Electron beam quality limitations and beam conditioning in free electron lasers

The operation of free electron lasers can be severely limited by the axial velocity spread of the beam electrons. We propose a method for reducing the axial velocity spread in electron beams by redistributing the electron energy via interaction with an axially symmetric, slow, TM waveguide mode. In this method, the energy redistribution is correlated with the electrons´ betatron amplitude. Reductions of more than a factor of 40 in the rms axial velocity spread have been obtained in simulations