Determination of vacuum pumping characteristics for an evacuated ECH circular waveguide system

The Electron Cyclotron Heating (ECH) waveguide transmission line, which is used to launch microwave power into the Advanced Toroidal Facility (ATF) plasma confinement experiment, was studied to determine the need for pumping along its length. Several constraints are imposed on the system; the two main constraints are: (1) gas released to the plasma device and (2) the maximum pressure allowed in the waveguide before gas breakdown occurs. The computer program Vacuum System Design-II was used to model the various vacuum components of the existing systems and to determine the gas flow vs. pressure. In addition, outgassing and a constant leak were used to realistically model the gas flow characteristics of the complete system. Pumping must be accomplished via small perforations in the waveguide wall to minimize microwave leakage and maintain the mode purity of the waves. Because the diameter of the perforations must be smaller than the screen thickness to prevent microwave power loss, each perforation was modeled as a long tube. The conductance, of this screen was found to reduce (about 30%) the effectiveness of the pump. In addition, the waveguide presents a significant reduction in the system conductance thereby increasing the pressure difference between the pump and the end of the waveguide. Several cases were run using various waveguide lengths and component configurations. Results of these cases are presented and conclusions made.