Alcator C-MOD Weekly Highlights Report October 27, 1993 Alcator C-MOD is now in the seventh week of the current run block. An extra run day (Monday) has been added to the usual four day a week schedule in order to complete the data taking for the upcoming APS meeting. Some runs are also being extended to 10 hours. The major accomplishment this week is undoubtedly the achievement of ohmic H-mode on Friday, October 22. Clear H-mode transitions were obtained at a field of 3.1 Tesla, with line density approximately 1e20/m3 and plasma current of 700kA. The threshold ohmic power of just under 1MW (surface power density .14MW/m2) is consistent with the ASDEX-U scaling for ohmic H-mode. The ELM-free period lasts over 50msec, during which the density approximately doubles and substantial increases in energy confinement time (H-factor > 1.5) are observed. In the same run, values of the safety factor q as low as 2.1 were obtained. Last Thursday's run was devoted to filling in gaps in our ohmic confinement database with D+ plasmas. The data continue to support the previously reported trend that energy confinement is more strongly correlated with current than with density, and is consistent with L-mode scaling (e.g. ITER-P) in regimes where the L-mode confinement is predicted to be higher than neo-Alcator. The run on Thursday, Oct. 21, was delayed for about 2 hours in order to remove a piece of a Molybdenum tile which had fallen onto the TCI window. Three tiles (out of over seven thousand) are known to have been damaged; two fragments ended up on windows located behind gate valves, and have been removed without breaking machine vacuum. All of the known damaged tiles have been identified as having originated from the "EF1 pocket" region, which is not normally subjected to direct plasma flux. No deleterious effect on plasma operation has been observed. Monday's run was dedicated to a continuation of the divertor gas puffing experiments and studies of the edge and divertor plasma. Particular attention was paid to the scaling of the "flame-out" phenomenon, in which the power conducted to the divertor plates drops and the radiating region near the separatrix changes shape and location. Scans of density were carried out at several currents. The density at which the "flame-out" occurs appears to be correlated with the input power. Half of a run was devoted to observations of high-n state transitions in He- and H-like Argon in the outer chords of the HIREX diagnostic. Some of these states are preferentially populated by charge exchange recombination and the enhancement of these lines can be used to infer the neutral deuterium profile. High elongation studies have been extended to higher currents and lower li values. Maximum elongations above 1.6 were obtained. Passive growth rates were measured by turning off feedback for a short time. The run on Wednesday, October 27, is devoted to ICRF heating experiments. The minority fraction is being varied to try to observe the production of high energy tail. ICRF power in the range of 0.5 to .7 MW is being injected using the moveable monopole antenna. Lithium pellet injection is being carried out in conjunction with the ICRF to evaluate the effect of "lithiumization" as a wall conditioning technique in our metal-wall machine. Use of lithium pellets is known to have beneficial effects in carbon machines such as TFTR.