Alcator C-Mod Weekly Highlights June 9, 1997 Plasma operations continued on Alcator C-Mod last week. The week was devoted to completing initial operation with the 40 MHz ICRF system. Plasma runs were carried out on Wednesday, Thursday, and Friday. Wednesday's run consisted primarily of clean-up discharges following last week's up-to-air. Transmitter #4 was operated into plasma for the first time, producing up to 0.3MW. By the end of the day, full length plasmas were being produced. Thursday's run continued conditioning of the antennas and increasing the power from the 40 MHz transmitters. Over 2MW (approximately 1MW from each transmitter) was coupled to the plasma. The target plasma was D(He3) at a field of 4.3T. Heating efficiency was low, but some electron heating was observed on several discharges. Two attempts were made to change the antenna phasing, but preliminary measurements suggested the decoupling stub position was incorrect. A scan of the He3 concentration was made. The H/(H+D) ratio climbed to > 50% after a few shots and remained high throughout the session. Friday was the final run with the 40 MHz ICRF system during this phase of the campaign. The RF ran reliably at this frequency at a power level of 2MW. The decoupler stub was rechecked with a vacuum load, and adequate isolation between the two main transmission lines was observed. However, the behavior of the relative phase between the antenna current straps connected to different transmitters observed on Thursday is not yet understood. In collaboration with PPPL, modelling of the transmission line system will be performed to determine what course of action should be taken. A preliminary exploration of heating scenarios using the 40 MHz system was carried out. Because of the high H concentration (over 10% on shot 1, increasing to about 50% later in the run), D(H) minority heating or standard D(He3) minority heating scenarios were not accessible, so the D-He3-H mode conversion experiment was carried out. Toroidal field and He3 concentration were varied, at currents of .8MA and 1MA. It was found that high He3 concentration (about 10 torr-l) and "high field" (around 5T) produced the best heating for both electrons and ions. Significant neutron rates were observed, possibly indicating generation of a deuterium tail. Cynthia Phillips and Randy Wilson of PPPL participated in the RF runs on Thursday and Friday, and will be involved in the data analysis. Additional diagnostics were also brought on line. The core Thomson scattering system, which is now using a 30 Hz NdYAG laser obtained from PPPL, became operational during Friday's run. The new inner cylinder flux loops, which detect radial fields and are sensitive to motion and alignment of the central solenoid as well as providing information on disruption dynamics, are now connected and producing data. The new arrays of XUV core and edge bolometers are operational. The gas-injecting fast-scanning probe drive was tested during the RF conditioning run last friday, injecting helium and nitrogen gas. The gas delivery system works according to design, depositing up to 10^19 molecules in the 10 ms deacceleration period during the scanning probe's end-of-stroke. Tests indicate that bypass leaks in the gas delivery capillary are small, about 10^18 molecules/second for the largest plenum pressures. The 'plume-camera' in F-top recorded some emission patterns in HeI, HeII and NIII. However, a combination of neutral density filters and/or greatly reduced gas injection rates will have to be implemented to reduce signal levels to the camera which is now saturated much of the time. Stewart Zweben and Raffi Nazikian of PPPL were at C-Mod for the first part of the week. Raffi is working on the reflectometer diagnostic, and Stewart will be involved in optical fluctuation diagnostics and studies of the L-H transition and H-mode pedestal region. Jim Drake of U. Md. also visited and participated in discussions on H-mode physics. Valdimir Fuchs from Tokamak de Varennes visited last week and worked with Paul Bonoli on some numerical problems in the ACCOME current drive and MHD equilibrium code. They also discussed the theoretical and numerical treatment of IBW mode conversion problems. Paul Bonoli also worked with Cynthia Phillips (PPPL) discussing modifications to the FPPRF code to improve its ability to analyze C-Mod RF heating cases. Cynthia also ran CARDS and METTS codes while here to analyze single pass absorption for the D-He3 experiments. On Saturday we began disassembly of the 40 MHz transmission line and tuners. The lines for the 80 MHz system which will be used for the bulk of the physics program for the Summer 1997 campaign will be reinstalled. Operation with the 40 MHz system provided an opportunity to debug the hardware and gave valuable experience with this new system, which will be the basis of current drive and heating experiments in the next campaign. This week will be a maintenance week. During this time we will also be carrying out a baking and ECDC campaign in D2 to reduce the hydrogen fraction for the upcoming runs. Physics run, using the 80 MHz ICRF system, will resume on June 16.