Comparison of Measured and Computed Loss to Parasitic Modes in Cylindrical Cavities with Beam Ports

The importance of parasitic mode losses for the design and operation of electron-positron storage rings is now well recognized. These losses at present set the limit on allowable beam current in the SPEAR II ring under some operating conditions. Parasitic mode losses and their potential deleterious effects are a prime consideration in the design of the PEP vacuum chamber. Too high a loss impedance can lead not only to overheating of individual components but to a reduced threshold for bunch instabilities. It is important therefore to have available adequate measurement and computational methods, both as an aid in the design of specific vacuum chamber components and to provide a better understanding of the nature of the loss impedance.