Alcator C-MOD Weekly Highlights Feb. 6, 1995 Plasma operations continued on Alcator C-MOD last week with a full four days of experiments scheduled and completed. The main experiments were concerned with effects of divertor geometry and ICRF heating. This was quite a productive week, with a total of over 100 successful plasma shots obtained over the four run shifts. Studies of the effect of divertor geometry on detachment were continued. A systematic investigation of the 'radial' extent of detachment, i.e. the flux surface location in the SOL (referenced to the midplane) where pressure is no longer constant along a flux surface, was carried out. The dependence of this quantity on strikepoint location was studied, for slot, inclined plate, and open divertor geometries; the strikepoint was positioned at different locations along the inclined plate to obtain more detailed data for this configuration. Data were taken with ramping and constant densities at two values of current (0.5 and 0.8 MA). For the inclined plate cases, detachment threshold at the separatrix does not seem to vary much as the strike point location is varied. Some differences were observed at different strike point locations depending on the rate of density rise; less density rise is needed to detach up to the vicinity of the divertor nose when the strike point location is moved closer to the divertor nose. There is a fairly narrow density range from the onset of detachment until the detachment reaches the nose of the outer divertor. At 0.8 MA this window is approximately between n~1.7 and 2.5e20 m^-3. The comparison of flat-plate and inclined-plate divertor continues. At 0.5 MA the detachment threshold for the inclined and flat-plate geometries were n~.8 and 1.5e20 respectively. This result is similar to the 0.8 MA results where the inclined plate and slot had a threshold of n~1.7e20 and the flat-plate at n~3e20. ICRF heating studies this week included scans of toroidal field (resonance location for our fixed frequency 80MHz sources) and minority concentration. The toroidal field was scanned between 3.5 and 5.5T at nebar~1.8e20/m3 and Ip=0.8MA with approximately 2.5MW of RF power. The ohmic stored energy (EFIT) was found to be independent of the toroidal field, at about 47kJ for the present condition. With on-axis heating, the stored energy increases to 80kJ during the L-mode phase and 90kJ during the brief H-mode phase (see below). The stored energy increment decreases as the resonance layer is shifted away from the center. With the resonance layer shifted half way out the stored energy (RF heated) was 60kJ, and no stored energy increase was observed when the resonance layer was shifted by r/a >= 3/4. H-mode transitions were observed on all shots with resonance layer located at r/a <= 1/4. We still have some difficulty maintaining constant power through L-H transitions, but we were able to keep reasonable power across the transition and observe the stored energy increase (from L to H-mode) on some shots. A hydrogen minority concentration scan was performed at two densities, nel_04=0.7 and 1.1e20 m^-2. Heating (stored energy increase) degraded as the H concentration was increased from 2% to 15%. At nel_04=1.1e20, the stored energy increased from an ohmic level of 46kJ to 80kJ with 2.5MW of RF power in L-mode with no H (concentration < 2%), but with 15% H (22% H mix in the plenum) the stored energy only increased to 52kJ. There is a definite indication that heating is better at nel_04=0.7e20, especially at higher H concentrations. The stored energy with 2.6MW of RF power at this lower density with 15% H was 69kJ. In support of the ICRF toroidal field scan, a new startup scenario was developed. Initiation was produced during the TF ramp-up, at a field of about 3.4 tesla. This technique allows the flattop TF to be changed shot-to-shot without affecting the breakdown. It also reduces the TF magnet heating at higher fields by decreasing the time the coil is at full current. The YAG Thomson Scattering diagnostic operated successfully this week, following a re-alignment carried out on Monday. Data are obtained for electron density and temperature every 20msec during the pulse; three spectrometers (spatial locations) are presently operational, with additional spectrometers in preparation. Electron temperatures obtained are consistent with the independently calibrated ECE system. Jim Bialek(PPPL,TPX) and Hutch Neilson(ORNL,TPX) visited MIT for the initial meeting of the CMOD Halo Current EM task group, which was formed to help resolve issues of halo and eddy current paths in the vacuum vessel and PFC's of ALCATOR-CMOD . Participants from MIT included Bob Pillsbury, Len Myatt, Jim McCarrick, Jim Irby, Dave Gwinn, Herb Becker, Jeff Freidberg and Joe Sorci. The fundamental mission of the group is to provide forces for stress analysis, based on actual 3D measurements of halo currents observed in the experiments. It is expected that there will also be insights gained as to the flow of these currents in the walls and other hardware, contributing to the development of a useful design and analysis tool applicable to other projects. On Monday-Wednesday, Jan. 30-Feb 1, Prof. Miklos Porkolab attended the US-Japan Workshop on Steady State Tokamaks, in Livermore, and presented a talk on Access to Advanced Tokamak Regimes in TPX through Profile Control. He also discussed possible collaborations between C-Mod and the LLNL Tokamak Physics Group. On Friday he attended the TPX Physics Workshop, in San Diego, and made a presentation on the latest profile control scenario work.