Nonlinear Field Effects and Longitudinal Emittance Growth of High Intensity Bunched Beams

For designing high intensity linear accelerators all known effects for emittance increase should be avoided, like mismatch, beam instabilities and reorganization of the particle distribution due to the field energy. In order to have a reasonable accelerating gradient and to avoid instabilities, in many cases the longitudinal acceptance is too small at the beginning. Therefore in high intensity bunched beams particles at the end of the bunch will see strong nonlinear field components. This nonlinearity causes emittance growth but no particle losses. In this paper a model is described by which the longitudinal emittance increase can be understood.