Alcator C-Mod Weekly Highlights March 14, 2011 FY2011 weeks of research operations Target: 15 weeks Completed: 12.7 weeks Plasma Shots: 1497 Operations ----------- Plasma operations resumed last week at Alcator C-Mod. Two run days were scheduled and completed. The runs supported Miniproposals in the Transport and Boundary Physics and Diagnostics areas. A total of 50 plasma discharges were produced with a reliability of 96%. These experiments were all carried out with "reversed" field and current, i.e. with the ion grad-B drift upward, primarily with lower single null equilibria. C-Mod operations are planned to continue this week. The frequency of the J-port ICRF system is being converted from 78MHz to 50MHz for this week's experiments. The field will remain in the reversed polarity. Operation Details ----------------- Tuesday's experiment was dedicated to MP#655 "Characterization of WCM behavior and estimates of transport at the edge". This MP comprises part of the thesis research of an MIT graduate student. The aim of this experiment is to study the relationship between the characteristics of the weakly coherent mode (WCM) and the energy and particle transport at the edge of I-mode plasmas, as well as the WCM dependence on edge temperature and pressure gradients. A number of fluctuation diagnostics, including O-mode reflectometry, Gas Puff Imaging (GPI), Phase Contrast Imaging (PCI), fast magnetics and ECE, were used to study the different characteristics of the WCM. The experiment focused on reproducing sustained I-modes at different values of density and current, during which the heating power was stepped in order to modify the temperature and pressure gradients close to the last closed flux surface. Data were obtained at currents of 1.0, 1.1, and 1.2MA (with B=5.8T) with ICRF power up to 4.5MW, with density in the range of 1.21.4e20/m^3 at 1.0MA, even at P>4MW. Data are being analyzed. Two experiments were carried out on Wednesday. The first two hours were devoted to completion of MP#635, "Effects of RF heating in edge/SOL poloidal velocities", which contributes to the thesis research of an MIT graduate student. The goal of this experiment was to complete the reversed field portion of the dataset, most of which was obtained on Feb 18, incorporating shots using the J-port antenna. We successfully completed a scan of q95 and ICRF power using the J-port antenna alone, and with a combination of J- and D-port systems. A comparison of USN and LSN configurations with 0 and 1MW (from J-port antenna) was also obtained at currents of 0.8 and 0.6MA (q95=4.7 and 5.5, respectively). The remainder of Wednesday's run was devoted to MP#610 "Investigate discrepancy between TS and ECE measurements at high Te". This experiment was carried out at 8T using He3 mode conversion heating. We were able to obtain several discharges with Te0 transiently reaching 7-8keV, but were not successful in reproducing the higher Te obtained during long sawtooth intervals observed in 2010. Those discharges had relied on on-axis MC heating using the J-port antenna in counter current phasing to significantly increase the sawtooth period and amplitude. On Wednesday we had difficulty maintaining steady RF pulses with the counter-current phasing. In addition we found that it was difficult to keep the density sufficiently low in the LSN reversed field configuration; the previous results had employed normal field polarity and USN equilibria. While we were not successful in extending the dataset to temperatures above 8keV, we did obtain enough data in the 5-8keV range to allow us to perform detailed comparisons of the TS and ECE data. ICRF Systems ------------ We have begun the frequency switch from 78 MHz to 50 MHz in preparation for this week's experiments. The transmitter settings (FMIT#3 and 4) and the control/diagnostics for 50 MHz have been set. The coaxial line network has been installed and pumping/purging has begun. The RF breakdown experiment has been disassembled and inspected following a series of experiments carried out during the recent C-Mod maintenance interval. We found that the frequency sensitivity of the experiment was due to an undersized center conductor, which has now been replaced. The tuned frequency of the device is now reproducible. We plan to resume these experiments following the completion of the current C-Mod operation period. Sixteen of 18 TZM tiles for the ARRA-funded rotated antenna have been machined and are now being deburred and cleaned. The Z plate has been polished. Consideration of the RF modeling indicated that the E-field parallel to B in the region between the grounding bridge and the Z-plate was marginal (6-10kV/cm). A shorting strap connection to remedy this issue has been designed and is under evaluation. Thirty-four Faraday rods have been ground to remove the high points resulting from the bending process, and we are polishing the rods in preparation for coating. The port extension has been received and has passed its initial leak tests. The boron carbide washers are being polished to improve the contact area. Two more of the bellows tubes have been completed. Two (of 8) of the top/bottom plates have been received and polished in preparation for plating. Lower Hybrid System -------------------- No high power LH operations were requested for plasma operation this week. The LH launcher was moved into position on Wednesday and Thursday to allow for density profile measurements with the X-mode reflectometer. Efforts were focused on the control system for the LH3 lower hybrid launcher, 4th cart, and TPS upgrades. Work on the TPS Collector Over-Temperature/Resistive Temperature Detector (COT/RTD) boxes continues. Knockouts for fans and connectors were drawn and implemented. Engineers continued working on wiring, labeling and fit up, and fiber optic cables and feedthroughs were procured. Progress continues on the control system for the LH3 launcher. Phase scans at constant amplitude were successfully achieved with ± 0.07 dB accuracy across a 15 dB range using electronic phase shifters and attenuators. Typical accuracy with the vector modulators is ± .5 dB. An improved lookup table generation algorithm has been developed for the vector modulators for better comparison with the discrete phase shifters and attenuators. Travel and Visitors ------------------- Robert Granetz attended the 1st Indo-US workshop on collaboration in fusion, held at the Institute for Plasma Research (IPR) in Bhat, Gandhinegar. The major US fusion facilities presented summaries of current research and near-term plans. During extended discussion periods, the most fruitful areas for collaborations were discussed. Martin Greenwald and Amanda Hubbard traveled to Washington D.C. to participate in a meeting of the Fusion Energy Sciences Advisory Committee. Martin chairs the committee, and Amanda is a member. Miklos Porkolab also attended the meeting. Paul Bonoli attended the US-Japan Workshop on Integrated Modeling and Simulation in Toroidal Plasmas that was held at Kyoto University from March 9-11, 2011. The meeting was jointly hosted by Professor Atsushi Fukuyama from Kyoto University and by Paul Bonoli from MIT. Paul Bonoli gave two talks at the meeting titled "Integrated Modeling Activities in the US" and "Recent Advances in Integrated Modeling in the SciDAC Center for Simulation of Wave-Plasma Interactions". David Mikkelsen (PPPL) spent the week at MIT discussing gyrokinetic turbulence calculations of C-Mod plasmas and a planned experiment on hydrogen-isotope particle transport. Glen Wurden, our collaborator from LANL, was on-site most of the week. Glen has supplied the primary IR cameras used on C-Mod. He was on-site to review the installation of the present IR camera and to begin investigations into the real-time analysis of signals from IR imaging cameras for the purposes of hot-spot detection and making control decisions on future devices like the stellarator W7-X. This involves both developing usable tests for hot-spots and ultimately intercepting the camera's data stream and performing suitable fast tests on it. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly