Alcator C-Mod Weekly Highlights Nov 1, 1999 Plasma operations continued on Alcator C-Mod last week. Three run days were scheduled and completed. A total of 42 plasma shots were produced, with a reliability of greater than 85%. Following the week's operation, the Alcator C-Mod bus was reconfigured over the weekend in preparation for a week of reversed field operation in which SOL, x-point, and pedestal measurements will be made during both normal and upper x-point operation. Wednesday's run was in support of MP#154, and involved a study of H-mode thresholds in helium discharges, with a goal of elucidating the role of neutrals in the L->H transition and pedestal formation. Although the RF was heating the core, it was very difficult to achieve H-mode. As expected we had a fair amount of radiation, but the edge remained pretty hot nonetheless. When the plasma did go into H-mode, the edge temperature threshold was found to be higher than in deuterium, with the exact ratio to be verified. We raised the current in an attempt to get more power with no obvious effects. H-modes were again very short. Helium rotation data were obtained, but further analysis is required. H-mode studies continued on Thursday with an investigation of low target density thresholds. No pre-operation discharge cleaning was done for this run, which followed the helium plasma experiments of Wednesday. As a result, the walls were unloaded, and there was a tendency for runaways to be produced early in the discharge. As previously observed, the power and temperature thresholds went up considerably at lower density. Nevertheless, we succeeded in obtaining H-modes for target densities down to ~5e19/m3, albeit with threshold values of P/nBS up to a factor of four higher than obtained at higher density. We observed small ELMs at very low density, seemingly type III. These may correspond to the low-density, high-temperature ELMS observed on DIII-D. Edge temperatures, and consequent pedestals were very hot. However, the density rise was slow and in some cases difficult to see. The second half of Thursday's run was devoted to studies using the divertor bypass (flappers). Localization effects were studied by opening and closing one bypass sector at a time. We succeeded in observing a diffuse glow on the TV camera view over the H-J flap when it alone opened. There was some evidence that the B-bottom pressure can be reduced using only the flappers on either side of B, but this was not completely clear. The purpose of Friday's run was to determine the profiles of the impurity diffusion coefficient and convection velocity during EDA H-mode. Complete radial brightness profiles for He-like, Li-like and Be-like calcium lines were obtained, in addition to a few time histories for Na-like calcium and Li-like fluorine, following a sequence of CaF2 injections. Bolometer and edge x-ray profiles were also collected. These data should allow for a complete characterization of the impurity transport coefficient profiles. Unfortunately, these were not very good EDA H-modes, with H factors only slightly above 1, since the walls were not boronized and the molybdenum and total radiated power levels were very high. Physics and Analysis -------------------- Investigations of ICRF minority heating with scans of the minority hydrogen fraction were carried out on Friday, October 22. In all cases, the hydrogen fraction was monitored with a high resolution spectrometer measuring the Balmer alpha multiplets from hydrogen and deuterium, viewing emission from recycling neutrals in the main chamber scrape-off plasma. The D and E port antennas were used together for this run, with net forward power in the range from 2 to 2.5 MW for most shots. The best heating and confinement, and lowest radiated powers, were obtained with no extra hydrogen puffing. The plasmas clearly degraded as the hydrogen fraction was brought above 0.05. Based on H-factor confinement enhancement, the optimum hydrogen level is at or below 0.025, the lowest we have seen during this run campaign. The sawtooth reheat rate (not adjusted for density differences) was also maximum for hydrogen fraction between 0.025 and 0.06. The shots with 0.04 < n_H/n_D < .06 were ok during the first part of the heating pulse, but eventually degraded from EDA toward type III Elmy or Elmfree, with increased radiation and particle confinement, and decreased energy confinement. Once the H fraction reached 0.08, even quasi-steady H-modes were impossible; the radiated power increased continuously, and the plasmas returned to L-mode. For fractions greater than 0.08, only the very briefest of H-Modes were obtained, with basically steady-state L-mode the result. At the highest H level reached (n_H/n_D ~ 0.25) there was little evidence of RF power absorption in the core plasma. Recently the views of two CCD cameras have been changed to record high resolution images of the J-port antenna. These views have been recorded since October 18th, 1999. The cameras primarily viewed unfiltered visible light, although some images were recorded with D_alpha or D_gamma bandpass filters. During operation, injections were observed from the antenna, with the majority of them originating from the top protection tiles. The injections were correlated with increases in radiated power, as measured by the bolometers, and with titanium influx, as measured by spectroscopy. Injections were also observed when the J-port antenna was not being used, but were notably worse when it was energized. In addition to the injections observed, the edges of the Faraday-screen box are seen to glow when the antenna is energized. It is still not clear what process is responsible for this glow. ICRF System ----------- On the last plasma shot of the day on 10/22/1999 we obeserved a fault and crowbar of ICRF transmitter #1. Pulsing into a dummy load revealed anode high voltage faulting of the output tube at decreasing levels of output rf power. A tube hi-pot indicated excessive leakage current at 35 kV (60 kV holdoff required), with no indication of poor tube vacuum, suggesting a damaged screen grid structure. This 2274 tube was placed in service early in September, and had been previously repaired and tested by the manufacturer. As a consequence of the extensive transmitter repairs and upgrades of this past half year, all of the tube protection circuits had been thoroughly checked out, and digitized waveforms showed proper circuit behavior. The tube is being returned to the manufacturer, since it is under warranty. It has been replaced in transmitter #1 by a lower power (EIMAC #8973) tube. With the improvements made over the summer we can still obtain relatively large power (~1.5MW), albeit for shorter duration. The limitation is presently plate disspation in the FPA tube. This configuration provided adequate power for the physics experiments run last week. The two ICRF antennas at D- and E-ports were used, coupled to transmitters #1 and #2, avoiding potential complexities from J-port antenna impurity generation and conditioning for the moment. Diagnostic Neutral Beam ------------------------- Progress on the DNB is consistent with the schedule presented at the September review. The high voltage modulator/regulator was installed this week and is being prepared for final testing prior to operation of the beam line in the test stand. The preparations for final installation in the C-MOD cell are underway during maintenance days when cell access is possible, and these activities are ahead of schedule by a few weeks. Travel and Visistors -------------------- Last week Paul Bonoli and Ron Parker attended an IAEA Technical Committee Meeting on Steady State Operation of Fusion Devices - Plasma Control and Facing Components. Bonoli presented an invited paper entitled "Modelling of Advanced Tokamak Scenarios with LHCD in Alcator C-Mod" (to be published in a special issue of the journal Nuclear Fusion).