Alcator C-Mod Weekly Highlights May 24, 1999 Alcator C-Mod resumed plasma operations last week. Three experiments were conducted. One run-day and part of a second were lost due to a malfunction on Tuesday of the solid-state interrupter circuit on one of the OH coils. This was repaired on Wednesday, and operations continued through the remainder of the week. A series of discharges were produced to determine if the regime of strong main chamber-wall recycling persists in very low core plasma density discharges. Specific measurements in these low (<6e19/m^3) plasmas included determining whether the flapper influences midplane neutral pressure and the measurement of parallel flows to the divertor with scanning Mach probes. A comparison to the ionization rate in the main chamber from H-alpha/Ly-alpha measurements was also made. Results indicate that the flapper does not influence the midplane pressure at all at lower densities. It does influence the divertor neutral pressure, although to a lesser degree (~25% drop in divertor pressure when flaps open compared to 50% drop at higher density). Data from the Mach probe and Lyman-alpha profile measurements are being analysed. Thursday's run was devoted to an examination of cold-pulse propagation and non-local transport effects. Ken Gentle (U.Tx.) proposed these experiments, and served as Session Leader. The objective was to inject sufficient quantities of carbon by laser ablation to drive significant, fast reductions in edge temperatues and cause prompt core temperature rises. The effect has been widely observed at low densities; the operating space in CMOD was to be explored. Low power, low density target plasmas were chosen to begin. Both diverted and limited configurations were run, with no significant differences. Weak, but clear evidence of prompt interior heating was obtained. The effect disappears at higher density and is strongest at low currents (low power.) Better, more conclusive experiments will require larger carbon injections: prompt peak radiated power on the 2pi bolometer greater than the 300 kW maximum in this series. Friday's run was dedicated to ohmic H-mode studies, specifically to determination of impurity transport in these discharges in support of MP#249. There were many extended periods of Ohmic ELM-free and EDA H-mode in several of the discharges. There was good correlation between the rotation velocities determined by Doppler shifts and from the fast magnetics. Many of the plasmas had large stored energy increases; shot 28 had a stored energy of 160 kJ, comparable to the highest stored energies in RF-heated H-modes during the current campaign. EDA H-modes were obtained at plasma currents as high as 1.44 MA. Edge Te and x-ray pedestals were measured. The impurity confinement during ELM-free H-modes were much longer than the discharge length, which led to the termination of the H-mode. For EDA Ohmic discharges, the confinement was shorter, but the injection often caused the plasma to return to ELM-free behavior. The main result is that the impurity confinement in Ohmic H-mode plasmas is very similar to ICRF H-mode discharges. Physics and Analysis --------------------- High resolution measurements of radial profiles of deuterium Lyman alpha emission have been performed near the outer midplane in L and H mode discharges. With knowledge of edge electron density and temperature we can determine the ionization profile (and also the neutral density profile). While the ionization profile is relatively broad across the separatrix in L mode, it appears to shift progressively inside the separatrix as the discharge makes a transition to H mode. This shift may contribute to the usual density rise observed during H mode, although further analysis is required in order to fully quantify this change. Ion impurities in the upper divertor scrape off layer plasma have been observed using the Omegatron probe. The Omegatron probe combines a gridded energy analyzer and an ExB ion mass spectrometer. Ion species with mass to charge ratio M/Z=2, 4, 6, 10, and 11 have been observed, with the M/Z=2 intensity always dominant in deuterium plasmas. The intensities of the M/Z=6, 10, and 11 species are larger after a limited plasma discharge than they are after a series of diverted plasma discharges. When present, the intensities of the M/Z=10 and 11 species appear in proportions that match the natural isotopic abundances of boron-10 and boron-11. An increase in the intensity of the M/Z=4 species is observed after puffing helium into a tokamak discharge, either through an inner wall capillary or through the helium probe at the midplane of an outer limiter. During a plasma discharge the evolution of the M/Z=4 intensity follows the inventory of helium injected; this is expected since helium is a recyling impurity. Impurity resonances can be identified unambiguously at intensities down to a hundred nanoamperes, which corresponds to approximately 0.5% of the total current in the collection cavity. Efforts to improve resolution further are underway. Travel and Visitors -------------------- Miklos Porkolab attended the National Academy of Sciences Review in San Diego on May 16-18 and made a presentation,"Wave Propagation and Interaction with Particles in Magnetized Plasmas:RF Heating and Current Drive". He also attended the FESAC meeting at PPPL on May 20,21, where the PPPL Program review was given as well as a review of the three Proof of Principle experiments. Earl Marmar was also at the NRC-NAS review in La Jolla; he gave a presentation titled "Diagnostics and Facilities Needs". Martin Greenwald travelled to PPPL to deliver a seminar to their theory group on C-Mod transport experiments. He took the opportunity to consult with the local staff on various physics issues, software and data sharing, and "Snowmass" planning. As noted above, Prof. Ken Gentle of the University of Texas visited C-Mod this week to direct the cold-pulse experiments.