Electromagnetic simulations and properties of the fundamental power couplers for the SNS superconducting cavities

The Spallation Neutron Source (SNS) makes use of superconducting cavities for the acceleration of negative H ions in the main linac. Two types of 6-cell Niobium cavities are used in the superconducting portion of the linac: 33 β=0.61 cavities and 48 β=0.81 cavities. Each cavity is powered via a coaxial fundamental power coupler (FPC) of a simple yet robust design. The electromagnetic design of the main components of that coupler has been modeled and some of those properties have been measured experimentally. Modeling includes impedance matching of the window and of the waveguide to coaxial doorknob transition; coupling of the coupler fields to the cavity fields; and multipacting behavior of the coaxial line and window. Various aspects of design, simulation, and testing on the coupler and cavity are presented.