Deformation of an electron bunch caused by free-electron lasers

This study reported the longitudinal compression and extension of an electron bunch by free electron laser (FEL) interaction. By observing coherent edge radiation generated by an FEL facility at Kyoto University, which has the highest extraction efficiency of an FEL oscillation, we found that an electron bunch interacting with a cavity-type linac FEL elongated when a detuning length of an optical cavity was positive. The transition from compression to extension of the electron bunch was bounded by a perfect synchronism condition between the electron bunch and the FEL pulse, and the change rate of the root-mean-square bunch length was as high as 14%. Therefore, it was experimentally demonstrated that a slight temporal difference between the electron bunch and the optical pulse dramatically changed not only the FEL power but also the outline of electron distribution in the bunch.