Alcator C-Mod Weekly Highlights Feb 09, 2004 Last week was a scheduled maintenance week at Alcator C-Mod. No run days were scheduled. Research operations are planned to resume this week. Physics ------- The FRCECE quasi-optical antenna system has been modeled using a ray-tracing method. Results from a high density, strongly peaked ITB at 5.4 T show that the FRCECE below midplane viewing geometry results in the measured emission being particularly sensitive to strong refractive effects for near cutoff densities. The modeled refractive effects can explain certain unusual features observed in the data, including a core mode at ~ 80 kHz that could result from small (~ 0.5%) density oscillations. Initial comparison with PCI density fluctuation data shows a feature with a time behavior similar to the observed FRCECE mode and an unusual feature in the f vs. kR analysis that may be related. Further analysis and comparisons with PCI are planned. ICRF System ----------- We have completed the replacement of the control wiring in the FMIT#4 crowbar cabinet. The shorting bar has been repaired and installed. The individual components and standoffs were high potted, the replacement wiring verified and a functional test of the control logic was completed. The new 65 A fuses and holders arrived . The holders have been installed, but high potting was delayed due to weather. In addition to high potting, the fuses need to be installed and the grounding straps removed. The remaining crowbar cabinet high voltage capacitors need to be installed and high potted. Final crowbar functionality and coordination between the vacuum breaker and the transmitter controls need to be verified. FMIT#3 FPA output cavity was disassembled to inspect the ferrites used to damp high frequency oscillations in the FPA. The ferrite spokes were realigned and securely mounted. The spokes were modified to improve position reliability and repeatability. We expect to return the FMIT#3 and 4 transmitters to service this week. The E-port antenna matching network was reassembled back in its standard resonant loop configuration. Lower Hybrid System ------------------- Tests of the circulators were carried out with the particpation of an engineer from the vendor. Two circulators are now fully tested to 250 kW, 5s pulse length, and a third was tested up to 250kW for 100 ms pulse length. In all tests the circulators performed well. The latest 3-window brazing test with copper plated waveguide (.0002") worked extremely well. We had good wetting of the surface, with no voids or cracks, and the window was leak tight. This week tests in which both the waveguide and the bricks are copper plated will be conducted. The plating thickness on the waveguide will also be increased. Diagnostics ----------- The FRCECE receiver, which has been out-of-service for the past several weeks, was delivered to UT following repair. Additional protection circuits were added last week. Alan Lynn (UT-FRC) will reinstall it at C-Mod this weekend, and will conduct tests early this week. All components except one stainless steel tube have arrived for the manufacture of a replacement DNB liquid helium fill tube. A solid model has been drawn and the final design of components is near completion. Some small stainless mating components will need to be machined, and then the new fill tube can be welded together. In the meantime, the beam is operating reliably into plasma, using only the turbopump system. Travel and Visitors ------------------- David Mikkelsen (PPPL) spent the week at MIT searching for recent C-Mod plasmas with density profiles suitable for fluctuation studies with the 132 GHz reflectometer fluctuation diagnostic operated by Nils Basse. L-mode plasmas from the January 6, 2004 experiment (MP# 363) met the requirements, and the signal exhibits a wide variety of spectral shapes for these plasmas. Significant scattered power is present to at least 400 kHz (on both sides of the zero frequency peak); the spectra are sometimes - but not always - asymmetric. Collection of specimens will continue along with preliminary attempts at classification. Robert Granetz attended a trio of MHD-related workshops and meetings held in Naka, Japan this past week. The ITPA MHD/Disruptions/Control meeting was primarily devoted to reviewing the initial drafts for chapters 3 and 8 of the ITER Tokamak Physics Basis document. Preceding that meeting were two other workshops: the US/Japan MHD workshop, and the Large Tokamak workshop on current holes. At the MHD workshop, recent C-Mod work on locked modes was presented.