Gain physics of rf‐linac‐driven XUV free‐electron lasers

In an rf‐linac‐driven XUV free‐electron laser oscillator, the gain depends on the details of the shape of the electron beam’s phase‐space distribution, particularly the distribution of electrons in the transverse (to the direction of propagation) position and velocity coordinates. This strong dependence occurs because the gain in this device is inhomogeneously broadened. Our previous theoretical studies have assumed that the transverse phase space distribution is a product of uncorrelated Gaussian functions. In the present work, we present the results of a theoretical study of the gain for non‐Gaussian phase‐space distributions. Such distributions arise either from a better representation of the electron beam from an rf‐linac or from an emittance filter applied to the beam after the linac.