Alcator C-MOD Weekly Highlights Dec 18, 1995 The operating campaign on Alcator C-MOD continued last week, with four runs scheduled and completed. Two of these runs were in support of graduate student thesis research. PEP modes with deuterium pellets were investigated on Tuesday. Previously, our PEP mode discharges had employed lithium pellets. The deuterium pellet injector can produce up to twenty pellets per shot, in four different sizes, corresponding to nominal particle inventories of 0.5, 1.0, 2.0, and 3.0e20 deuterons. Most of this run employed two pellets in succession. PEP modes were obtained with single pellets of the largest size, and with a combination of a 2.0e20 size pellet fired 10msec after a 1.0e20 size. Peak densities of over 1.7e21/m3 were obtained. Comparing to the Li PEP modes, the length of the modes are comparable (50 ms), but Li gave higher neutron rates than D2. Aside from the isotopes, one difference between the best Li PEP shot and the best D2 PEP shot that could be significant is the relative timing of the injection and the RF. The Li pellets were fired in about 50 ms before the RF turned on, while the D2 pellets were fired only 10 ms before the RF. This could indicate that the plasma needs time to "settle down" before the reheat. This will be investigated in future experiments. Mode conversion direct electron heating in hydrogen/He3 plasmas at a toroidal field of 6.4Tesla was used in an experiment to investigate transport with perturbative techniques. The RF power was square wave modulated at a frequency of about 40Hz to allow analysis of heating profiles and transport coefficients. All shots in this run were at 800kA plasma current with a nominal line average density of 1.5e20/m3. The He3/H ratio was varied to optimize the heating. On Friday, a run was carried out to investigate the effect of helium plasma operation on divertor and SOL characteristics, as compared to our standard operation in deuterium. Through that comparison we hope to better understand various aspects of SOL transport, recycling, impurity transport and detachment. Note that even though charge exchange is much reduced as a momentum-loss mechanism, ion-neutral elastic collisions still remain. Several significant differences were noted for the helium discharges: - The SOL was hotter - detachment appears to be at higher density (about 50% higher than w/D2) - The transition was slower than recent D2 discharges (as a function of density). - The pressure drop observed was small (upstream pressure over downstream of order 4-5). The corresponding Te at the plate was slightly higher. - no "death ray" was observed. This may be related to the reduction in CX which results in momentum-carrying neutrals directed along a flux surface. Elastic collisions would result in a 'spray' of neutrals in all directions. - The ratio of main plasma radiated power to total input power was reduced, resulting in higher powers flowing into the SOL. The divertor radiated power as measured by bolometry was lower than equivalent densities with D2. This is consistent with the hypothesis that cx contributes significantly to this measurement. - Measured XUV radiation at the x-point is low for all impurities and decreases with density. Helium dominates the XUV emission spectrum. Recombination edge in hydrogen-like He is very strong. The moly source as measured by the Chromex was quite significant but decreased with increasing density. - The neutral pressure at the edge (and divertor) was lower than with D2 by up to a factor of 5. - Measurements using the Fast Scanning Probe as a Mach probe indicate edge flows similar to D2; they exist at low density and disappear as the density is increased. Spectra were obtained from 200 nm to 1 micron at three different densities. A number of different spectra were also obtained with the McPherson VUV spectrograph looking at the x-point. At the end of the run the McPherson was scanned poloidally up from the x-point for further comparisons with D2 plasmas. The safety review for the boronization system was held on Wednesday. The boronization PLC program has now been loaded and debugging of the program will begin this week with helium as a test gas. The Paragon display screen is also up and running. Procedures for installing the diborane bottle, purging the gas system, boronization, and handling emergency conditions are in the final review stage. The boronization mini-proposal has been written. The new switchgear for the third and fourth ICRF transmitters has been installed in the high-yard. The new dummy load for these units has also arrived on-site. Miklos Porkolab, Ian Hutchinson, and Earl Marmar attended the meeting of the FEAC Strategic Planning Subcommittee at PPPL last week. Porkolab and Hutchinson made presentations on the ALcator C-MOD program. Dr. Mike Knotek, head of the FEAC Strategic Planning Subcommittee, visited the PFC on Friday. He observed C-MOD operation, toured the facility, and held discussions with Alcator scientists and students. Joel Hosea of PPPL also visited C-MOD on Friday for discussions on the ICRF collaboration between MIT and PPPL.