Nonlinear theory and experiment of collective free electron lasers

A theoretical and experimental study of the nonlinear performance of a free electron laser (FEL) amplifier operating in the collective (Raman) regime is reported. The FEL generates up to ∼100 kW of rf power at a frequency of 9.3 GHz and an efficiency of ∼10%. Power saturation, efficiency, and synchrotron oscillations are studied as a function of rf input power, electron beam energy, current, wiggler field amplitude, and axial distance within the helical wiggler. The influences of the nonlinear electron motion in the ponderomotive potential and space‐charge waves are studied by measurements of the dependence of gain and efficiency on the initial radiation intensity. Good agreement with a nonlinear theory that takes cognizance of electron trapping in the combined ponderomotive and space‐charge potential well is obtained.