Gain measurements on a waveguide FEL amplifier with pre-bunched electron beam

A recently proposed theory suggests that a synchronous, pre-bunched electron beam should amplify radiation with a power gain which is inversely proportional to the square root of the input power. We have measured the power gain experimentally for a waveguide FEL system using a 1 mA nominal low-voltage (55.5 kV) electron beam, pre-bunched in a waveguide cavity. The gain in the wiggler has been observed as a function of the beam energy, beam current and wiggler waveguide input power; and good agreement with theory was obtained.