Alcator C-Mod Weekly Highlights May 17, 2004 On this first week of our up-to-air period we began maintenance of Alcator C-Mod power and alternator systems, the machine was warmed up to room temperature, and glow discharge cleaning was used to minimize the amount of heavy boron hydrides in the vessel. Work continued on the ICRF and Lower Hybrid systems. Physics ------- Recent work has revealed a connection between self-generated core and SOL flows, topology, and the L/H threshold. For the conditions studied, 0.8 MA, 5.4 T, and 1.2-1.6 x 10e20 /m^3, the transition occurs when the core toroidal rotation approaches 0 m/s for USN, LSN, and DN plasmas. To make a connection to the broader issues of the L/H threshold, we ran experiments to determine if this picture holds for other plasma conditions and particularly to measure dependence on parameters which are known to be important for the L/H power threshold, in this case a toroidal magnetic field of 3.4T using ICRF at 50 MHz. Given the power available at this frequency, we were able to look for the threshold only for lower and double null configurations. In the lower null case, the power threshold was with Prf between 0.5 and 0.6 MW. Ohmic power was about 0.5 MW. Rotation was slightly counter (but less than 10 km/s) before RF. Little change was observed with RF. Core rotations increased to about +20 km/s during H-mode. With 1.4 MW of power, the threshold occurred with ssep between +1 and +2 mm. Though small, this difference gave repeatable results, indicating how sensitive the threshold is to the topology. With this geometry, vphi was -20 km/sec in the target plasma. Rotation increased with RF and was slightly co at the time of transition. An experiment was performed to examine the scalings of H-mode pedestal parameters, and the associated H-mode regimes, at fields lower than those routinely exploited on Alcator C-Mod. Previous data analysis performed on EDA H-modes had suggested a trend toward wider density pedestals as toroidal field (or possibly edge safety factor) was lowered. The tuning of the J-port antenna to 50MHz allowed for near-axis D-H minority heating at fields as low as 2.9T, while 80MHz ICRF could still be used in discharges at higher fields. Using both sets of antennae, a number of H-modes, and accompanying edge Thomson scattering profiles, were obtained over a wide range of conditions, with fields ranging from 3.0 to 5.3T and plasma current from 0.5 to 1.2MA. H-modes typically were short and ELM-free even at higher edge safety factor. Differences in profile characteristics due to changing field are subtle, and a systematic analysis of the data is necessary before a definite conclusion can be made. Operations ---------- Following the last plasma discharge on May 7th, the liquid nitrogen cooling system was shut down, and the 10 day process of warming up the machine began. A glow discharge in deuterium was run for two days last week to prepare the vessel for entry. This process breaks down heavy boron compounds. Preparations are being made to go in-vessel. The first tasks will include careful documentation of the status of in-vessel components and the vacuuming of the vessel to collect all particles down to 1 um in diameter for analysis (composition and size distribution). Lower Hybrid System ------------------- A strip line probe is being designed for installation on the lower hybrid launcher. These probes will be installed at four locations on each rear waveguide channel and will monitor forward and reflected power at critical locations. After careful inspection of both the 11-window Ti/Cu and 12-window Cu/Cu braze samples we have decided to make the production coupler brazes using bare titanium walls with copper plated windows. This combination of surfaces produced leak free windows with excellent braze characteristics. The critical path for completing the brazes is now procurement of the new windows. Alumina coupons documenting each stage of window production from metalization to the nickel and copper plating have been produced by our vendors. These coupons have been baked at 500C, carefully inspected, and have passed peel tests. Production of twelve sample bricks will start this week. High power testing of the rear waveguide is continuing. After ensuring that the gasket between the waveguide and the load was compressed, one pair of waveguides was successfully tested to 100 kW input power. The next steps will include mating of the rwg and fwg components, and additional high power testing. ICRF Systems ------------ FMIT#3 and #4 were operated last week into both a dummy load and the j-port antenna with a vacuum tune as we tried to understand the limitations in operation we have experienced at 50 MHz. However, we were unable to reproduce the effects seen during plasma operation. Work will continue this week into the dummy load with a modified stub design. Diagnostics ----------- We are designing new CPCI based integrator boards for the Thomson scattering system. The old CAMAC based modules can no longer be easily maintained, and new components will allow better specs to be obtained. Diagnostic Neutral Beam ----------------------- Disassembly of the short pulse diagnostic neutral beam is nearly complete. This system was on loan from the RFX project in Padua, Italy, and will now be sent to Italy. All power system and vacuum chamber components have been removed from the experimental cell and are ready for packing and shipping. The shipper arrived on Monday to begin crating and packaging the parts for shipment. Power Systems and MIT Alternator -------------------------------- We have begun maintenance on the TF magnet power supplies. Gate drive boards that caused some difficulties during high field operation during the last campaign are being modified to improve reliability and immunity to vibration. Maintenance also began on the MIT Alternator support systems. Oil, water, air, and backup systems are being checked out and cleaned. Preparations are being made to improve the stator grounding system. Inspections have begun on primary 13.8 kV breakers. Travel and Visits ----------------- Perry Phillips, UT-FRC, was at the PSFC this week for the end-of-campaign shutdown of the FRCECE system. Some electronics was removed for testing and repair in Austin. Measurements were collected for design of the temperature control upgrade for the next campaign. Calibration of all remaining data for the last run campaign was finished. Steve Wukitch was at ORNL as part of the ORNL Theory Group Review from May 12 to 13. Martin Greenwald recently traveled to Santa Fe for a meeting of the ESnet Steering Committee. Of particular note were discussions of one-time password schemes for authentication; site upgrades; and migration to H.323 (tcp/ip) based video conferencing.