Coherent transition radiation diagnosis of electron beam microbunching

The action of the free-electron laser instability (FEL) on an electron beam produces a longitudinal density modulation with a periodicity near the resonant radiation wavelength. This modulation, which can produce femtosecond or shorter microbunches inside of a macroscopic picosecond electron pulse, has been proposed for use as a prebunching injector for both higher harmonic FELs and short wavelength accelerators. Standard methods involving streak cameras or beam sweeping with dipole mode cavities will certainly fail to provide information about this longitudinal microstructure, however. In this paper we explore the use of coherent transition radiation generated from a foil at the exit of an FEL undulator to diagnose both the longitudinal and transverse electron microbunch structure.