Methods for conditioning electron beams in free-electron lasers

The operation of free-electron lasers can be severely limited by the axial velocity spread of the beam electrons. In this Letter we propose methods 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 the first method, the energy redistribution is correlated with the electrons' betatron amplitude, while in the second method it is correlated with the electrons' synchrotron amplitude. Reductions by more than a factor of 40 in the rms axial velocity spread have been obtained in simulations.