Alcator C-Mod Weekly Highlights June 5, 2006 FY2006 weeks of research operations: Planned: 14 weeks Completed: 12.7 weeks With the completion of the reversed-field portion of the experimental campaign on Friday, C-Mod has now surpassed 90% of our planned 14 weeks of Research Operations in Fiscal Year 2006. We anticipate that we will meet or exceed the planned 14 research weeks by the end of FY06. Last week was the final week of reversed field operation for this campaign, and this week has been scheduled for maintenance. The magnet current bus will be returned to the forward field direction, and the FMIT#3 and #4 ICRF transmitters will be tuned for 50 MHz operation. Operations ---------- Plasma operations continued at Alcator C-Mod last week with three run days scheduled and 2.8 completed. The loss of a instrumentation control power breaker for the ICRF system caused us to loose some time on Friday. The breaker panel will be replaced this week. On Wednesday MP409 on Sol Phase Space Studies was continued. On Thursday MP453: A Study of QC Mode with Unfavorable (for H-mode) Grad B Drift was run for the first time. And on Friday MP452: Reversed Field Pedestal Studies was begun. Also on Friday, more time was given to MP409, MP404a: Parametric Dependence of Moderate n Alfvén Eigenmode Damping Rates, as well as for commissioning of the new lower hybrid launcher protection instrumentation. A full boronization of the machine was carried out successfully on Wednesday evening in preparation for the QC Mode work. Run summaries ------------- On Wednesday we were able to collect edge plasma profile and flow data in matched LSN/USN discharges under the reversed field conditions as part of MP409. These data will be key to sorting out effects of topology on scrape-off layer flows and related core flows. The significance of the profile and flow results will require a detailed analysis, but based on initial results it is clear that the SOL flows are behaving in a symmetrical way with respect to B-field reversal and x-point location. Core plasma rotation is also exhibiting a nearly symmetrical swap. These results indicate that differences in upper/lower divertor geometries and/or small differences in USN/LSN flux surface shapes are not of great importance. Thursday was devoted to the study of Quasi-Coherent Modes (QCM) with unfavorable Grad B drift. The QCM amplitude was observed (by the PCI) to decrease to zero as it approached the X-point (in this case the upper X-pt). The radial distance from the X-point to the radius at which it disappeared is 8-12 cm. This "disappearance point" was observed to move (inward), tracking the radial position of the X-pt as it was moved inward. We caution that if k_radial is increasing to >/~10 this could also cause an apparent decrease in the amplitude because of the finite wavenumber resolution of the diagnostic (further analysis will be done). For the QCMs obtained in USN the dominant k_theta (near the outboard mid-plane) was about 2 cm-1. The propagation direction (down at the outboard mid-plane) is as expected, since this is equivalent to the normal-field LSN case for which the propagation direction was up. Interestingly, in addition to the strong feature at k_theta~-2 cm-1 and f~100-150 kHz, there were weaker features at the QCM frequency with k_theta= 3, 5, and 6.5 cm-1. We do not yet understand the origin of these k_theta>0 features. In the reversed-field USN configuration, ELMs and the QCM appeared in plasmas with upper triangularity >0.72 and lowish pedestal collisionality. ELMs were detected on the fast magnetics and fast-diodes. The short ELMing period was followed by a QCM whose frequency was observed to sweep down after onset. Friday's run began with MP452 to acquire and characterize H-modes and pedestals with the direction of field and current reversed, in order to make a comparison with similar discharges in the normal configuration. The plan was to run a range of currents and densities in this configuration, tracking pedestal parameters (both dimensional and dimensionless), and confinement and edge fluctuations. In particular, we sought the quasi-coherent mode, and the possible appearance of ELMs. Also of interest was comparing local L-H threshold conditions with those in the forward-field case. Before an ICRF breaker fault ended work on this miniproposal, a density scan at 0.8MA, 5.4T was