Alcator C-Mod Weekly Highlights August 18, 2003 Work on ICRF and lower hybrid systems continued this week. The vessel was brought up to nitrogen so that maintenance could be performed on some vacuum components. Physics ------- Daniel Andelin, a National Undergraduate Fusion Fellow from Brigham Young University, completed his 8 week visit at C-Mod working under the supervision of John Rice, with help from Martin Greenwald. He worked on the creation of a database to study ICRF absorption under a variety of H-Mode conditions in C-Mod. The technique used to evaluate absorption involves examining the time derivative of stored energy in the plasma, derived using EFIT, shortly after fast transitions of rf power. The results show possible trends with plasma temperature and density, and the database should prove useful for guiding future studies. Operations ---------- The vessel was vented to atmospheric pressure with nitrogen gas on Thursday morning. Two leaks that had been temporarily fixed during the run campaign have now been permanently fixed. Maintenance on ionization gauges, gatevalves, and other vacuum components is also being performed. Invessel pictures were taken using a borescope and digital camera. High-pots of the alternator stator have been completed successfully with very little leakage current. Inspections of the alternator bearings indicate no detectable changes since the last inspection and that they are still in excellent condition. ICRF Systems ------------ We continue preparations for operation of FMIT #3 and #4 at 50 MHz. Changes required to the dummy load design have been finalized. A model for the J-Port antenna loops in vacuum at 50 MHz has been developed from the simulation results. The model shows that the required changes in length needed for 50 MHz operation can be easily implemented. Modifications to alter the FMIT #2 phase shifter/stub tuner end plates for better containment of the insulating gas were completed. This change should eliminate trace amounts of SF6 in the cell during operations. Lower Hybrid Systems -------------------- Polishing and testing of the forward waveguide components continued last week with very good results. Losses below 0.5 db are expected from all forward waveguide channels, with most below 0.4 db. Assembly of the waveguide stacks will begin this week (2 stacks of 48 guides each). The stacks will be shipped to MIT for high power testing within the next two weeks. Work also continued on developing techniques to remove the vacuum windows from the lower hybrid couplers. Several techniques including chemical etching and etching followed by mechanical stressing of the windows are being pursued. Simulations of the coupler during the brazing operation are being carried out to model stresses on the windows. These simulations indicate that small modifications to the coupler body will eliminate chipping seen on the outer waveguide windows during earlier brazing operations. Brazing tests conducted at one of our vendors and at PPPL have quantified differences in the coefficient of thermal expansion of the alumina and titanium used in the couplers. These measurements were consistent with values used in the simulations. The prototype circulator installation has been completed at MIT. The circulator will be tested to full power this week. The vendor has sent a representative to witness the test. The high power waveguide run from the cell to the high power testing area has been modified with some new components that replace suspected arc locations. Work continued on the PLC programming needed for analog scaling of the lower hybrid control signals. The printed circuit board layout for the control system test chassis is almost complete. Testing of the SFOL transmitter was completed successfully this week. The SFOL boards are being populated by the vendor and are expected in-house this week. The low power microwave relay chassis has been installed and cabling is being labeled and installed. The heater control chassis for the LH launcher has been installed in the control rack. Travel and Visits ----------------- Huang He, UTexas, was here through Wednesday of last week to help continue the disassembly of the UTexas DNB power system and controls. This work is now complete.