METHODS FOR CONDITIONING ELECTRON-BEAMS IN FREE-ELECTRON LASERS

The operation of free-electron lasers can be severely limited by the a xial velocity spread of the beam electrons. In this Letter we propose methods for reducing the axial velocity spread in electron beams by re distributing the electron energy via interaction with an axially symme tric, slow, TM waveguide mode. In the first method, the energy redistr ibution is correlated with the electrons' betatron amplitude, while in the second method it is correlated with the electrons' synchrotron am plitude. Reductions by more than a factor of 40 in the rms axial veloc ity spread have been obtained in simulations.