AMO Experiments with Seeded FELs

Free electron lasers (FELs) originated with the idea of John Madey that stimulated emission, normally associated with discrete transitions of bound electrons, can also occur with free electrons undergoing bremsstrahlung, the process responsible for synchrotron radiation [1]. Because the wavelength can be continuously tuned by adjusting the electron beam energy and the magnetic field that causes bremsstrahlung, a characteristic advantage of FELs is their easy, extremely wide-range wavelength tunability, presently ranging from a few μm to below 0.1 nm [2]. Indeed, the first FEL was operated at 3.4 μm and, like a conventional laser, relied on a two-mirror cavity to provide the necessary feedback (i.e., gain) [3]. The long-wavelength range takes the most credit for the remarkable development of FELs (seeded FELs in particular, because of the availability of feedback optics and/or seeding sources) and, in that range, FELs still have some desirable features, such as pulsed operation, compared to laboratory lasers.