Measurements of amplification and phase shift (wave refractive index) in a free-electron laser

Measurements of the amplification and phase shifts (wave refractive index) in a free-electron laser (FEL) are reported. The studies have been carried out at microwave frequencies (7–16 GHz) in a free-electron laser operating in the collective (Raman) regime, using a mildly relativistic electron beam with energy of ∼160 keV, and current of ∼5 A. The observations are found to be in excellent agreement with theoretical predictions based on the full three-dimensional FEL dispersion relation.