Alcator C-Mod Weekly Highlights July 14, 2008 FY2008 weeks of research operations: Target: 15 weeks Completed: 15.7 weeks Physics ------- Regression analysis being developed by Dennis Whyte of the L-H power threshold in enhanced L-mode shows a significant dependence on elongation and triangularity. This result points to the possibility to extend these results to higher input power and absolute performance by avoiding a sudden L-H transition in enhanced L-mode with increased shaping. Analysis of time-evolving, toroidally resolved emission data from new AXUV arrays during a mitigated disruption is proceeding. These data show emission local to the gas jet interacting with the edge plasma and spreading over the plasma surface at neutral transport speeds prior to the start of the thermal quench. Additionally, radiative power loss from the bulk plasma increases, lagging the loss from the gas jet. Both are thought to contribute to the energy loss that drops the edge temperature and destabilizes the MHD responsible for the thermal quench. During his visit last month to Tore Supra, Orso Meneghini, as part of his MIT graduate work, worked with Julien Hillairet, CEA ,Damien Vojer, ECL, Marc Goniche, CEA, and Roberto Bilato, IPP, to compare the lower hybrid coupling codes TOPLHA and ALOHA. When simulating the same geometry and plasma conditions the two codes compared remarkably well. Orso also discussed the lower hybrid induced MHD activity seen on some C-Mod discharges with Girardo Giruzzi, CEA, and received the Tore Supra perspective on these results. Operations ---------- Disassembly of C-Mod continues. All spring-plates have been removed as have all TF vertical legs. Removal of the upper TF arms is underway. The location of an intermittent ground fault experienced during the FY2007 and FY2008 campaigns has been found on a lower arm, outer finger. We will know more about this issue when the lower arm is removed from the machine and the insulation around the TF plate can be removed. We continued work in preparation for the alternator inspection. The grating around the 2000 HP motor was removed, and the control wiring from the wiring enclosure was disconnected and removed. The drive coupling brushes and associated wiring have also been removed. City water piping that supplies the alternator hot water system has been disconnected and capped off. ICRF Systems ------------ The new Intermediate Power Amplifiers (IPA) have been shipped. We continue to specify cooling and wiring components for the new IPA system. Lower Hybrid System ------------------- Phase and amplitude calibration checks of the 1st lower hybrid launcher continued last week. Results are generally in-line with the calibration data taken before the start of the FY2008 campaign. Diagnostics ----------- Perry Phillips, UT-FRC, continued repair work on the FRC-ECE RF units. The high frequency RF stage is still not operational, and an alternative source for this equipment has been found. Evaluation of FIR detectors for the C-Mod polarimeter is underway. Two detectors from two vendors are currently under test, with detectors from a third vendor expected next week. Work continues on procurement of parts for a feedback control system for the FIR laser fill gas. Travel and Visitors ------------------- Several visitors were at MIT last week from PPPL. Stewart Zweben visited to continue analysis of the GPI turbulence data and to help with GPI hardware upgrades. Mark Smith visited to discuss the upgrades to the MSE diagnostic needed to eliminate stress-induced birefringence, and Dennis Mansfield visited to discuss design, FIR laser, and detector issues involving the new C-Mod polarimeter. Dennis also discussed recent lithium dust injection experiments on NSTX. Steve Knowlton, Auburn University, visited last week to discuss the new lower hybrid launcher. Paul Bonoli hosted a visit of Dr. John Cary (Tech-X Corporation and the University of Colorado) at the PSFC on July 9, 2008. John had physics discussions with Paul Bonoli, Martin Greenwald, Miklos Porkolab, and Abhay Ram concerning high performance computing and simulation. Bruce Lipschultz attended the ITPA Coordinating Committee meeting held in Aix en Provence, France for 2 days, June 30-July 1. He represented the ITPA SOL/Divertor group, reporting on the activities of the past year. The meeting also involved discussions of the new tasks and goals of the different ITPA groups. He also visited the ITER staff at Cadarache on July 2 and had discussions on a number of topics including ITER diagnostics and D retention studies. Finally he gave a seminar about Alcator C-Mod to the ITER and Tore Supra staffs. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly