Alcator C-Mod Weekly Highlights June 18, 2012 Fy2012 weeks of research operations Target: 18 Completed: 7.65 Plasma Shots: 1250 Operations ---------- Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and completed. A total of 100 plasma discharges were produced with a reliability of 83%. Research operations were devoted to investigation of lower hybrid current drive using the MSE and Polarimetry diagnostic systems, evaluation of the field-aligned antenna before and after boronization, and investigation of dilution effects on turbulence and transport in low density Ohmic plasma. On Wednesday night a boronization was carried out for the first time since the last up-to-air. On Thursday, the plasma impurity levels and hydrogen fraction were dramatically reduced, and good ICRF heating from all antennas was obtained. The upper divertor cryopump was successfully operated on Friday for the first time since the up-to-air. Operational Details ------------------- On Tuesday morning we continued work on MP705, "current profile measurements in plasmas with strong LHCD". This MP uses the MSE diagnostic and diagnostic neutral beam to make careful measurements of the plasma current profile changes with the application of over 800 kW of lower hybrid power. A shot-to-shot scan of increasing density in diverted plasmas was performed until current drive was reduced to below a detectable level. A limited and an almost limited shot were also taken. These two shots had the same density and LH power as the high density diverted case and allow comparisons between magnetic geometries. These data will be used to reconstruct the driven current profile and to compare to models. This work comprises part of the PhD thesis research of an MIT graduate student. On Tuesday afternoon work continued on MP704, "observation of current density profile changes in LH discharges with the polarimeter system". The best current drive case from the morning was determined and re-run for several discharges to judge reproducibility of the polarimeter signals. Several Ohmic discharges without LH were also taken for comparison. The inner two polarimeter chords showed significant reductions in rotation during the lower hybrid pulse. It was also clear from these data that changes made to the position of the polarimeter detectors relative to the focal plane of the focusing mirrors had reduced the vibrational noise levels. This process continued during the rest of the week, and by Thursday all three polarimeter chord noise levels had been significantly improved. In addition, small changes to the TF current programming improved the signal baselines and reduced the drift. This work comprises part of the PhD thesis research of an MIT graduate student. On both Wednesday and Thursday we ran MP681, "evaluate field-aligned antenna performance", to measure antenna performance both before and after boronization. On Wednesday, we compared D&E and FA-J into L-mode discharges. A current scan [600, 800, 1000 kA] and a density scan at 600 kA were completed. Two discharges were run at near-monopole phasing which resulted in strong interactions at the plasma limiter and little heating. Core radiation from impurities, when using the FA-J antenna, was much reduced compared to that when using the D&E antennas at the same power levels. On Thursday, boronization recovery went quickly for all antennas. A current scan was done while also obtaining GPI data. The differences in core radiated power in H-mode were neglible when comparing the FA-J and D&E antennas. However, the dominant radiating impurities under these conditions do not appear to be Hi-Z, such as molybdenum. The LH system was also run on Thursday, at the beginning of the day, and the results showed that the launcher coupling and power handling have not been affected by the boronization. MP696, "continued investigations of dilution effects on turbulence and transport in low density ohmic plasma", was run on Friday. Previous studies of turbulence and transport in high-density plasmas have quantitatively verified the role played by ion temperature gradient (ITG) turbulence. In these regimes, with strong electron-ion coupling, experimentally measured transport and simulation (GYRO) agree when the profile gradients are varied. In the case of the low density linear-ohmic regime, where electron transport dominates, the turbulent energy transport predicted by GYRO does not generally agree with the experimentally measured transport. In this experiment, density scans were done both with and without nitrogen seeding to see the effects of the dilution on the transport. An unseeded ohmic scan was run from nebar = 0.7 to 1.4 x 10^20 /m^3, and the seeded shots were done from nebar = 0.8 to 1.4 x 10^20 /m^3. Rotation reversals were observed after nitrogen seeding on discharges which were slightly above the transition density from the linear to the saturated Ohmic regime. These data are being analyzed. This work comprises part of the PhD thesis research of an MIT graduate student. Research -------- A new diagnostic that analyzes probe signal spectra near the lower hybrid launch frequency has measured parametric decay instabilities (PDIs) away from the LH launcher, correlated with the onset of MHD activity during the injection of LH waves. The amplitude of the sidebands was typically 20 to 30 dB below that of the pump wave. The relative amplitudes and the frequency difference suggest that the sidebands are generated at different radial locations. However, it is not yet clear how the MHD activity changes the behavior of the waves. The paper "H-mode threshold power reduction in a slot divertor configuration on the Alcator C-Mod tokamak" by Y.Ma, J.W.Hughes, A.E.Hubbard, B.Labombard, and J.Terry has been published in Plasma Physics and Controlled Fusion on June 12, 2012. H-mode power thresholds (Pth) are examined in Alcator C-Mod lower single null plasmas with standard vertical-plate and slot-type divertor configuration (BT = 5.4 T, Ip = 0.9 MA, q95 ~ 4.0 and ne_bar = 1.3-1.6 X 10^20 m-3). The total required power to access H-mode is found to be significantly reduced when the plasma is operated with a slot divertor. The lowest Pth achieved is ~0.7 MW, about 1/3 of the normal level and 40% of the multi-machine scaling law prediction. Pth does not explicitly correlate with plasma shaping parameters (X-point locations, elongation, triangularities, etc.), but generally shows an overall inverse correlation with outer divertor leg length (the poloidal distance between X-point and outer strike point) and/or the outer divertor connection length. This implies that another, as yet unidentified parameter, related to the outer leg of the separatrix, might be responsible for the observed differences in H-mode access power. Despite these large differences in Pth, the edge electron temperature (Te) and density (ne) profiles just prior to L-H transition are not strongly affected by the divertor configuration. Values of Te,95 (i.e. Te at 95% normalized poloidal flux surface) at the L-H transition are similar for the two divertor configurations. The paper can be found at http://iopscience.iop.org/0741-3335/54/8/082002 ICRF Systems ------------ All ICRF transmitters and antennas performed well last week. Lower Hybrid System ------------------- The LHCD system was used successfully during plasma operations on Tuesday and Thursday. Testing of the newest transmitter protection system (TPS) firmware has begun on the Cart 4 TPS boards. The new firmware fixes a resolution problem encountered for high heat capacities. The new firmware will be downloaded to the boards for Carts 1-3 after testing of the Cart 4 boards is complete. The coupler protection system (CPS) user interface was simplified to ease operation, and to provide additional software interlocks to prevent certain potential operator errors on the fault masking screen. Remote Participation, Travel, and Visitors ------------------------------------------ David Mikkelsen worked at MIT all week on turbulence simulations for C-Mod plasmas and discussed plans for simulations in which electrons may dominate the heat flux, as they do in TRANSP analyses of experimental plasmas. He also added a page to the C-Mod wiki describing how to use the GYRO pre-processor profiles_gen on C-Mod computers. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly