MEASUREMENT OF HOMOGENEOUSLY BROADENED SPECTRA OF SPONTANEOUS EMISSION AND SMALL-SIGNAL GAIN IN A LOW-ENERGY, WAVE-GUIDE-MODE FREE-ELECTRON LASER

The spontaneous emission spectrum and the small-signal gain have been measured in a low-energy, waveguide-mode free-electron laser. Measurements are carried out at microwave frequencies (25-40 GHz) in an X-band rectangular waveguide with a low-energy (450-600 keV, γ∼2), low-current (∼1.5 A), low-energy spread (ΔE/E≪10-4) relativistic photoelectron beam. The measured spontaneous emission spectrum agrees well with the homogeneously broadened spectrum of the TE10 waveguide mode. The measured small-signal gain spectrum is in good agreement with the theoretical prediction calculated under the assumption of operation in the low-gain Compton regime.