Alcator C-Mod Weekly Highlights June 30, 2008 FY2008 weeks of research operations: Target: 15 weeks Completed: 15.7 weeks Physics ------- Dennis Whyte traveled to DIII-D last week to participate in experiments on fuel retention. The DIII-D experiments repeated the fuel retention measurement technique started on C-Mod, where all active pumping is closed off during the plasma pulse and for 5-10 minutes afterwards; this results in very high accuracy measurements of the retained fuel in the vessel walls. Surprisingly, the DIII-D ohmic discharges exhibited nearly identical retention to that seen on C-Mod: ~1% of incident D ion fluence to the wall was retained repetitively on each shot, without any sign of the wall saturating. This result is contradictory to simple expectations of the difference between the C-Mod metal molybdenum wall and the DIII-D graphite wall. Analysis of C-Mod flow drive experiments using detailed full-wave simulations suggest that the ion heating from the mode converted wave may be important in driving the observed poloidal flow. Turbulence frequency as measured by the phase contrast imaging system was broadened at the time of strongest flow, indicating a change in the bulk ion flow also. Perry Phillips, UT-FRC, has analyzed ECE data from core channels, Rmid ~ 0.74 m, that show broadband fluctuations extending up to 600 kHz. These fluctuations have short radial correlation lengths of ~1 cm, and occur early in the formation of Internal Trasport Barriers. Fluctuations later in the ITB could not be seen due to the high densities and the resulting cut-off of the ECE signals. Operations ---------- Removal of all PF coaxes and curved bus was completed last week. The TF coax and first-to-last turn plates were also removed and found to be in very good condition. The bus tunnel has been pulled back away from the retaining cylinder. A total of 96 draw-bars and pre-tensioned nuts hold the C-Mod domes to the retaining cylinder. The upper 48 nuts were loosened last week, the torque required to do so documented, and the top dome was prepared for removal. On Friday the dome was removed exposing the the upper TF arms and the top of the TF core. Extensive documentation of this area is underway. ICRF Systems ------------ Work continued on the new 10 kW solid state IPA last week. Transmission line components are being selected and procured. Control and protection systems are being reviewed. Lower Hybrid System ------------------- Work continued on klystron filament supply upgrades, improvements to the VacIon controllers, and procurement of components for the 4th lower hybrid cart. Analysis of the new launcher 4-way splitter phase and power balance as a function of reflected power at the output ports is making progress. A prototype 4-way splitter is nearly complete and plans are being made to test it at high power. Diagnostics ----------- Work continued on a new DNB high voltage power supply invertor module test-stand. The transformer for the stand is being designed, and an implementation similar to the lower hybrid power supply module test-stand is being considered. An optical bench is being developed to test retro-reflectors to be used in the polarimeter diagnostic. Both mechanically assembled and electro-formed retro-reflectors are being fabricated and will be tested on this bench. A PLC controlled gas flow system is being developed for the polarimeter FIR laser system. This system should allow the FIR lasers to run indefinitely without purging the FIR cavity fill gas. The polarimeter PLC also now maintains the FIR laser at line center. Travel and Visitors ------------------- Joshua Stillerman and Thomas Fredian held meetings at Consorzio RFX in Padova Italy on future MDSplus development. The main topics discussed were object based APIs for data access and event based data acquisition for long pulse experiments. They also attended the EFDA remote participation workshop. Fredian presented a summary of MDSplus usage in the community. Stillerman presented an overview of US remote collaboration efforts, the ESnet RCWG (Remote Collaboration Working Group), as well as remote collaboration in use by Alcator C-Mod. David Mikkelsen, PPPL, continued his visit to MIT last week primarily working on selecting discharges for a study of density peaking in C-Mod H-mode plasmas. Shots were selected for transport analysis, to be followed by turbulence simulations, and for evaluation of the uncertainty in the ion temperature gradient. Paul Bonoli attended the Joint IEA Workshop (W68) on Development of high beta-N scenarios for ITER. It was held at General Atomics on June 24-25, 2008. The title of his talk was "ITER Relevant Physics Studies in the Lower Hybrid Range of Frequencies on Alcator C-Mod". Dennis Whyte traveled to DIII-D last week to participate in experiments on fuel retention. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly