Alcator C-Mod Weekly Highlights July 27, 2009 FY2009 weeks of research operations Target: 9 weeks Completed: 1.7 weeks Discharges: 280 Operations ----------- Plasma operations continued on Alcator C-Mod last week. Four run days were scheduled, but two were shortened in order to deal with several operational issues. A total of 60 discharges were produced, with startup reliability of 75%. The runs continued the ICRF commissioning process, and also supported two miniproposals, in the Boundary and Operations/Diagnostic Development areas. Tuesday's experiment supported MP#480 "Topology effect on SOL total flow vector". This work comprises part of the doctoral research of an MIT graduate student. Analysis of previous experiments resulted in the discovery of a significant particle pinch inside the last closed flux surface on the high field side (HFS). This has been proposed as a mechanism for closing the HFS particle flow loop implied by the near-sonic parallel flows observed on the HFS midplane. Most of the previous database is from the Upper Single Null (USN) configuration. The present experiments aimed to confirm the previous observation, extend the dataset to LSN discharges, and investigate the dependence of the pinch on discharge density. In addition, the probes were electrically configured to run in flow mode, to verify flows on identical discharges, and also to investigate the sensitivity of fluctuation-induced flux measurements to electrode configuration. Experiments were carried out in both LSN and in USN equilibria, and fluctuation data were obtained for analysis. The runs on Wednesday and Thursday were scheduled to support continued conditioning of the D- and E-port ICRF antennas, and to begin commissioning of the J-port antenna. Diagnosis and repair of several power and gas system faults resulted in reduction of the available experimental time on both days, and only ten plasma discharges were produced. Some progress was made on conditioning the D- and E-port antennas. No conditioning of the J-port antenna was accomplished due to faults in the ICRF control system. Friday's run supported MP#534 "Foil/AXUV bolometer calibration in helium" from the Diagnostic Development group. In addition to cross-calibrating the AXUV and foil-based bolometers, and testing the ability of the AXUV diode arrays to obtain absolute emissivity measurements for various radiating impurities, the experiment is also intended to check the influence of impurity line emission of NV on the Lyman-alpha diagnostics. We also developed a helium startup scenario in support of several planned helium majority experiments proposed by the ITER H-mode Scenarios task; previously we have relied on breakdown in D2 for majority helium plasma experiments. The run began with 800kA, LSN discharges intended to reproduce the density scan carried out during Tuesday's run, but with helium discharges. These shots provided data showing significant signal reductions on the Lyman-alpha diagnostics, proving that the measurements in D plasmas are indeed Lyman-alpha emission and not scattered light. The divertor AXUV radiation was increased by about a factor of two due to the change in main ion species. The main focus of the experiment, calibration of the bolometers in USN discharges with nitrogen and xenon impurity seeding, was not accomplished due to insufficient ICRF power; both D- and E-port systems operated independently, but we were unable to combine them without faulting. The helium breakdown scenario development was largely successful. The helium startups were found to be less susceptible to runaway geneneration than the standard D2 scenario, as reported previously on JT-60U. However, disruption recovery was found to be less robust using helium. Plasma operations are planned to continue at Alcator C-Mod this week. Physics -------- Analysis of measurements from the 2008 run campaign of hard X-rays during high density L-mode discharges with ~700kW of LHCD indicate a significant reduction of fast electrons in the plasma above 1e20/m^3 line averaged density. Above this same threshold, large current densities, of ~500 kA/m^2, are observed in the SOL, indicating that power may be absorbed in the SOL rather than inside the separatrix. Ongoing analysis of these high density L-mode discharges with the GENRAY/CQL3D simulation package shows that collisional absorption in the SOL may be a sufficient loss mechanism to reduce core absorption by the amount observed. Low density H-mode discharges at similar density to the high density L-modes (~1.5e20 m-3) show an increase in X-ray emissivity as compared to the L-modes. These H-modes also show considerable changes to their density and temperature pedestals caused by the application of LHCD. An article by J.C. Wright, et al., entitled "An assessment of full wave effects on the propagation and absorption of lower hybrid waves" is featured on the cover of the July 2009 issue of Physics of Plasmas. Quarterly Review ---------------- A C-Mod quarterly review was held on July 21 by tele-video. Presentations can be found at http://www.psfc.mit.edu/research/alcator/pubs/Q_Reviews/Q_rev_jul09/Q_rev_jul_09.pdf Attendees included: MIT - Earl Marmar, Bob Granetz, Amanda Hubbard, Jim Irby, Steve Wukitch, Jerry Hughes, Martin Greenwald, Miklos Porkolab, Ron Parker, David Terry and Rui Vieira OFES - Rostom Dagazian, Ed Synakowski, Curt Bolton, Erol Oktay, John Mandrekas, Sharon Stevens, and Steve Eckstrand. PPPL - Chuck Kessel, Steve Scott, and Randy Wilson University of Texas - Bill Rowan ICRF Systems ------------ Testing and conditioning of the J-port antenna system was begun. We experienced significant control issues that have appeared only during operation into plasma. The D- and E-port antennas have been operated to 1.5MW independently, but interfere with each other when operated simultaneously at high power. DNB System ---------- The repaired arc source was leak checked successfully and mounted on the system. Testing of the arc power supply into a dummy load was begun. Diagnostic Systems ------------------- Initial thermocouple measurements of the temperature of the MSE thermal shield, the MSE optics canister, and the lens mount for the MSE lens that is most affected by thermal birefringence indicate that the temperature slew rate has been decreased significantly compared to previous experience. Qualitatively, this suggests that the thermal shield is functioning properly. A more definitive analysis will be performed when the complete suite of ~20 MSE thermocouples is connected to the C-Mod data analysis system this week. Travel and Visitors -------------------- Manfred Bitter and Ken Hill from PPPL visited July 22 and 23 to discuss replacement of 2 Pilatus detectors in the HIREX Sr spectrometer and construction of a prototype velocity fluctuation diagnostic. Rich Hawryluk, Randy Wilson and Chuck Kessel visited the PSFC Wednesday for discussions of the engineering of the advanced outer divertor upgrade. The discussions related to the personnel requirements and the timescales for its design and manufacture. The PPPL staff will have discussions in the next few weeks about the best ways for them to participate, and the MIT personnel will work towards a set of goals needed to be achieved leading up to the Conceptual Design Review. While the PPPL personnel were at MIT, an audio conference call was held with engineering staff at PPPL to review the thermal calculations being done there. It was found that the addition of a dome between the inner and outer divertors had a very positive effect on reducing temperatures at the inner divertor. Future work will include higher temperatures as well as heating more of the outer divertor. Paul Bonoli and Robert Granetz attended the APS-DPP sorters meeting on Thursday and Friday at the APS headquarters in Maryland. Paul Bonoli is on the MFE theory sub-committee and Robert Granetz is chair of the MFE experiment sub-committee. Stewart Zweben (PPPL) was at C-Mod last week to work on the diagnostic check-out of the upgraded Gas Puff Imaging (GPI) system. Good images of the background Dalpha light were seen using the new 'turbo' mode in both Phantom 7.3 cameras at 250,000 frames/sec at an exposure time of 1 microsec/frame. The motion of the turbulence in the low-level recycling light could be seen even without a GPI gas puff in both the midplane and X-region GPI cameras. Stewart also finished an MP proposal "GPI measurements of SOL turbulence and comparisons to the SOL width". _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly