Synchrotron radiation
A free-electron laser (FEL) is a (fourth generation) synchrotron light source producing extremely brilliant and short pulses of synchrotron radiation. An FEL functions and behaves in many ways like a laser, but instead of using stimulated emission from atomic or molecular excitations, it employs relativistic electrons as a gain medium. Synchrotron radiation is generated as a "bunch" of electrons passes through a magnetic structure (called undulator or wiggler). In an FEL, this radiation is further amplified as the synchrotron radiation re-interacts with the electron bunch such that the electrons start to emit coherently, thus allowing an exponential increase in overall radiation intensity.