Alcator C-Mod Weekly Highlights November 17, 2008 FY2009 weeks of research operations: Target: TBD Completed: 0 weeks Physics ------- Lower hybrid waves, when applied to EDA H-mode plasmas, have been observed to promote reduced plasma density and increased temperature, through regulation of the edge pedestal. Jerry Hughes has begun examining the temporal response at the edge in H-modes with applied LH power, to better understand the mechanisms that are governing the pedestal modification. Edge and core phenomena are seen to occur on varying time scales, perhaps indicating an evolution in the deposition location and/or damping mechanisms of the LH waves. Prompt increases in Ly-alpha brightness and electron density are observed in the scrape-off layer when LH is applied, with the drop in both pedestal and core densities occurring on approximately 100 ms time scales. Concurrent changes in the density fluctuations measured by phase-contrast imaging are also observed, pointing to an enhancement of existing radial particle transport, which allows the relaxation of the pedestal density gradient. Previously reported core toroidal flow modification occurs following this pedestal relaxation. Jim Terry has been investigating if blob/filaments reach the divertor target. The filaments are the turbulent structures present in the far-SOL and are primarily responsible for the radial particle transport there. Previous analysis of data from Gas-Puff-Imaging (GPI) at the outboard mid-plane, and outboard of the lower x-point region, has shown that filaments map magnetically between the two regions. This result was predicted by theory. In doing so, the filaments are elongated and sheared in cross-section in the x-point regions. The new work compared the statistics of the SOL fluctuations at the mid-plane with those at the divertor target. Since the filaments dominate the density fluctuations in the main chamber and show strong intermittency there, similar statistic signatures at the divertor targets would indicate the connection of the filaments all the way to the targets. Upon examination, the statistical signatures were quite different, implying that the filaments may be "disconnected" from the targets, a phenomena described in the theoretical work on filaments by Myra and D'Ippolito. The elongation and shearing of the filaments through the x-point region plays a key role in the disconnection. The influence that magnetic shear and perpendicular plasma flow shear may have on controlling edge pressure gradients near the last-closed flux surface (LCFS) has been investigated by Brian LaBombard. In ohmic L-mode discharges, where magnetic topology was varied systematically from double-null to inner-wall limited, scrape-off layer plasma temperature and pressure profiles are found remarkably invariant. Normalized pressure gradients, as characterized by the MHD ballooning parameter, alpha_mhd, remain persistently in the range of ~0.5 at a location 1 mm outside the LCFS, regardless of whether the LCFS is defined by the inner-wall (low magnetic shear case) or by a magnetic separatrix (high magnetic shear case). A key piece of the puzzle appears to be perpendicular plasma flow shear just outside the LCFS independent of magnetic topology, it is very strong, comparable to the ideal ballooning growth rate; the physics of flow shear may dominate over magnetic shear in these discharges. In discharges where the x-point topology was varied from lower-null to upper-null, edge pressure gradients show a systematic decrease. Coincidently, perpendicular plasma flow shear decreases, with reduced shear associated with reduced alpha_mhd values. Thus it appears that perpendicular flows play a dominant role in all these discharges, impacting the pressure gradients (alpha_mhd) that can be attained near the LCFS, and differentiating between the physics of the near scrape-off layer region (steep gradients) and that of the far scrape-off layer region (flattened profiles with ballistic, blob-like transport). Operations ---------- All TF arm and leg feltmetal soldering has been completed. The central column TF finger plating has been successfully completed. This work started on Monday and was finished on Friday. Removal of the masking that protected un-plated parts of the TF fingers and insulation system was begun on Saturday and should be completed later this week. Work continued on fabrication of the spring-plate packs that provide pressure across the finger joints. All heaters for the inner cylinder have been installed, cured, and tested. Lower Hybrid System ------------------- Work continued on fabrication of the 4-way splitters, waveguides and vacuum windows, rf probes and electronics needed for the 2nd lower hybrid launcher. ICRF Systems ------------ Upgrades to the ICRF programmable logic control system PC software continued last week. The ICRF system will become part of the control system network and much better trending of system parameters will be possible. Simulation of the new 4-strap antenna and coupling loops continued. Travel and Visits ----------------- Last Wednesday, Scott Rau, from Structural Integrity (SI) Associates, came for a site visit of our generator, made a brief presentation on the analysis of rotor NDE data, and participated in a detailed question and answer session. SI is one of the companies being considered for possible analysis of NDE data from our generator rotor. Dave Terry was in California last week at CPI inspecting klystrons before shipment back to MIT, and discussing the status of the new klystrons and ICRF final amplifier tubes being fabricated. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly