Alcator C-Mod Weekly Highlights October 10, 2011 FY2012 weeks of research operations Target: TBD Completed: 0 weeks Plasma Shots: 0 Research -------- The Fusion Energy Sciences FY11 Joint Research Target in Pedestal Physics has been completed and the final report, which includes contributions from more than 60 individuals representing 15 institutions, is being submitted to FES this week. The goal of this combined experiment/theory milestone was to improve our knowledge of the physics processes that control the H-mode pedestal by applying models of these mechanisms to experimental data. C-Mod made substantial contributions to the JRT, having devoted more than 16 run days in FY2011 to pedestal studies in both H-modes and I-modes. Profile and equilibrium data from these and earlier experiments have been analyzed and processed into standard formats that were agreed upon by the JRT collaborators early in FY11. This has enabled numerical simulations of pedestals in C-Mod discharges with a number of codes, including ELITE, BOUT++, M3D, SOLT, 2DX and XGC0. The JRT provided excellent motivation for enhanced collaboration between experimentalists and modelers, and opened pathways for this collaboration to proceed quickly. Initial ELITE calculations (P. Snyder, GA) show that C-Mod ELMy H-modes operate near stability limits for ideal peeling-ballooning modes, as expected from the current understanding of Type I ELMs. Peeling-ballooning stability was also calculated for "identity" discharges run on DIII-D to match the C-Mod ELMy H-mode shape and dimensionless parameters atop the pedestal. Stability diagrams, and proximity of the experimental data to the peeling-ballooning boundary, were quite similar in these cases. EDA H-mode discharges, on the other hand, are found ideal MHD stable by both ELITE and BOUT++ (X. Xu, LLNL, E. Davis, MIT). However, BOUT++ does identify finite growth rates for edge modes with toroidal mode number n less than 30 when realistic values of resistivity and diamagnetism are included. An investigation of the full non-linear dynamics in BOUT++ is underway. Additional work to simulate the edge modes responsible for the quasi-coherent fluctuations in EDA H-mode are underway with M3D (L. Sugiyama, MIT) and codes developed by Lodestar: SOLT and 2DX (D. Russell, J. Myra). Pedestal width scalings predicted by neoclassical simulation with a Monte Carlo neutral model in XGC0 (C.S. Chang, NYU) systematically underestimate the pedestal width, and are used to identify the amount of addition anomalous transport that is needed to match the experimental pedestal profiles. In our JRT studies, we have provided additional supporting evidence for pedestal width (in ELMy H-mode) scaling as the square root of poloidal beta at the pedestal top, a result that has now been confirmed on a number of devices. This is the dependence that would be expected from theory if kinetic ballooning modes (KBMs) were responsible for limiting the pedestal width. Since both the assumed pedestal height (PBM-limited) and width (KBM-limited) constraints in the EPED model appear consistent with experiment, the EPED model predicts C-Mod pedestal height and width reasonably well (within 20%); these results increase confidence in the applicability of EPED to a wide spectrum of tokamaks, including ITER. Significant progress has been made on the accelerator-based in-situ plasma-facing component diagnostic for Alcator C-Mod (AGNOSTIC). In addition to magnetic beam steering and particle transport simulations, we have brought our data acquisition online, begun assembling detectors, and taken our first experimental data on the benchtop. Engineering for B-port radial flange in support of AGNOSTIC installation in fall'11/spring'12 is complete and waiting for completion of upgrades to the accelerator. Jim Terry presented results from analyses of C-Mod's heat-flux profiles on the outer divertor. The profiles are measured using the IR thermography system. Profiles from experiments that were run as part of the 2010 JRT have been re-analysed using a new fitting scheme suggested and used by scientists at JET. Scientists at DIII-D are also re-fitting heat-flux profiles to the new scheme, continuing the collaboration that began with the 2010 JRT. The results show that the variations in the C-Mod profiles occur in the characteristic profile decay length into the common-flux region, consistent with previously published observations, while the decay length into the private-flux region appears to be relatively constant. In addition, characteristic widths of I-mode confinement shots were analyzed for the first time and compared with those occurring in discharges with EDA H-mode confinement. The initial result is that the widths in I-mode are similar to or somewhat larger than widths in EDA at similar plasma currents and stored energies. Operations ----------- Preparations for resumption