Alcator C-Mod Weekly Highlights August 31, 1998 Engineering: Soldering of 17 out of 20 lower TF arms has been completed. The delivery of feltmetal which was initially slowing the soldering process down is no longer a problem, and the soldering is proceeding at a very good rate. We also expect to complete silver plating of the TF core and arms this week. Following cleanup, inspections, and hi-pots, we will be ready to begin machine reassembly. Checkout of all the vessel heaters, thermocouples, and flux loops has been completed. Some minor repairs are in progress as is replacement of insulation over heaters and coils. Our new spring plate design is being finalized even as some less critical production steps are beginning. Tests of the new design confirm both the expected factor of two increase in compliance and the increase in pressure needed for more reliable high field operation. We continue to move forward with testing of the new transmitter control system. The malfunctioning 2274 (the filament showed an open circuit) has been replaced with an older 8973 tube. While the 8973 can not be tuned to quite as high a frequency as the 2274, this tube is adequate to complete tests of the transmitter control system up to the 1 MW level. Full power tests of the transmitters will be possible as soon as one of our 2274 tubes returns from repair. We continue to discuss modifications required to the antenna vacuum flange (J-side). A plan to weld four 20" long, 4" diameter extensions directly to the flange appears to be acceptable. Installation and wiring of the diagnostic neutral beam accelerator power supply control panels are complete except for the final connections to the fast control logic chassis. Boards for control and monitoring of this supply are under construction with some ready for test. A vacuum oil dryer system to condition the oil in the power supplies was completed and installed. Work continued on the fast control logic for the accelerator supply and on construction of the F-Port flange. Software for control and data acquisition for the CXRS spectrometer was completed and successfully tested. We have started moving C-Mod computers out of the MITnet address space and into the new CIDR block in ESnet address space. Name service for this address space has now been established. This change will improve our connectivity to other fusion labs in the US and overseas. Physics: Analysis of CCD images of D_gamma from the divertor region has produced interesting results. These images are inverted to obtain the local emissivity through a method developed by Chris Boswell. In general, D_gamma, in the divertor region, is dominated by recombination, as opposed to excitation radiation. We thus find it to be a measure of the volume recombination of ions and electrons away from the divertor plates. During detachment the recombination region expands from the outer divertor strike point and private flux regions towards the x-point. We have found, that as the density is increased in detachment, this expansion continues. At high enough densities the recombination crosses the separatrix into the core near the x-point. The distance into the core corresponds to less than 1 cm referenced to the midplane. The core Te profile is depressed such that the edge Te, according to ECE, is 20-25 eV approximately 2 cm inside the separatrix, again referenced to the midplane. As the plasma current or input power is increased, the density at which this recombination region crosses the separatrix increase (as does the detachment threshold). Analysis of data from two reciprocating probes show systematic differences during ohmic operation, suggesting a possible poloidal asymmetry in the scrape off layer conditions. The A-port probe (scanning horizontally, 10 cm above the midplane) indicates a much flatter electron temperature profile than the F-port probe (scanning vertically upwards, from near the divertor), reaching a maximum of only 30 eV at the EFIT separatrix. Such low values of electron temperature at the separatrix are inconsistent with power balance estimates based on the standard two-point model. In addition, the radial electric field derived from electron temperature and floating potential profiles appears to not map between the probes, being not more than 500 V/m at the location of the A-port probe, but up to 10kV/m at the location of the F-port probe. The possible influence of a poloidal limiter located near the horizontal probe is being considered. Plans are underway to repeat the measurements with nearly identical probe geometries. During the last week of their visit here at MIT, Pablo Acedo, Horacio Lamela, and Ernesto Garcia continued work on the new differential interferometer as part of our collaboration with Carlos III University, Madrid. Early tests had used separate plate beamsplitter and mirror components to form the interferometer. This technique was good enough for initial tests of the concept and detector, but suffered from more vibrational effects than could be allowed for use on C-Mod where measurements at the 0.001 fringe/mm level are required. Replacing these components with beamsplitting cubes and prisms made a dramatic improvement in the system stability and increased the signal-to-noise by more than a factor of 10. Inserting a shearing plate into the beam allowed quantitative measurements of the phase gradient to be made. This work will be continued at Carlos III with installation of the final instrument on C-Mod expected during the next run campaign. The development of the 2D feedback system that will keep both this system and the tangential two-color system (TTCI) aligned while in operation is progressing. This system uses a piezoelectrically driven mirror mount and a position sensitive detector to control the beam position with a several hundred Hertz bandwidth.