Alcator C-Mod Weekly Highlights June 21, 2004 FY 2004 weeks of operation planned: 18 weeks, completed: 18.9 weeks. We are currently up-to-air with the primary goal of installing the lower hybrid launcher. In addition, a large number of invessel activities are ongoing in support of ICRF and diagnostic systems. Maintenance and upgrades to diagnostic and power systems are also in progress. Invessel Work ------------- The inner wall ultrasonic testing (UT) survey has been completed. Of all 1920 tiles that were inspected only one was found to show delamination and has to be replaced. Modules with loose tiles have been removed to the cleanroom to have the tile mounting screw re-tightened and secured. Removal of the ceiling tile modules also proceeded last week. Some ceiling tiles show delamination and melting at the leading edge from high heat loads most likely applied during upper null operation. UT inspection of lower EF1 conic heat shield began late last week. Lower Hybrid Project -------------------- As we await the new alumina windows needed for the lower hybrid coupler braze, tests using incusil braze were begun. This braze material has a lower melting temperature than the cusil braze used for the primary braze and will allow repairs to be made if required. All of the new reinforcing bars have been installed on the rear waveguide assembly. Waveguide #23, which had previously developed a discharge at the 75 kW level, was successfully tested to full power (100 kW for 0.5 s). Several waveguides were then set up to allow rapid testing. Seven channels were tested on Friday to full power and pulse-width. However, one guide showed high reflected power and tripped protection circuits at very low power levels. This guide appears normal from visual inspection. Borescope and time domain reflectometry measurements will be used this week to help determine the cause of the high reflected power. Two more of the new klystron solenoid power supplies have been installed and tested. We will begin replacement of the klystron vac-ion power supplies this week. Work continues on improvements and refinements to the amplitude and phase control system software and hardware. ICRF Systems ------------ The installation and calibration of the digital tuning indicator upgrade is complete. Inspections of the transmitter, crowbar, filament power, and soft-start cabinets have also been completed. We have begun verifying the control logic functionality of all the ICRF systems. A new euro card cage is being configured to test and calibrate RF demodulators that will be used for the Fast Ferrite Tuner prototype. Power Systems ------------- Ninety out of 120 SCR gate drive boards have been removed from TF #3, convertors 5 and 6, for upgrade work. Most of the boards have already been upgraded and are ready to be tested on the bench. This testing process will begin this week. Diagnostics ----------- Calibration of the MSE diagnostic has been completed in its up-to-air state (partially coated plasma facing lens) with linearly polarized light. The system's spatial resolution and its response to unpolarized light have also been measured. The measured pitch-angle is an approximate linear function of the actual polarization direction. Deviations from linearity are of order 0.3 degrees at the system's optical axis and increase to about 1-1.5 degrees at the plasma edge. These deviations have a clear periodicity in cos(2 theta) and cos(4 theta). Numerical simulations of the diagnostic response indicate that such error periodicities could be caused by imperfections in the three internal mirrors. A phase shift generates a cos(4 theta) error term, while unequal reflection coefficients for S- and P-polarized light generates a cos(2theta) error term. The measured amplitude of the cos(4 theta) error term implies a significant mirror phase shift in excess of 10 degrees. The measured amplitude of the cos(2theta) error term implies a small deviation from equal S/P reflectivity, ranging from 0.998 at the optical axis to 0.965 at the plasma edge. Upgrades to the two-color interferometer, including cabling with better rf shielding, new signal monitoring electronics, new vacuum windows, and improvements to the density feedback electronics continued last week. A single chord polarimeter/interferometer is being developed for installation on C-Mod during the next run campaign. This system will determine what minimum Faraday rotations can be measured and also assess the ability of inner wall retro-reflectors to stand up to plasma operation. We expect the metal retro-reflectors to be in house by the end of this month, and orders are being processed for the required optical components. Information from this diagnostic will be used to help design and fabricate the full multi-chord system. Travel and Visits ------------- Dylan Brennan, General Atomics, was at MIT during the past two weeks as part of an on-going collaboration on MHD stability modeling of resistive and neoclassical tearing modes using the Nimrod code. Dylan is analyzing a series of high-beta C-Mod discharges, some of which exhibited beta-limiting m=2/n=1 MHD instabilities. He finds that non-linear coupling of energy from the omnipresent 1/1 mode (sawtoothing discharges) is very important in the dynamics of the 2/1 instability. Dylan will also be examining other discharges in the same run with even higher beta, but which didn't exhibit beta-limiting MHD.