Alcator C-Mod Weekly Highlights October 14, 2008 FY2009 weeks of research operations: Target: TBD Completed: 0 weeks Physics ------- Further analysis has been done on data obtained in experiments performed last April investigating the effects of LHCD on subsequent H-mode evolution. This effort was led by George Sips, IPP Garching, as part of an ITPA Joint Experiment in the SSO (now Integrated Operation Scenarios) group aimed at comparisons of the "hybrid" scenario. In these experiments, 800 kW of LHCD was injected into the current ramp phase of 600 kA discharges, and reproducibly delayed sawtooth onset was observed until well into the flat top, indicating significantly modified j(r). Three MW of ICRH was then added, at varying times in different discharges, not only triggering H-modes, but also initiating sawteeth. Stored energy was found to be transiently higher in H-modes for which the target j(r) was modified by LHCD. Recent comparisons show that in these cases the L-H transition is significantly delayed, occurring typically 60-70 ms after ICRF turn-on, in contrast to ~ 20 ms with ohmic targets. Rates of density rise are also slower. In all H-modes, the stored energy correlates well with edge temperature, suggesting that differences in pedestal evolution (higher Tped) are primarily responsible for the improved confinement. Modeling of LHCD in these discharges is in progress to assess the degree of j(r) modification. These results will be presented by Sips at the upcoming IOS ITPA meeting in Lausanne. Stewart Zweben, PPPL, has made a direct comparison of the SOL turbulence measured in C-Mod using the Gas Puff Imaging diagnostic (GPI) with the SOL turbulence calculated for these SOL conditions using the GEMR turbulence simulation model developed by Bruce Scott, IPP Garching. There is reasonably good agreement on several points, e.g. the correlation lengths and shape of the frequency spectra, but less good agreement on the fluctuation levels and k-poloidal spectra at low k. Stewart is also working on new analysis in this area and on hardware upgrades for the GPI diagnostic. Analysis of Reversed Shear Alfven Eigenmodes (RSAEs) data is being used to follow the evolution of the current profiles during the sawtoothing phase of Alcator C-Mod plasmas. Two distinct groups of RSAEs are observed between sawteeth crashes, the standard up-chirping group, and a less common down-chirping group. Investigation of these branches with the ideal MHD code NOVA is being used to put limits on the q-profile immediately before and after the sawtooth crash. We find from the up-chirping branch that a reversed shear q-profile must exist immediately prior to the sawtooth crash, and that from the down-chirping branch a q-profile with a local maximum must be present. These results are consistent with a Kadomtsev-like magnetic reconnection model. Operations ---------- All TF arm feltmetal soldering has now been completed. Preparations are being made to begin work on the TF vertical legs. We continued to fabricate new insulation for the back of the TF arms that will better protect the magnet from ground faults. Cleaning of the TF column sliding joint fingers continued. New inner wall heaters arrived in-house last week. These new heaters have embedded thermocouples that will allow better control of the machine temperature during both normal operation and high temperature bake. We continued to upgrade the PC to PLC control software for major engineering systems. Lower Hybrid System ------------------- Low power rf testing of the copper plated 4-way splitter have been completed successfully. We continue to prepare for high power testing with the addition of several new thermocouples to the test stand. ICRF Systems ------------ We continued work on the new Intermediate Power Amplifiers (IPA). Front panel wiring is complete as are the rf coaxial connections. All remaining wiring from the old IPA installation needed for the new installation has been refurbished and secured. Diagnostics ----------- A realistic velocity distribution function for fast-wave heating of minority ions was added to a simulation of the emission spectrum observed by the fast ion charge exchange diagnostic by Bill Rowan and Igor Bespamyatnov, UT-FRC. An IDL program was developed to evaluate an analytical representation for the velocity distribution function and bench-marked against two examples in the literature. The simulation will be used for S/N estimates of this new diagnostic. Perry Phillips, UT-FRC, has continued assembly of the new shielded box for the FRCECE system. Fabrication of a heat sink for the low frequency unit is also underway. This heat sink will allow the use of additional magnetic shielding. The Polarimetry Diagnostic FIR lasers are back in operation on the new optical table. The new gas control system for the lasers is being used. The lasers are being carefully aligned to produce coaxial beams over a several meter path-length. Travel and Visits ----------------- Stewart Zweben, PPPL, visited MIT last week to continue analysis of GPI data. Jim Zaks traveled to Chicago, Ill last week to discuss production of vessel heater components with our vendor. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly