Alcator C-Mod Weekly Highlights Nov. 20, 2000 Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled, but only one-and-a-half days were completed, owing to a combination of computer problems and a power outage. A total of 15 plasma shots were obtained. Experiments conducted included a study of xray pedestal asymmetries in reverse-field H-modes, and additional data on SOL fluctuations and transport. A problem with a RAID (Redundant Array of Independent Disks) controller serving the disk array which contains C-Mod shot data resulted in loss of one run day last week. The faulty controller has been replaced, and online data archives are being restored from backup tapes. During Thursday's run a broken lug on the 4160V AC power feed to the alternator drive motor resulted in an arc which caused an interruption in service from the electric utility. Repairs have been completed, and service has now been restored. The reverse-field phase of the campaign has been extended in order to complete the experiments which were not accomplished last week. Physics -------- During previous experiments, it was observed that the xray pedestal location at the plasma top is found to be very close to the separatrix (typically 1-2 mm), in contrast with the xray pedestal on the outboard side, which is usually 10-12 mm inside of the separatrix. The emissivity is from recombination of highly charged impurities drifting out from the core, and therefore may be subject to the same up/down asymmetry observed for Ar+17, which was attributed to Bxgrad(B) effects on neoclassical transport. This explanation was tested during experiments last week with reverse-field H-modes, for which the Bxgrad(B) drifts are reversed. Contrary to expectations, the observed relative displacement of the x-ray pedestals at the top and outboard were the same as with normal field operation. The up/down argon asymmetry, on the other hand, was reversed from the normal field case, as had been observed previously. These results indicate that the mechanism for the shift of the x-ray pedestals is not the ion grad-B drift effect. Further experiments are planned to test the scaling of the shift with q95 in reverse-field. We have now observed what appear to be beta-limiting MHD modes at high power on C-Mod. By combining all three ICRF antennas, we have a number of shots with up to 4.5 MW of ICRF power, and we have achieved normalized beta values up to at least 1.6 for several hundred msec. At moderately high density, 3 - 4 x 10^20 m^-3 in EDA H-mode, small chaotic ELMs appear on top of the enhanced D alpha emission. At lower density, less than 2.5 x 10^20 m^-3, we sometimes find large amplitude low frequency MHD activity that begins as sawtooth precursors or postcursors, but then continues throughout many sawteeth. The frequency may reach 20 kHz or so, but when the amplitude becomes large enough ~ 3 G or more at the wall, the mode begins to slow down and finally locks. The confinement is substantially degraded when these large modes appear and the appearance of the modes is well correlated with the peak in normalized beta ~ 1.4 - 1.6. The poloidal beta values at the same time reach values of 0.8. Mode analysis has shown that in some cases, the modes have m=2, n=1 or m=3, n=2. The low m,n, large amplitude, and correlation with the peak normalized beta suggest that these modes may be the first neoclassical tearing modes observed on C-Mod, particularly since they are also correlated with lower density operation at high power in a lower collisionality regime. However, more analysis is required to confirm this speculation. ICRF Systems ------------- The J-Port ICRF antenna remained in its 2-strap configuration this week for C-Mod physics experiments. Following machine boronization after the DNB bellows replacement, up to 3 MW of heating power have been launched into the plasma using all three antennas. H-modes were obtained in reversed-field configuration, with the ion grad B drift direction away from the the X-point, at 0.8 MA, and an ICRF power threshold of 2.2 MW. In addition, we have prepared for the 2-strap, 70 MHz operation by tuning FMIT#3 to 70 MHz (2 MW into dummy load) and identified the coax required to change the transmission line configuration. Thia will be used for 2 frequency heating experiments to investigate on-axis heating of core barriers which are triggered by high-field side off-axis ICRF. In preparation for 4-strap operation, we have modified the 4-port splitter to ease the assembly by bolting the center conductor together. Diagnostic Neutral Beam Systems -------------------------------- Testing continued on the new interlocks on the fizzle detector and magnet current in both conditioning and plasma modes. The magnet interlock remains somewhat too sensitive due to the slow response time of the PLC, and some further refinement will be necessary to reduce false alarms. The fizzle interlock may be somewhat slow after disruptions and new PLC programming will be tested at the next opportunity. The pressure in the DNB has been measured when using only neutralizer gas, only source gas, and a combination, in preparation for tests to increase the neutralization efficiency. Travel and Visitors ---------------------- Miklos Porkolab attended the FESAC meeting in Washington on Nov. 14,15. Norton Bretz came to MIT on 11/13 to work on the MSE diagnostic. Gerrit Kramer came from 11/13-15, also to work on the MSE. David Mikkelsen came for the week and continued work on C-Mod discharge transport modeling. Christopher Watts of Auburn Univ. and Romik Chatterjee of U. Texas visited last week to work on commissioning the Te fluctuation portion of the ECE heterodyne radiometer. The diagnostic is now fully operational and 32 channels of T_e data are now being taken at 20 kHz. An additional 32 channels of fast (>100 kHz sampling) data were successfully added to allow better observations of fast MHD. A four channel correlation system dedicated for fluctuation studies was also added with its own 1 MHz digitizer. Bill Dorland (U. Md) visited the PSFC for two days to work with the C-Mod staff on implementation of the gyrokinetic stability code - GS2. This code is being used to calculate the critical gradient length for the plasma core and to search for the instabilities responsible for EDA H-mode.