of C-Mod operations continued this week. Maintenance work is underway on the commutation switch capacitor banks, including inspection of all electrical connections, capacitors, and support structure. Testing of the new data transmitter and receiver boards in the EF1 commutation switch control cabinet was successfully completed. The backplates for the new ARRA-funded rotated ICRH antenna have been successfully re-machined. We have assembled the back and center plates with the rest of the antenna (except the antenna straps) and evaluated the assembly. One shim required for the mating surface between the back and center plates has been specified based on the results of the evaluation, and is in fabrication. The antenna has been disassembled and the back and center plates are being cleaned and baked. The outer wall of the vessel has been cleared for installation of the antenna. Two magnetic field probes were removed from the installation site and replacements installed at a different toroidal location. The feedthroughs have been assembled and are leak tight. The flange installation fixture has been modified and is ready for installation. In-vessel activities last week included accurate positioning measurements of the Correlation ECE (CECE) and Fast Ion Loss Detector diagnostics, replacement of the polarimeter protection tile and installation of the inner-wall polarimeter retro-reflector assembly, securing of fiber and wiring conduits for several diagnostics, installation of the MSE heat shield, and installation of protection tiles for the inner-wall periscope at K-port. We successfully performed tests of the MIT alternator emergency diesel generator start procedure and the alternator emergency lube oil and seal oil procedures. The alternator was then run to speed 1 (500 RPM) and some preheating of the field coil was performed. Additional alternator testing is planned for this week. Tests on the transformer proposed for use in the EF2 power supply upgrade were successfully completed at our vendor's site. Preparations to return the transformer to MIT and install it in the C-Mod power room are in progress. ICRF System ----------- We have successfully operated the FMIT#1, #2, and #4 transmitters into dummy loads up to 2MW. FMIT#3 testing has been delayed pending replacement of an ignitron in the driver stage. The in-vessel waveguide for the new SOL reflectometer has been installed. Lower Hybrid System -------------------- All transmitter protection system (TPS) boards have been completed by our SBIR vendor. Firmware on the TPS boards was successfully updated. The source of TPS board communication issues observed during testing was identified. Interrupts under certain conditions result in a reboot of the master CPU. The SBIR vendor has replicated the issue and is working on a solution. Removal of old TPS cabling on carts 1 and 3 is in progress. The old cabling was tagged and stored after removal. The old TPS chassis were removed and re-wiring of the equipment racks is under way. New wiring for solenoid power supplies on carts 1 and 3 is complete. New cabling has been connected at carts 1 and 3 for waveguide switched, arc detectors, and press switches. HV control wiring is complete in cart 1 and in progress in cart 3. Travel, Visitors, Remote Participation -------------------------------------- Jerry Hughes represented MIT at the 21st ITPA Pedestal Topical Group meeting in York, UK. He presented, "Assessing pedestal structure and stability: Progress in three confinement regimes on Alcator C-Mod." Also at the ITPA meeting, he led a discussion of a number of joint experiments in which C-Mod participates, and also a discussion on status and plans for the ITPA L-H Transition Working Group. Amanda Hubbard and Yunxing Ma also participated in the meeting using remote conferencing tools. On Thursday, Oct 6, Amanda Hubbard, Steve Wolfe, and Dennis Whyte participated remotely in a briefing session organized by the BPO for the US ITER STAC members in preparation for the upcoming STAC-11 meeting. Wolfe, along with Mickey Wade (GA) and Michael Bell (PPPL), helped prepare a briefing on the potential impact of deferrals proposed by the IO on the ITER Research Program Plan. John Rice attended the 7th Meeting of the Transport and Confinement ITPA Group, 5-7 October, 2011, in Cadarache, France. Bill Rowan presented a talk on "Boron Impurity, Helium Impurity, and Helium Main Ion Profile Measurements in C-Mod" at the the TC Group meeting as a remote presentation. Igor Bespamyatnov presented "Core-CXRS system on Alcator C-Mod and DNB simulation by ALCBEAM code" at the 2011 ADAS Workshop 6-8 Oct 2011, Auburn, AL. Robert Granetz attended the ITPA MHD meeting in Padova, Italy (site of the RFX fusion project). At this meeting, the agenda was particularly focused on disruption mitigation, including runaways. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly