Alcator C-Mod Weekly Highlights September 8, 1998 Engineering: We continue to solder feltmetal to the TF arm fingers. We have completed this work on all the lower arms. Thirteen out of twenty upper arms have also been completed. Out of the several hundred feltmetal pads soldered on thus far, more than 90% have passed inspection; a small number will need reworking. All silver plating on the TF core has been completed. However, during the last stages of the plating activity a small quantity of liquid found its way into some core cooling passages. Reassembly of the machine has been delayed as we flush and dry out these passages. We have begun the cleanup process of the core fingers, insulation, cooling passages, OH coax connections, and inner TF core bore. Following cleanup and inspection, we will begin resistance checks and hi-pots of the TF core and OH coil connections. We continued work on the DNB accelerator supply. Testing of the power supply control panels is nearly complete. Boards for control and monitoring of this supply are now undergoing testing. Initial tests of the vacuum oil dryer system indicated a minor problem in the control logic which will be corrected next week. Work continued on the fast control logic for the accelerator supply and on construction of the F-Port flange. Preparation for the installation of the in-cell low capacitance transmission line was begun with completion of the design for the last transition box. We continue testing the control system for FMIT#3 and FMIT#4 at low power. Steve Wukitch visited PPPL last Friday to discuss the J-port antenna and RF physics plans for the upcoming campaign. We discussed the modified flange cover drawing and the new design looks very good. A computer model of the resonant loops has been created by Gerd Schilling, PPPL, which agrees with test stand results. The pacing item for installation is the Faraday screen which is projected to be ready by the beginning of October. Once the screen is ready, the antenna needs to be assembled on the test stand with all its components including the bolts needed to align Faraday shield elements within modules. The antenna will be shipped to MIT with the FS modules assembled. The antenna components will then be baked before installation which is expected near the middle of October. We have received requests from readers of the weekly report who are outside the fusion community for more general information about what C-Mod is all about. A very good place to start learning about C-Mod and plasma physics in general is our PSFC web site at: http://www.psfc.mit.edu Links to C-Mod and other fusion related sites can be found there. Physics: Analysis has been carried out by Joe Snipes of ICRF H-modes during the current rise to determine how early H-mode could be obtained, and whether or not the H-mode threshold is increased by an upward current ramp. Several current rise H-modes were seen during runs in which ICRF heating was applied in the current rise in an attempt to obtain reversed shear conditions. The earliest H-mode found had an L-H transition at about 0.24 sec. However, the global power threshold during the current rise was as low as P_th/(nebar*B_T*S) = 0.037, which is higher than the typically low C-Mod values of 0.022, but still lower than the ASDEX-Upgrade values of 0.044. A systematic power scan needs to be done during the current rise to be sure that these are indeed the lowest threshold values. These examples of current rise H-modes show that it is possible to obtain H-modes quite early in C-Mod discharges. They provide a good starting condition for the studies of confinement and fueling in the current rise for comparison with various ignition scenarios. A technique has been developed for using neutral molecules and atoms to filter out the neutral deuterium atom flux from the plasma from being measured by the bolometers. This technique, implemented by Spencer Pitcher and John Goetz, is based on increasing the neutral pressure inside the bolometer camera housing until the neutral-neutral mean free path is short compared to the length of the housing. Since the bolometers are sensitive to both neutral and photon fluxes from the plasma this enables us to filter out the neutral component for portions of the shot. We have used no-plasma shots to measure the cooling effect of the injected neutrals on the bolometer. This cooling or pickup shows up as negative power and is very repeatable. It is due to the conduction of heat from the camera housing tho the bolometer foil which is of order 20 degrees C hotter. We find that the neutral component can be as large as 30% of the measured bolometer power in the divertor. The percentage does not seem to be strongly affected by density variation or detachment. We have some limited data with H-modes which shows a similar effect. The neutral component location does seem to vary. The observations made using the divertor viewing, filtered TV have shown that some high density, detached discharges have a region of strong volume recombination just above the X-point, inside the last closed flux surface. This means that temperatures of ~1eV exist on closed flux surfaces. This recombining region maps to about 5 mm inside the separatrix at the midplane. Evidence from other diagnostics support the observation of a very cold region at and above the X-point in these discharges. The VUV spectrometer, viewing across this region, observes emission lines from O+5, N+3, N+2, N+, and D0. The time histories of those lines is consistent with a stong cooling above the X-point to a temperature at which deuterium atoms exist (by recombination). It is also observed from the midplane ECE emission that the temperature ~2 cm inside the separatrix at the outer midplane falls to ~30 eV when this occurs. Thus a temperature gradient is sustained along closed flux lines. However we calculate that only a modest amount of radiational cooling (~200 kW) is required along the appropriate flux tube to maintain such a gradient. Joaquin Sanchez, Asociacion EURATOM-CIEMAT, completed his four week visit with us last week. He has worked closely with us on hardware issues involving the C-Mod double sideband reflectometer. He has also learned all the details involved in running and modifying our 2D full-wave reflectometry code. This code will be used to model the reflectometer system coming on line at CIEMAT on the the TJ-II sellarator. Joaquin also plans to simulate reflectometer signals returned from turbulent plasmas.