Alcator C-Mod Weekly Highlights May 10, 1999 Last week was a maintenance week at Alcator C-Mod. No plasma operations were scheduled. This week will also be a maintenance week. Physics and Analysis: --------------------- The high resolution (0.7 mm at plasma edge) toroidal Z-Meter array, which images free-free bremsstrahlung emission from the plasma, has been used to look at EDA H-mode discharges. A clear pedestal in the emission forms, with tanh fits giving the pedestal foot location at or near to ( within about 2 mm) the outboard midplane location of the last closed flux surface derived from EFIT reconstructions. Typical emissivity full-widths range from 4 to 6 mm. Analysis of the rangefinder data indicates a linear trend in differential motion of the inner wall with halo current. We have measured up to 47 um of movement with a halo current of 170 kA. These results are being incorporated into the simulations needed to guide the design of the inner divertor upgrade. ICRF Systems: ------------- As a result the tube failure in FMIT#2 and another suspected failure in FMIT#1 (tests are still ongoing), a review was conducted to investigate whether the tubes are adequately protected and whether our operating practices were a contributing factor in the loss of these two tubes. This review was undertaken in collaboration with PPPL (Chris Brunkhorst, Joel Hosea, Gerd Schilling, and Randy Wilson). As a result of this review, several modifications and additions to the protective circuitry are being undertaken, including installation of an additional "soft-start" resistor, a screen crowbar circuit, and a low-stored energy grid bias regulator circuit. In addition, a circuit was installed to disable the rf drive during a crowbar. These modifications are being made to all four transmitters. We plan to have FMIT#3 and #4, which are connected to the new J-port antenna, prepared for testing early this week with the aim of beginning operation into plasma next week. FMIT#1 has had an 8973 tube installed (should provide ~ 1 MW) and FMIT#2 has had a reconditioned 2274 tube installed (should provide ~2 MW). Both of these units will also operate once all of the above modifications have been made to them. In addition to the ICRF engineering issues, Joel Hosea and Randy Wilson visited to discuss the ICRF priorities for the remainder of the current campaign. Diagnostic Neutral Beam ------------------------ During the past week it was necessary to divert technical and engineering resources from the DNB to the ICRF transmitter modifications. This resulted in a schedule slip of nearly one week for the completion of the DNB. We still anticipate that the DNB will be ready for testing at the commencement of the scheduled outage this summer and that the installation onto the tokamak and final comissioning of the beam will be completed in time for the beginning of the FY 2000 operating campaign. Travel and Vistors ------------------- Marco Brambilla has been visiting the PSFC and working with the Alcator C-Mod group since April 19, 1999. His visit will continue until May 14, 1999. He has been working mostly with Paul Bonoli on the implementation of a new version of the toroidal ICRF code TORIC on our local workstation cluster. The improved TORIC module now computes the electron damping of ion Bernstein waves using a new formulation which relies far less on a numerical broadening (ad hoc) of the ion-ion hybird layer as did the previous version. The code has also been coupled to an ICRF current drive module that was developed at MIT. The new code is being tested and compared with the old code as as well as with experimental data from mode conversion electron heating in C-Mod. Some effort has also been devoted to the implementation of an IDL post-processor for TORIC that can be used both at Garching and MIT. This will facilitate the transfer of future versions of the ICRF code between MIT and the US under the ASDEX Upgrade - US collaboration agreement. Perry Phillips (U. Texas) will be at M.I.T. for the rest of this month to work on the high resolution ECE system.