Several different mechanisms and schemes for attaining a high trapping fraction by rapid (nonadiabatic) changes in parameters involved in the interaction have been investigated. These nonadiabatic changes were allowed to occur not only at the entrance of the interaction region but also within it. The experimental setup is described, and results for electron trapping at the entrance of the interaction region, induced by the bending radius of the electron trajectory, are discussed. Trapping was detected also within the interaction region by inserting a section of large gradient in the DC axial electric field. The place where resonant condition is attained is controlled by means of the initial acceleration voltage of the electron beam. The electron's energy profile is analyzed by a time-of-flight-type analyzer.