Alcator C-Mod Weekly Highlights Nov 13, 2000 Plasma operations continued on Alcator C-Mod last week. Four run days were scheduled, but only three were completed owing to a computer problem. A total of 35 plasma shots were obtained. All runs were carried out with reversed (positive) toroidal field and current. One run day was devoted to a triangularity scan with upper single null plasmas, to investigate the effect of shaping on the EDA/ELM-free boundary. Another run continued investigations of the SOL fluctuations and corresponding transport. An investigation of H-mode pedestal properties in lower single null plasmas was attempted, but was not successful due to inadequate conditioning of the RF antennas. This experiment has been rescheduled for this week. Reverse-field plasma operations are planned to continue this week. Physics -------- It has been established that triangularity, especially of the active x-point, affects the character of the H-mode, with higher triangularity lending itself to the advantageous EDA operation, and lower triangularity to more ELM-free behavior, with the associated difficulties of impurity accumulation. The closed geometry of the lower divertor makes triangularity scans equivalent to scans of divertor geometry, from nearly touching the inner nose through to operation on the top of the outer divertor. Operating the plasma into the upper divertor chamber, which is open, avoids most of the strike-point geometry changes, and enables us to establish much more clearly whether the effects we observe are a consequence of plasma shape or are linked more intimately to divertor configuration and recycling location. Reverse-field operation is required to maintain the sense of the grad-B drift with respect to the active x-point. A series of dynamic triangularity scans were carried out, with the upper triangularity varied from 0.8 to 0.4. The variation of xray pedestal width as well as the appearance of the quasi-coherent mode, as observed on the PCI diagnostic, confirmed our previous results. The "EDA-ness" depends on the triangularity. Ramps of current and toroidal field were employed to maintain the q approximately constant during the triangularity variation, confirming that the shaping rather than the safety factor was the dominant effect. It therefore appears that the upper diverted case is the same as the lower diverted case, and the effects we see are not a divertor hardware proximity effect, but truly shaping. The Divertor/Edge Group continued their program exploring the dependence of SOL fluctuation characteristics and attendant cross-field particle transport on the collisionality of the SOL. Reversed field/current allows some important measurements to be made on particle transport in the SOL and the associated levels of plasma turbulence. Experiments being conducted in this configuration aim to determine whether the field direction affects the 'scaling' of Deff with collisionality. We will also get data on in-out asymmetries. The outer divertor conditions change in reversed B, which may impact the transport through changes in the sheath conditions at the divertor plate. In addition to the fluctuation and Lyman alpha measurements, we are using the 'burping probe' system to produce and image C2D4 plumes in order to observe ExB flows in the SOL region. During the run day devoted to this program last week a coarse density scan was carried out at 0.5MA and 1.0MA currents. We anticipate completing these experiments this week. ICRF System ------------ With J-port antenna configured as a 2-strap, 1.4 MW were coupled to the plasma after conditioning. Total RF power to reverse field experiments last week was limited by impurity influx. The maximum net power obtained without injections was ~2MW, which is below the L-H threshold for reverse-field lower single null operation at the operating field of 5.3T. This impurity influx is most probably attributable to recovery from the up-to-air required to fix the DNB beamline. We have pursued developing a plasma test for automatic antenna matching. The system being considered is a double stub matching system using the recently developed fast ferrite tuner (FFT) prototypes. These were developed by Advanced Ferrite Technologies (AFT) with GA and tested on ASDEX-U. This is a critical technology for C-Mod which relies solely upon ICRF power for auxiliary heating. If the FFT application to C-Mod is successful, we could purchase FFTs for all the transmitters and greatly improve C-Mod's experimental productivity. In addition, the FFT would decouple the transmitter from plasma variations that could reduce the lifetime of the high power tube. Currently we use power thresholds and dissipation limits to limit the tube exposure to events that result in large reflected power to the transmitter and tube. The FFT will allow the transmitter to be matched to 50 ohms by adjusting the matching network to compensate for the load variations. DNB Systems ------------ The beam was conditioned last week; the highest voltage was 47 kV at 5.1 A. The interlocks for divertor magnet, fizzles, and disruptions were fully tested in DNB conditioning mode, i.e. without opening the DNB gate valve. The beam is now working sufficiently well to begin operating into C-Mod discharges with low neutral pressure. At the next available opportunity, the DNB will be operated with HV only (no production of beam) to test the new interlocks with the beam synched with C-Mod. Lower Hybrid ------------- Work is proceeding on the modification of the klystron carts in preparation for painting. A phase-controller module was tested successfully that will be used in the klystron filament power supply circuit. The modified filament supply circuit allows for remote turn-on/off and limits the cold filament surge current. Travel and Visitors ---------------------- Earl Marmar attended the 2000 Conference on the Applications of Accelerators for Research and Industry, held in Denton, Texas, where he presented an invited talk entitled "Diagnostics on the Alcator C-Mod Tokamak using Neutral Hydrogen Beams". The presentation was on Saturday, November 4. Miklos Porkolab and Dmitri Mossessian participated in the DIII-D Research Opportunities (aka "Brainstorming") Meeting at General Atomics in San Diego last week. Amanda Hubbard and Martin Greenwald also presented research proposals via the videoconferencing system from MIT PSFC. Stewart Zweben (PPPL) came to MIT for the week to reinstall the control box for the fast camera and take new data for the edge turbulence imaging diagnostic. A new series of images was obtained using a helium gas puff. Gerd Schilling spent the week at PPPL working on collaboration budget and manpower issues.