First observation of laser–beam interaction in a dipole magnet

As a new-generation light source,free-electron lasers(FELs)provide high-brightness x-ray pulses at the angstrom-femtosecond space and time scales.The fundamental physics behind the FEL is the interaction between an electromagnetic wave and a relativistic electron beam in an undulator,which consists of hundreds or thousands of dipole magnets with an alternating magnetic field.We report the first observation of the laser–beam interaction in a pure dipole magnet in which the electron beam energy modulation with a 40-ke V amplitude and a 266-nm period is measured.We demonstrate that such an energy modulation can be used to launch a seeded FEL,that is,lasing at the sixth harmonic of the seed laser in a high-gain harmonic generation scheme.The results reveal the most basic process of the FEL lasing and open up a new direction for the study and exploitation of laser–beam interactions.