The electronic conversion efficiency of a free-electron laser using an intense relativistic electron beam in a rippled magnetic field as its active medium is limited mainly by the destruction of the coherent slow space-charge wave through particle trapping and the depletion of the available beam energy. It is shown that by proper stepping up of the ripple magnetic field strength prior to electrostatic wave saturation, the efficiency can be substantially increased along with some bandwidth improvement and shortening of the device length.

• Received 17 January 1979

DOI:https://doi.org/10.1103/PhysRevLett.42.1670