  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. Here, we report the first observation of the laser-beam
interaction in a pure dipole magnet, in which the electron beam energy
modulation with 40-keV amplitude and 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.