begun. Programmed target line integrated densities for these shots were in the range of 0.8 to 1.3e20 m^-2, and there was generally a fueling enhancement above this whenever RF power was applied to the discharge. Te,95 at the transitions was in the neighborhood of 400 eV, with grad-Te of 50--100 keV/m. The edge temperature increase after the L-H transition was substantially reduced as the density was raised, and the H-mode Te,95 was barely above 400--500 eV. This type of discharge had a relatively collisional edge (nu*>~2), and exhibited ELMy or dithering features, which were more prevalent at lower Te. It is possible that these were Type III or similar ELMs. Less collisional (nu*<~1) discharges showed signature of a weak quasi-coherent mode. There is insufficient data to firmly establish either an upper or a lower density limit in these discharge conditions. Discharges were also run in support of MP#409 on Friday, completing the density scan started on May 31 (between 1.0e20 and 1.4e20 /m^2 for both LSN and USN). The A-port fast scanning probe ran well, and this new data in combination with that from the previous run should give a more complete picture of how the edge flows and profiles behave in reversed field. Progress was also made on MP404a: Parametric Dependence of Moderate n Alfvén Eigenmode Damping Rates. During an earlier run, the Active MHD system was used to drive sustained stable Alfvén eigenmodes at q=1 and q=1.5, and higher damping rates were observed at the lower q value. The dependence of damping rate on B X Grad(B) into and out of the divertor was also studied and no difference was found. The RF power was also scanned to study the effect on damping rate. Preliminary analysis suggests that the damping rate increases up to about 2 MW and then decreases for higher RF powers, though plasma density and hydrogen minority fraction may also influence the results. Diagnostics ----------- The Active MHD amplifier was upgraded last week with a more sophisticated tuning routine. It now switches its tuning capacitors only at the extrema of the frequency sweep, keeping the center frequencies clear of glitches. Leonid Dorf from the LANL P-24 Plasma Physics group and Jim Terry worked last week to troubleshoot and upgrade a new infrared spectrometer diagnostic installed at A-Top. A new alignment procedure using a heated molybdenum tile was implemented. An infrared lens focusing the light onto the entrance slit of the spectrometer was installed. Additional measures to increase the signal level are still required. With the help from Josh Stillerman, the software integration is complete and the data acquired using the LabView program is now stored automatically to the MDSplus tree. Lower Hybrid System ------------------- On Thursday and Friday the lower hybrid system was operated into plasmas while tests of the new interlock system continued. The data taken will allow trip levels to be set more precisely. Several hardware issues discovered during the tests will be resolved during the maintenance week. Travel and Visitors ------------------- Stewart Zweben, PPPL, was at MIT this week to continue analysis of edge turbulence data during wall limited plasma operation. The analysis this week focused on the correlation lengths in the fast diode data. He has also been discussing the simulation of this experiment with theorists. Miklos Porkolab attended the FESAC meeting on June 1, in Gaithersburg, Md. Martin Greenwald traveled to Washington to attend a meeting of the Fusion Energy Sciences Advisory Committee. As reported last week C-Mod had a very active participation in the PSI meeting in Hefei, China. A list of talks and posters presented includes: LaBombard (talk)- 'Operational phase space of the edge plasmas in Alcator C-Mod' Lipschultz (invited talk)- 'Operation of Alcator C-Mod with high-Z plasma facing components' J. Terry (talk)- 'Investigation of ELMs on Alcator C-Mod' Whyte (UW) (talk)- 'Disruption mitigation on Alcator C-Mod using high-pressure gas injection: experiments and modelling toward ITER' Wukitch (invited talk)- 'RF-Plasma Edge Interactions and Their Impact on ICRF Antenna Performance in Alcator C-Mod' Y. Lin (poster)- 'Hydrogen Control in Alcator C-Mod Walls and Plasmas' Wright (UW) (poster)- 'Dynamics of Hydrogenic Retention in Molybdenum: First Results from DIONISOS' _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly