Microwave Measurements of Energy Lost to Longitudinal Modes by Single Electron Bunches Traversing Periodic Structures

In the design of future linear colliders, it will be important to minimize the loss of beam energy due to the excitation of higher-order modes in the accelerator structure by single bunches of electrons or positrons. This loss is not only detrimental in itself but also gives rise to energy spectrum widening and transverse emittance growth. This paper describes microwave measurements made on disk-loaded and alternating-spoke structures to determine the loss to the longitudinal modes. In these measurements the Gaussian bunch is simulated by a current pulse of the same shape transmitted through the structure on an axial center conductor. Results to date are presented for the total longitudinal loss parameter per period K in volts per picocoulomb.