Alcator C-Mod Weekly Highlights July 30, 2007 FY2007 weeks of research operations: Target: 15 weeks Completed: 11.25 weeks Operations ---------- Research plasma operations continued at Alcator C-Mod last week, with all experiments carried out in the reverse-field configuration (ion grad-B drift upward). Two and a quarter research run days were completed, supporting miniproposals from the Transport and Divertor Edge physics groups and the H-mode Integrated Scenarios thrust. A total of 64 plasma discharges were produced, with a reliability of 75%. Two boronizations were carried out in support of last week's experiments. During operation on Thursday, a magnetics sensor (flux loop) failed, necessitating modifications to the plasma control algorithms and equilibrium reconstruction routines. These changes were accomplished and successful plasma operation using the remaining sensors was demonstrated on Friday. Preparations for refurbishment of the C-Mod magnetics diagnostic set during the next machine opening are underway. Plasma operation is planned to continue next week. Our power utility has scheduled a work order for the initial phase of work to remove the high impedance component from our 13.8 kV AC line, which has restricted simultaneous operation of C-Mod auxiliary heating systems. This work is planned for next Saturday, and will entail a brief interruption of service. Completion of the work is anticipated during the following week without interruption of C-Mod operation. Quarterly Review ----------------- A DoE quarterly review (FY07 3rd quarter) was held on Thursday by videoconference. Attending were: Earl Marmar, Brian LaBombard, Bob Granetz, Jim Irby, Ron Parker, Miklos Porkolab, all from MIT, S. Scott, PPPL, W. Rowan, U. Texas, and Rostom Dagazian, Steve Eckstrand, Erol Oktay, Ray Fonck, and Curt Bolton, OFES. Presentations may be found at http://www.psfc.mit.edu/research/alcator/pubs/quarterlies.html Operation Details ----------------- Tuesday's run was devoted to MP#511 "Study of ELM Structure, Dynamics and Energetics on Alcator C-Mod", continuing experiments in the reversed field and current orientation begun the previous week. Experiments were run in a slightly modified upper-null "JFT2-M shape" that avoided the disruptions encountered previously. Large discrete ELMs were obtained in this configuration with a range of outer gaps, enabling observations with the upper divertor probes, A-port scanning probe (in fixed position "dwell" mode), edge soft x-ray array, beam emission spectroscopy (BES), and reflectometer, as well as the gas-puff imaging (GPI) fast diodes, magnetics, phase contrast imaging (PCI) and ECE diagnostics which had been employed in previous C-Mod ELM studies. In addition, we also succeeded in producing ELMs in the lower null JFT2-M shape, i.e. with unfavorable grad-B drift direction. The run on Wednesday was dedicated to MP#510 "Topology effect on SOL Total Flow Vector with Reversed Field". The goal of this experiment, which comprises part of the thesis research of an MIT graduate student, was to obtain SOL flow data in the reverse field orientation in ohmic plasmas to complement the database already assembled in the normal field configuration. Scans of plasma density in upper and lower null equilibria were carried out. Despite a delay due to a power system fault in the morning, the run was successful in accomplishing most of the goals of this experiment. Thursday's run supported MP#504 "Slow Evolution or Two Phase L-H Transitions", which is intended to follow up on recent observations of slowly evolving H-mode transitions in unfavorable drift configurations, i.e. with the ion grad-B drift away from the active xpoint. In cases with the applied power just above the transition threshold, we have observed a slow evolution of the pedestal temperature and a reduction in edge fluctuations prior to the final transition, which is marked by a drop in D-alpha light and formation of the density pedestal. These "two stage" transitions appear to indicate a separation of the energy and particle transport. Thursday's experiment, which contributes to the thesis research of an MIT graduate student, was performed with lower null equilibria in reverse field, a configuration which optimizes diagnostic coverage of the pedestal region. We succeeded in creating clear two stage transitions, although a complete dataset was obtained at only one of the five desired target densities. The edge temperature pedestal formed and evolved prior to the transition, with little density evolution and a suppression of edge fluctuations. Although the RF power was constant the stored energy and the H-factor rose steadily for 200ms before the actual L-H transition. H-factors (H89) over 2.5 were obtained in the fully developed H-mode, with stored energy up to 150KJ, with 2MW total input power and 800kA of plasma current. The temperature pedestals and edge toroidal velocity shear were sustained even after the H-L back transition, and the H-factor stayed around 1.4. Also on Thursday, a piggyback experiment (MP#494a, "Fast Electron Driven Alfvén Eigenmodes" was carried out, employing LHRF very early in the startup phase (0.02 < t < 0.1 sec). As in previous experiments with the normal field orientation, high frequency modes were observed in the 300 - 700 kHz range. The toroidal mode numbers were found to increase with increasing frequency and time from n=1 to n=11. These modes were observed to rotate in the co-current direction, in contrast to those obtained with normal field and current, which rotated in the counter-current sense. Lower Hybrid System -------------------- The lower hybrid system was employed in support of the C-Mod experiment on Thursday of last week. Preparations are underway to ship two klystrons to the vendor for rebuilding in order to improve their performance. Design of a new 4-way splitter is underway. This new design should significantly simplify the fabrication of the new couplers. Diagnostic Neutral Beam System ------------------------------ The long-pulse diagnostic neutral beam performed nominally in support of experiments last week. Typical parameters were 50kV and 6 amps of ion beam current. Travel and Visitors -------------------- Martin Greenwald, Jim Terry, and Ian Hutchinson were in College Park Md. for the "Sorters Meeting", which assembles the program for the 2007 APS-DPP meeting. Prof. Eugenio Schuster of Lehigh University visited C-Mod on Monday for discussions relating to plasma control. During his visit he met with Steve Wolfe, Ian Hutchinson, Marco Ferrara, Ron Parker, Bob Granetz, Sara Angelini, and Josh Stillerman. He also made a presentation at the weekly C-Mod Science meeting on "Model-based current profile control during the ramp-up phase". _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly