Temporal evolution of electron-bunch length in macropulse of resonator-type free-electron lasers

Microbunching caused by free-electron laser (FEL) interactions in an electron bunch deforms the overall bunch shape. Recent reports indicate the timing of the electron bunch overlapping with the FEL micropulse affects deformation in resonator-type FELs. The electron bunch shape is expected to change with the FEL micropulse energy because the FEL micropulse energy is enhanced within the electron beam macropulse; however, this has not yet been investigated. We evaluated the temporal evolution of the electron bunch length during FEL oscillations by observing the coherent edge radiation generated by the resonator-type FEL device, KU-FEL, Kyoto University. Experimentally verification showed that the expansion and contraction of the electron bunch changed more clearly when the energy and net gain of the FEL micropulse were high. Furthermore, the expansion and contraction behavior was consistent with a one-dimensional FEL simulation based on a well-established unaveraged method.