Alcator C-Mod Weekly Highlights August 30, 1999 Plasma operations continued at Alcator C-Mod last week. Three runs were carried out, although the run on Tuesday was terminated after a single shot, so that further work could be carried out on the ICRF system. A total of 37 plasmas were produced, with typical currents of 1MA. Start-up reliability was 95%. The run on Wednesday was dedicated to MP#208, which is designed to study transport effects in the SOL using the fast-scanning gas-injection probe. The probe was used to inject trace amounts of ethylene (C2H4) for plume imaging and impurity screening studies. A video system was used to capture images of CII and CIII "plumes" to provide information on parallel and cross-field transport in the SOL in a variety of ohmic plasma conditions. The objectives of this experiment were: (1) Investigate the dependence of cross-field spreading of C+1, C+2 (i.e., cross-field transport) at fixed locations in the SOL as a function of plasma density (or collisionality) (2) Investigate dependence of cross-field impurity transport on SOL location (rho) in fixed discharge conditions (3) Investigate parallel flow of background plasma (via parallel C+1, C+2 spreading)as a function of SOL location and plasma collisionality. At a core density of 1.4e20, with deep insertion of the probe (close to the LCFS), reversed flows were seen and plumes exhibited a corresponding parallel shift away from the divertor. Good CII and CIII plumes were seen on most of the shots. From analysis of the difference in CII and CIII emission patterns, we hope to learn something about the cross-field and parallel transport of impurity ions in the SOL. These data are expected to form part of a doctoral thesis. Thursday's run was devoted to continuation of the ICRF commissioning activities. RF operation was carried out with the E-port dipole antenna and with the J-port (PPPL) four-strap antenna. Peak combined power delivered to the plasma was 3MW. Up to 1.5MW was coupled to the plasma from the E-port antenna, driven by FMIT#2. However, later in the day this transmitter exhibited an oscillation which led to persistent fault conditions. This behavior is being investigated. Over 1.5MW was coupled through the J-port antenna as well. Heating efficiency appeared to be lower with the J-port antenna than with E-port, and impurity production was higher. Spectroscopic diagnostics indicated influxes of iron, nickel, and titanium associated with high-power operation of the J-port antenna. Further conditioning of this antenna, both in vacuum and into plasma, is planned. Physics and Analysis -------------------- Perturbations of floating potentials in the far scrape-off layer plasma have recently been observed during ICRF antenna conditioning discharges in which the J-port antenna has been operated. These perturbations appear to be caused by RF-sheath effects localized to antenna surfaces. During unfavorable phasing of the J-port antenna with 1.3 MW of input power, the F-port scanning probe records floating potentials as high as +200 V on portions of the profile which connect along field lines to J-port antenna protection tiles. These discharges correspondingly show large core impurity increases in response to the RF power pulse. In contrast, during favorable phasing at the same power level, floating potentials exhibits a much smaller increase (+50 V), the impurity influx is less severe, and these discharges are seen to transition into H-mode. Further evidence for RF-sheath effects localized to antenna surfaces is supplied in data from the A-port scanning probe. Unlike the F-port scanning probe, at no point in its trajectory does the A-port probe connect along magnetic field lines to J-port antenna surfaces. Consequently, only a small decrease (-30 volts) in the far scrape-off layer floating potential is seen during J-port antenna operation. This relatively small change is independent of antenna phasing, impurity influx, and attainment of H-mode. These observations suggest that ICRF antenna operation can be greatly improved by replacing the molybdenum antenna protection tiles with electrically insulating boron nitride tiles. This idea is presently being considered for the next in-vessel maintenance period. Recent experimeents have improved our understanding of the source of molybdenum impurities in ICRF-heated discharges. In the past, the inner wall has been identified spectroscopically as the source of the molybdenum found in the plasma core during current ramp up, when the plasma is limited by the inner wall. However, until the beginning of the current experimental campaign, there had not been a consistent correlation between the spectroscopically measured molybdenum sources and molybdenum core densities during the steady state portion of the plasma discharges and especially during RF heating. This was the case despite systematically monitoring a number of surfaces including the outer divertor (biggest source observed), the inner divertor, the inner wall, the antenna protection limiters, and the ICRF antennas at the midplane. During the vacuum vent, significant erosion had been found along the top protection tiles of both (D & E) antennas, which prompted us to redirect two spectroscopic views to look at the top of the two antennas instead at the midplane. Our measurements since then have