Alcator C-Mod Weekly Highlights May 17, 1999 Last week was a maintenance week at Alcator C-Mod. No plasma operation was scheduled. Plasma operations will resume this week. Physics and Analysis -------------------- The impurity injection system is fully operational and running reliably. CaF2 is now the material of choice, with several central and edge charge states available in the VUV and x-ray spectral regions. MIST has been updated to include calcium, and modelling of x-ray spectra shows good agreement with the observations. Time histories of VUV and x-ray lines are also in good agreement with simulations for L-mode discharges. Comparison of the Edge Thomson measured density profiles and scanning F-probe profiles shows good agreement. Some modifications to the Edge TS system are being made. The width of spatial observation range is being increased to get better coverage of temperature pedestal. In addition, the spatial resolution in the density pedestal region will be improved to about 1 mm. The upgraded system should be operational this week. Data obtained with the AXUV edge array have been examined for the various experiments of this run campaign. The array measures the radiated power profile at the edge with a nominal 2mm resolution, and its coverage has been changed from a previous campaign to look further in the plasma. Results now show a pedestal in edge radiation profile during H mode. Standard hyperbolic tangent fit show a full width of 4-5 mm, with a midpoint present approximately 8 mm inside the reconstructed separatrix. Further analysis is proceeding to compare this data with other high resolution pedestal diagnostics, such as the X-ray, edge Thomson, and visible bremsstrahlung emission. ICRF Systems ------------ We have implemented changes to FMIT#3 and FMIT#4 DC power supply circuit as recommended from the review of FMIT transmitters (review was conducted in collaboration with PPPL). This review was undertaken to analyze the safety of the high power vacuum tubes. The recommendations included installation of a second soft start resistor, adding a screen crowbar and low energy storage grid bias regulator, reducing a 150 ohm resistance to 40 ohms, adding an ignitron holding anode power supply, and additional instrumentation. A testing and debugging phase is in progress. Remaining tests include measurement of the anode bus voltage during fast RF interrupts, and operation of the transmitters into a dummy load. Conditioning of the new RF antenna can then proceed. In parallel, parts required for FMIT#1 and #2 have been ordered. Ross dividers for the new instrumentaiton upgrade are being provided by PPPL until new ones are received. Visits and Travel: ------------------ Marco Brambilla (IPP-Garching) ended his one month visit at the PSFC last Friday and has returned to Garching. His trip was very successful given that we implemented a new version of the TORIC ICRF code on the C-Mod computer cluster. This code computes the electron Landau damping of ion Bernstein waves (IBW) without the use of a numerical broadening technique that was used in the previous version of TORIC. The code was also coupled to an ICRF current drive package and an IDL post processor was written for use both at MIT and in Garching. An interesting prediction of the code for off-axis mode conversion in D(He-3) plasmas in C-Mod is that some ELD can be expected near the plasma center in addition to the ELD that is expected off-axis near the ion-ion hybrid layer. This occures because an IBW is also excited on the tokamak LFS where f_o < 2f_cd. A preliminary inspection of old MCEH data by Eric Melby indeed shows some evidence of this secondary on-axis peak in electron damping.