Observation of self-amplified spontaneous-emission-induced electron-beam microbunching using coherent transition radiation

We report the measurement of electron-beam microbunching at the exit of a s elf-amplified spontaneous-emission free-electron laser (SASE FEL), by obser vation of coherent transition radiation (CTR). The CTR was found to have an angular spectrum much narrower than spontaneous transition radiation and a narrow-band frequency spectrum. The central frequency of the fundamental C TR spectrum is found to differ slightly from that of the SASE, a finding in disagreement with previously invoked CTR theory. The CTR measurement estab lishes the uniformity of microbunching in the transverse dimension, indicat ing the SASE FEL operates in a dominant transverse mode.