shown that indeed the top protection tiles seem to be the main source of molybdenum during RF heating. In L-mode, we have found clear correlations between the antenna Mo sources and the core Mo levels. The antenna source rates (and the core Mo density) correlate with the RF power levels. The correlations can be mainly attributed to a local effect (e.g. local sheath acceleration of ions) rather than to a global effect through the heating of the plasma, although the latter has been also observed. Other sources have been excluded from being the main contributors to the observed Mo core levels because of lack of correlation between Mo sources and Mo core density. Specifically, it has been observed that the outer divertor and inner wall source levels decrease significantly during elm-free H-modes in contrast to the unaffected antenna Mo source and to the increasing (due to transport) molybdenum core density. Recent runs have provided additional examples of the "Enhanced Neutron" mode, characterized by neutron rates that increase by up to a factor of 3 just after the H-L transition. We have previously attributed this effect to spontaneous formation of an internal transport barrier (ITB) triggered by the profile changes following the H-L transition. Analysis of results from runs on 990818 and 990819, which had mostly ohmic H-modes with some ICRF at the end of the H-mode, show that the increased neutron rate can be well fitted using the measured central electron temperature from the ECE GPC system and the central density calculated from the visible bremsstrahlung array. Most of the increase in neutron rate comes from the temperature increase in these discharges; the central density is in fact falling at the time of the increase in neutron rate. This result appears to be in contrast to the the older ICRF dominated H-modes where the central density remains high after the H-L transition for a brief period when the neutron rate increases. Large low frequency (2 - 10 kHz) MHD modes appeared just before the peak in the neutron rate on most of these discharges. They appear to be m=1, n=1 in the core and m=2, n=1 on the edge magnetic pick-up coils. These modes become so large as to lock to the wall, but so far, they have not led to disruptions. The large MHD can be present even when there is not a large increase in neutron rate. While there are also cases in which there is no clear MHD mode on the magnetics and there is a large increase in the neutron rate, the central soft x rays do see an m=1 mode. Calculations of the profile of the density from the visible bremsstrahlung indicate that the large m=1 mode begins when a steep gradient in the density and pressure arrives at the sawtooth inversion radius. Work continued on development of the FRC ECE radiometer. Perry Phillips (UT-FRC) was on site and continued work on the ECE profile calibration. In addition, the two channel ECE fluctuation system was re-installed with additional gain and will be used in the next week. ICRF System ------------ FMIT#2, connected to the E-port antenna, had a high power 2274 tube on loan from General Atomics installed and tested up to 2 MW into dummy load. After initial plasma operation with power up to 1.5MW, an excessive self-oscillation was observed in the FPA stage. We have begun adjusting the screen, bias, and filament voltage to eliminate this problem. Further progress on J-port plasma operation was initially hampered by an intermittant gate, which prevented FMIT#3 from pulsing reliably. A problem with the gate reset was identified and corrected. The ICRF antenna power waveforms now show good tracking of the desired waveform, even in the face of antenna load changes resulting from plasma L-H transitions. This indicates that good progress has been made in the rebuild of the transmitter feedback circuits. The FMIT#3 and #4 transmitters now appear to be operating reliably at moderate power levels. Conditioning of the J-port antenna will continue this week. A rebuilt 2274 tube on loan from PPPL arrived late last week and the installation process began immediately. The tube was inspected and successfully high potted. Initial testing into dummy load should happen by the end of this week. Once this tube is installed in FMIT#1 (connected to the dipole antenna at D-port), all four transmitters will have high-power 2274 FPA tubes, which will bring the rated total source power up to 8MW. Travel and Visitors --------------------- Joel Hosea and Randy Wilson visited C-Mod last week to participate in the new antenna startup experiments. Dr. Sakae Besshou of Kyoto University and the Heliotron E project has been visting C-Mod for the last two weeks. Dr. Besshou is in charge of designing the magnetic and bolometric diagnostics for the new heliotron at the institute affiliated with Kyoto University. He is visiting in order to consult with C-Mod scientists and observe C-Mod operations. This is the last week for three summer students who have worked productively at C-Mod since the end of May. Two, Dominique Huebner and Stefan Krotz, are from the the University of Wurzburg, Germany. They have worked on spectroscopic diagnostics and RF systems respectively. The third student, David Smith, is from Northwestern University. David has improved and updated the MIST impurity transport code and worked on other spectroscopic analyses.