Nonlinear Power Saturation and Phase (Wave Refractive Index) in a Collective Free-Electron Laser Amplifier

We report measurements of the nonlinear radiation intensity and phase (wave refractive index) of a free-electron laser amplifier operating in the collective (Raman) regime. The laser generates up to {~} 100 kW of rf power at a frequency of 9.3 GHz and with an efficiency of {~} 10%. Power saturation, efficiency, synchrotron oscillations, and the rf phase are studied as functions of electron beam energy, current, and axial distance within the helical wiggler. Excellent agreement with nonlinear theory that takes cognizance of electron trapping in the combined ponderomotive and space-charge potential well is obtained.