Alcator C-Mod Weekly Highlights April 12, 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 Research -------- We have investigated high frequency (400-900 kHz) modes observed during ITB discharges. These modes occur during ICRF-heated EDA H-modes and ITB discharges. The features are observed both on magnetics and phase contrast imaging (PCI) diagnostics. Notable features include: (a) the presence of multiple, closely spaced modes; (b) cyclic chirping of the mode frequency over ~25 kHz range, typically at ~60 Hz; (c) persistence of the modes for 5-10msec after termination of the RF pulse. In I-mode discharges which transition into H-mode, these features are observed only during the H-mode segments. In a modulated ICRF heating experiment, a single 20-50 kHz wide fluctuation between 800 and 1000 MHz was observed. The mode appears during each RF pulse, persists for 5-10 ms after RF is turned off, and is more prevalent at higher power, consistent with fast-ion drive. In all cases, the mode frequencies are higher than the center of the gap in Alfven continuum associated with TAE's at the q=3/2 surface. MP 602, "Core Fluctuations and Transport in Two-Frequency ICRF Heated ITB Plasmas with Jogs and Modulated ICRH" was conducted on March 24. The purpose of this experiment was to produce strongly peaked ITB plasmas, fully documented with all available fluctuation and profile measurements, and controlled with modulated on-axis heating. This work continues previous studies focused on observing TEM turbulence through direct comparison of phase contrast imaging density fluctuation spectra with gyrokinetic simulations, using a synthetic PCI diagnostic in GS2 as described at http://www.psfc.mit.edu/research/alcator/pubs/iaea/2006/Ernst-paper.pdf The goal of MP602 was to use modulated on-axis heating to separate core and edge fluctuations, using PCI to document core fluctuations, and PCI, reflectometry, Mirnov coils, and gas-puff imaging to document edge fluctuations. Five strong ITB's were produced with off-axis ICRH, with two successfully maintained in steady state with modulated on-axis ICRH. A full suite of profile data, including Thomson scattering, ECE, and HIREX toroidal and poloidal rotation measurements was obtained. Initial analysis of fluctuation data shows a strong increase in density fluctuations measured by PCI during on-axis heating. This is accompanied by a strong reduction in edge fluctuations associated with the quasi-coherent mode, as shown by Mirnov coils and reflectometry. On the basis of previous studies, trapped electron modes are expected to be driven unstable by the on-axis heating inside the ITB foot. Detailed analysis will be carried out, comparing nonlinear gyrokinetic simulations with fluctuation spectra. DoE Quarterly Review -------------------- A quarterly review was held on Tuesday, 4/6, at MIT via video conference with DoE (Germantown and GA), and U. Texas. At MIT were Bob Granetz, Rui Vieira, Jim Irby, Brian Labombard, Amanda Hubbard, Jerry Hughes, Dan Brunner, David Terry, Ron Parker, Anne White, Martin Greenwald, Miklos Porkolab, and Earl Marmar. Participating From U. Texas (Austin) were Bill Rowan and Igor Bespamyatnov. Representing DoE were Mark Foster (at GA), Steve Eckstrand, Erol Oktay, John Mandrekas, Nirmol Podder, and Curt Bolton. Presentations were given on the status of the fy10 run campaign, the status of the lower hybrid launcher, plans for the current up-to-air period, and status and plans for the the Joint Facility Milestone. These presentations may be found at http://www.psfc.mit.edu/research/alcator/pubs/Q_Reviews/C-mod_quarterly_review_FY10_Q2_draft-2.pdf Operations ---------- Horizontal ports C, G, and B were removed last week, and, following a radiation survey, in-vessel work began. An extensive inspection of in-vessel components was made, and pictures were taken to document the state of the machine. These pictures may be found at http://www.psfc.mit.edu/research/alcator/photos/cmod_images.php?subdir=/INVESSEL/2010/Invessel%204-7-2010 Lower Hybrid System -------------------- Following a successful leak-check, the new lower hybrid launcher was removed from the vacuum test stand, and low power rf measurements were made. These measurements were performed at several different waveguide pressures, and effects on amplitude and phase were documented. These data will be needed as part of the overall calibration of the lower hybrid launcher system, which will operate in dry nitrogen or SF6 at a pressure of approximately 25 psi absolute to increase the fault tolerance of the waveguides and 4-way splitters. Work continued on the Coupler Protection System, rf probe calibrations, and procurement of components needed for the 4th klystron cart. ICRF Systems ------------ We have completed the in-vessel inspection of the ICRF antennas. The D- and E-Port antennas show minor erosion of the boron coating on a few front protection tiles. On the J-Port antenna, the TiCN coating has eroded at a few locations on the Faraday screen and plans are being made to polish these areas to minimize further erosion. Some antenna protection tile damage will also be addressed during the up-to-air period. We continue to prepare FMIT#4 for use on the ICRF breakdown experiment. The driver has been plumbed into a 125 kW dummy load, and the transmitter is being tuned for 78 MHz. Preparation for power testing the 5" vacuum feed-throughs for thenew advanced 4-strap antenna is nearing completion. A 9" coaxial switch is being re-assembled for use in the test following refurbishment. Coaxial adapter components are being prepared. Diagnostics ----------- Work continued on installation of the C-Mod Polarimeter. The alignment of the upper optical table relative to the lower table was documented so that installation of the beamlines that connect the two table enclosures could be completed. Work also continued on PLC wiring and cabling, new high gain detector amplifiers, and FIR laser frequency control hardware. Travel and Visits ----------------- Yijun Lin traveled to the Southwest Institute of Physics (SWIP) and the Institute of Plasma physics of Chinese Academy of Sciences (ASIPP) from March 20 to April 10. At SWIP, he discussed possible collaboration in the ICRF area for their next upgrade HL-2M. At ASIPP, he participated in the first ICRF experiments on the EAST tokamak. Jim Zaks visited vendors in San Francisco and Los Angeles California last week to discuss components required for our lower hybrid system. David Mikkelsen was at MIT all last week to develop plans for an experiment on C-Mod and collaborate on GYRO turbulence simulations of momentum and impurity transport in C-Mod plasmas. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly