Theory of high gain free-electron lasers operating with segmented undulators

The dynamical behavior of high gain free-electron lasers, operating with segmented undulators in the self-amplified spontaneous emission regime, is studied using a set of semianalytical formulas, which reproduce the induced energy spread, the bunching coefficients, and the field evolution along the longitudinal coordinate. The proposed technique is applied to the design of segmented free-electron laser devices, employing undulators with helical or linear polarization. The case of biharmonic sections is discussed too, as well as the possibility of extending the analysis to this "exotic" case.