Enhancement of the efficiency of a free-electron laser in a longitudinal magnetic field

An analysis is made of the possible enhancement of the efficiency of a free-electron laser in a longitudinal magnetic field by a reduction in the nonisochronism parameter of the electron oscillations in a wave field. The efficiency enhancement is due to an autoresonance of electrons and a Raman wave which increases both in the absence of profiling of the wiggler parameters in the Compton and Raman regimes and with profiling of the parameters in the trapping and reflection regimes. Numerical calculations confirm that the efficiency tends to increase as the nonisochronism parameter decreases.