Alcator C-Mod Weekly Highlights April 30, 2012 FY2012 weeks of research operations Target: 17 weeks Completed: 5.6 weeks Plasma Shots: 759 Operations ----------- Plasma operations resumed at Alcator C-Mod last week, following a planned vent. High power testing of the C-Mod magnet systems was begun on Thursday and completed on Friday morning, following which plasma startup was successfully achieved. By the end of the day we had obtained a series of three consecutive full length (over 1.7 sec) ohmic discharges, including two at 600kA and one at 800kA plasma current. Plasma operations for machine conditioning and diagnostic checkout are planned to continue this week. Quarterly Review ---------------- The C-Mod Quarterly Review for the 2nd quarter of FY2012 was held on Wednesday, April 25, via video conference. Presentations from MIT were made by Jim Irby, Martin Greenwald, Bruce Lipschultz, Ralph Kube, Catherine Fiore, Darin Ernst, and Amanda Hubbard. DoE participants included Mark Foster, Jim VanDam, John Mandrekas, and Nirmol Podder. Viewgraphs from the presentations can be found at http://www.psfc.mit.edu/research/alcator/pubs/Q_Reviews/Q-rev_April_2012/Q-rev_April_2012.pdf Physics ------- A new model (FM3 model) has recently been developed by W.Fundamenski, et al, (to be published in Nuclear Fusion, June 2012) aiming to make a physics based prediction on H-mode threshold power. This model has been tested in the assembled C-Mod L-H transition database, using the well characterized local plasma conditions near the magnetic separatrix as suggested by the model. The model was first 'calibrated' against the C-Mod data with normal field (5.4T) by setting two adjustable numerical parameters to reproduce the experimental trend of Pth with plasma density. Then, keeping the calibrated numerical coefficients, the model has successfully recovered the trend in a lower magnetic field of 3.5T, as well as the divertor geometry effect observed from recent C-Mod experiments. As an important implication, the model indicates that the local minimum of Pth appears at the density where the SOL-divertor transport transitions from the sheath-limited to the conduction-limited regime. This prediction was also seen to be consistent with C-Mod experiments. Geodesic Acoustic Mode (GAM) oscillations have been observed in C-Mod I-mode discharges using the Gas Puff Imaging (GPI) diagnostic. A time delay based routine has been developed to derive velocity fluctuations from the GPI APD array data. The poloidal velocities at the radial location of the weakly coherent mode (WCM) in I-mode discharges exhibit an oscillation at the frequency characteristic of geodesic-acoustic modes (GAM). Velocities along a flux surface are all in phase (within the 4.4 cm GPI view), as is expected for GAMs. Bispectral analysis of the GPI brightness- and velocity-fluctuation signals show that these GAM oscillations are strongly coupled with the WCM and cause significant energy redistribution within the mode. The GAMs are observed in a large number of C-Mod shots in the frequency range 10-40kHz approximately doubling the highest frequency of GAMs recorded on other tokamaks. The vast majority of these discharges are in the I-mode regime and the time history of the GAM power follows that of the WCM. Tuesday April 24, Darin Ernst presented a talk at the MIT PSFC Turbulence Simulation User's Group meeting entitled, "Nonlinear upshfit of the TEM critical density gradient: Experiments and Simulation." This was an extension of a plenary talk presented at the US TTF meeting on April 10. A detailed gyrokinetic study of the parametric scaling of the nonlinear upshift of the TEM critical density gradient revealed a strong increase with collisionality, saturating at seven times the linear critical density gradient. Pulsed on-axis heating in Alcator C-Mod ITB's was used to drive strong density fluctuations, consistent with TEM turbulence in the ITB. Core electron temperature modulations as large as 50% were produced. The strong temperature scaling of the nonlinear TEM threshold, together with gyroBohm scaling of the expected transport, could account for the observed bursts of PCI fluctuations. A full suite of edge measurements reveals that the edge fluctuations are out of phase with the bursts observed on PCI, which closely follow sawtooth modulations of core electron temperature. A new limit cycle stability diagram shows that the density gradient during on-axis heating appears to be clamped by the new nonlinear TEM critical density gradient, rather than the linear threshold. Finally, the parallel mode structure of "double peaked" edge TEMs, found with steep density gradients, was derived analytically. ICRF Systems ------------ The H-port SOL reflectometer external waveguide run was assembled and found to require modification of two lengths of waveguide. Steve Wukitch discussed potential ICRF collaborations with the JET ICRF team last week, and Yijun Lin had similar discussions with the K-STAR ICRF team. Lower Hybrid System ------------------- Two klystrons were pulsed for 5 sec at 30 kV and 20 kW to test the asymptotic behavior of the downstream collector coolant temperature. The rise in coolant temperature agrees well with the Collector Over-Temperature System (COTS) model assuming an aggregate heat capacity of 35 kJ/K. A full CAD model of the LH3 8-way splitter prototype was produced. The model includes WR187 to reduced height waveguide tapers on the input and all 8 outputs for testing at high power. Diagnostics ----------- One of the Thomson lasers which had been operating at reduced pulse energy was repaired on Wednesday, under coverage of vendor warranty. A representative from the vendor replaced a damaged oscillator head, and successfully returned the unit to full power operation. Data acquisition for the Two Color Interferometer (TCI) system was successfully upgraded from CAMAC to cPCI, supporting both density profile and fast fluctuation measurements (FTCI). Ten CO2 and four HeNe channels are presently digitized at up to 5MHz. The new data acquisition configuration was operational for the first plasma operation. Travel and Visitors -------------------- Istvan Cziegler (UCSD) visited MIT last week. He made a presentation to the C-Mod Science meeting on observations of GAM oscillations in I-mode discharges. During his visit, he also worked with Steve Wukitch and Jim Terry to finalize their paper on the broadening of the sheath recitification region in the SOL in the presence of ICRF heating. The paper will be submitted to PPCF. They also reviewed an MP for the follow-up work for this paper. Earl Marmar traveled to PPPL to represent Miklos Porkolab at a meeting of Lab and Program Directors, which was held on Thursday and Friday. At the invitation of Korea Institute for Basic Science, Miklos Porkolab participated in the First Symposium of the Institute of Basic Science, Daejeon, Korea, where he presented an invited lecture, "Transport and gyrokinetic analysis in ohmic plasmas in Alcator C-Mod". He also visited K-Star where he discussed potential collaborative research, and also gave a seminar: "Alcator C-Mod Research Highlights and Opportunities". Ron Parker traveled to Princeton last week, where he chaired the meeting of the PPPL Advisory Committee. On Thursday members of the KSTAR and C-Mod research teams met via video conference to discuss possible areas of collaboration. The Korean delegation was led by J.S. Ko, a former C-Mod graduate student. The C-Mod delegation was led by B. Lipschultz. Present from the C-Mod team were J. Rice, A. White, S. Shiraiwa, G. Wallace, D. Ernst, P. Bonoli, and Y. Lin. Common areas of interest in transport/turbulence, LHCD, ICRF, and edge/divertor/PMI were identified. A list of interested parties from both institutions for each topic was created and circulated following the meeting. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly