Efficiency of free-electron lasers with a scattered electron beam

The quality of the relativistic electron beam in a free-electron laser is an important factor in determining its efficiency of radiation production. In this work, the problem of beam quality is addressed by considering an electron beam that has a scattered velocity distribution but is otherwise monoenergetic. A newly developed particle-in-cell plasma simulation code is used to study the nonlinear stage of the free-electron laser; in particular, the efficiency of energy extraction is determined as a function of the amount of beam scattering. It is found that the mean scattering angle of the electron beam of a free-electron laser in the Raman regime is a much more sensitive parameter than expected. The efficiency of radiation production decreases rapidly for electron beams with finite scattering angles.