ensemble data sets from groups of identical H-modes, we intend to relate the onset and evolution of the QCM, and perhaps other turbulence characteristics, to critical values of pedestal gradients. Detailed analysis of the data set is to be completed, but preliminary observations indicate that the measurable onset of the QCM is roughly contemporaneous with the roll-over in the pedestal pressure gradient. On Friday, we carried out MP#587a, an investigation of "The Parallel Structure and Dynamics of Blob/Filaments". Olaf Grulke (IPP-Greifswald) served as co-Session Leader for this experiment using remote participation tools. The goal of these experiments was to obtain cross-correlations between the outer midplane Gas Puff Imaging (GPI) array with probe-based fluctuation diagnostics (F-port Scanning Probe (FSP) and fixed divertor probes). Magnetic mapping between the GPI array and the probe locations was successfully established by tuning the plasma current and equilibrium in a series of ohmic discharges over a range of densities corresponding to 0.2< n/n_Greenwald < 0.5. The analysis of the time-delayed cross-correlations is underway. Physics -------- Results from C-Mod ICRF operation with high Z plasma facing components were included in a presentation given by M.-L. Mayoral at at the European Fusion Physics Workshop (7-9 Dec, Velence Hungary) ICRF Systems ------------ All four ICRF systems operated nominally in support of C-Mod experiments last week. On Tuesday a fault in the FMIT#2 interlock system permitted the transmitter to operate into vacuum despite the presence of a "shutdown" signal from the C-Mod fizzle detector system. The interlock was reset and the system is again operating reliably. Lower Hybrid System -------------------- The LH SOL reflectometer and vacuum feedthroughs have been installed on the LH launcher; installation of the rf probes and associated cable runs is proceeding. The shroud that protects all these components along with top and bottom support plates has also been installed. The launcher is being prepared for installation into the vacuum test stand where it will be baked and leak-checked. Work continued this week on the assembly of the LH jungle gym in the cell. All of the support structures are in place and all WR187 waveguide components from intermediate directional coupler forward have been mounted and clamped into position. The high power water loads were fit up to determine hose routing, but were then removed until needed for high power tests. New DC break components were received and installed. The patch panel for the microwave connections and diode stacks was put in place and Panduit runs were determined and support pieces are being fabricated. Water loads were pressure tested to 100psi. Diagnostics ------------ The C-Mod Active MHD Spectroscopy system is being returned to service in preparation for upcoming experiments. This instrument is used to investigate the damping of stable Alfven eigenmodes, which can be driven unstable by fast ions which can be produced in C-Mod by ICRF minority heating. The system was successfully operated in open-loop mode in conjunction with the C-Mod shot cycle last week. Feedback control using the C-Mod Digital Plasma Control System to track the Alfven frequency and provide the AMHD waveform is under test. Travel and Visitors ------------------- Greg Hanson (ORNL) visited as part of the MIT/ORNL reflectometry collaboration to inspect the LH SOL reflectometer. David Mikkelsen spent the week at MIT helping out with an experiment on low collisionality H-modes, teaching three new users about GYRO, and discussing plans for an experiment to measure the H/D ratio in the core of C-Mod plasmas. Rich Groebner (GA) visited MIT last week. In addition to serving as Session Leader for Thursday's experiment, he participated in discussions of H-mode pedestal issues with several C-Mod scientists, and presented a PSFC Seminar on Friday entitled "H-Mode pedestal physics research on DIII-D". Bob Granetz participated in the ITER Control System Workshop in Cadarache, France last week. Martin Greenwald traveled to Washington to brief William Brinkman and Pat Dehmer of the Department of Energy on the Fusion Simulation Program. Bruce Lipschultz attended the FOM/Rijnhuizen Scientific Advisory committee meeting December 8-9. He then traveled to IPP-Garching primarily to discuss the construction of SLR-lens based spectrographs. Ralph Dux and Thomas Putterich have constructed several such spectrographs for CXRS and impurity influx measurements. The intent is for several such instruments to be built at C-Mod. While at the IPP Dec. 10-11 Bruce Lipschultz also discussed with H. Zohm and A. Kallenbach the plans for T. Putterich to visit C-Mod, had discussions with J. Roth on the summary report of the July 2008 meeting on analysis of retention data for ITER, and discussed with M. Wischmeier the modelling of divertor physics. Bruce Lipschultz then traveled to San Diego to chair the ITPA SOL/div meeting December 14-17. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly