Alcator C-Mod Weekly Highlights Jan 8, 2001 Last week the C-Mod vacuum vessel was subjected to a standard pre-entry de-boronization procedure consisting of one night of glow discharge cleaning followed by backfilling and an overnight purge with room-air. No diborane was detected at any point during the up-to-air process, and invessel work began on Thursday morning with an inspection and photographic survey. As expected, the D and E-port ICRF antennas, wall components, and most diagnostics were found to be in good condition. Details of observations made of the J-port antenna and MSE diagnostic collection optics are discussed below. Work continued last week on the Lower Hybrid system, upgrades to the RF control system, and general maintenance of engineering systems. Physics -------- More analysis has been carried out on the data from the two-frequency ITB experiments carried out on Alcator C-Mod on December 20, 2000. In order to reliably produce the density barrier mode with off-axis ICRF heating only, it was found that a high enough plasma target density was essential, in this case a core nl = 1.0X10^20. Using the higher target density produces a sustained EDA H-mode phase with an early L->H transition at (0.7-0.8) s, and void of any back (H->L) transitions. It has been suggested that the higher target density effectively dilutes the plasma impurities, leading to less radiation, and thus avoiding a collapse of the H-mode phase. The density increases were quite strong in many of these higher target density plasmas with ne0 (from Thomson scattering) rising to 8 X 10^20. The density profiles as measured from visible bremsstrahlung also exhibited clear peaking. Successful on-axis heating was also carried out with the 70 MHz J-port power during the density barrier phase, at different rf power levels and at different times. Increases in the neutron rate and central Ti were produced by injecting J-port power late in the discharge (1.25 - 1.5 sec) after the density barrier formed. The density increase during barrier formation was found to be arrested in some shots by the injection of the J-port power. Interestingly enough, on other shots the density increase was also found to continue during J-port heating. Thus it is possible that the on-axis heating causes the density barrier to "leak" under certain plasma conditions. A scan of the radial position of the HIREX diagnostic was also carried out to get data for Ti and plasma rotation profiles during on-axis heating of the density barrier mode. The data obtained during this day of running should be adequate to carry out a careful transport analysis in order to determine if heating inside a thermal energy barrier was occurring in these plasmas. Mode-converted ion Bernstein waves (IBW) near the ion-ion hybrid layer between Hydrogen and Helium-3 were again observed with the Phase Contrast Imaging (PCI) system on the 22 December 2000 run. Plasmas were run with high field (5.9 T), low density (ne0 ~ 1 10^20) and low current (400 kA), and a comparable mix of H, He-3 and D in order to have mode conversion occur near the plasma center. Strong signals (5 to 6 times background) were seen for both the D and E port antenna launch (the PCI is directly in front of the E port antenna). rf wave code results from TORIC suggest that the mode-conversion layer and associated electron Landau damping should be more aligned along a vertical chord at lower plasma current, hopefully simplifying the PCI observation, which vertically integrates along 12 channels. Preliminary results indicate that there is still IBW signal in several distinct major radial locations, but possibly more localized than in the high current case from 23 June 2000. General Engineering ------------------- Following warm-up of the machine last week, maintenance of the cryo system was begun. LN2 control valves are being reworked, filters will be checked and cleaned, and the LN2 sump will be opened up and inspected. The LN2 manifold that distributes the liquid nitrogen to the magnets has been checked for loose connections with nothing found. Invessel video is now available via the web. During manned entries either the wireless cameras or the wide angle view can be found at: http://www.psfc.mit.edu/cmod/online/control_room_rv1.html or rtsp://www.psfc.mit.edu/encoder/cmod.rm Pictures from the invessel survey can be found at: http://www.psfc.mit.edu/cmod/operations/EngImages/CmodImages.asp?ref=/INVESSEL/2001/Up_to_air_survey/ ICRF System ----------- Arc sites found on the J-port (four-strap) antenna were consistent with along-field-line paths between exposed metal components. This observation is also consistent with expectations based on video images of the antenna during plasma operation. Changes now being implemented to the boron nitride and moly tiles on the antenna should ameliorate this problem. Impurity injections from these sites had limited coupled rf power during four-strap operation to <= 2.5 MW if plasma disruptions were to be avoided. This level of operation was much better than obtained before the July modifications to the antenna (<=1.3 MW). We are hopeful that changes made during the current up-to-air will allow us to step up to somewhat higher levels of reliably coupled rf power without disruptions. Work was begun last week on installation of digital panel meters for the transmitter tuning circuits. A layout for the interface board required has been produced and should be ready to send out for fabrication soon. Work was also started on design of the remote control and instrumentation of the coax switches and dummy loads. Work continued on the design of the new rf demodulators. Design of a prototype pc board layout will begin soon. The National Instruments PXI hardware for the Fast Ferrite Tuner Control system is now in-house. This equipment will be mounted in a temporary rack early next week to allow setup and software development to begin. Assembly of some of the patching facilities for the test setup have been started. Lower Hybrid System ------------------- Work continued last week on the electronics needed for the LH system. PXI controller equipment ethernet connections have been established and Labview RT as been downloaded to the PXI embedded controller. A test frequency generator virtual instrument has been downloaded and tested in the controller equipment. Various test patching facilities for the PXI controller are being built to facilitate testing of analog and digital input and output circuitry. A test panel has been designed to allow characterization of the vector modulator. An rf test circuit will be built for the vector modulator and other components, and we will use the NI test panels to help characterize its operation. A VSWR circuit for protection of the LH sources was breadboarded and measurements made. These measurements agreed well with what was expected. Work is progressing on the design of the new two channel serial fiber optic transmitter/receiver boards for analog signals to be used in the control system and elsewhere. DNB and Related Diagnostics --------------------------- Glass dust from at least one of the optical components of the MSE diagnostic invessel assembly was found at the bottom of F-port. This damage is under evaluation; some repair work for this diagnostic will be required during the present opening. Visitors and Travel ------------------- Ian Hutchinson has begun a 5 month long sabattical at the Austalian National University. Earl Marmar has taken on his responsibilities while he is away.