Radiated and nonradiated electromagnetic fields in an FEL amplifier

For a free-electron laser (FEL) amplifier model, which incorporates not only potential but also rotational nonradiated electromagnetic fields, we study the repercussions of account for those fields on the beam-microwave interaction. Equations of excitation for the radiated and nonradiated parts of the driven electromagnetic field are derived. A new analytical technique of approximate integration of the nonradiated part along the interaction region in a regular waveguide is proposed and its accuracy is numerically verified. The investigation shows that the force caused by the nonradiated magnetic field partially suppresses the defocussing influence of the transversal (to the direction of propagation of the electron beam) part of the force caused by the potential part of the nonradiated electric field while its axial component is partially compensated by the force caused by the rotational part of the nonradiated electric field. It is found that the nonradiated electromagnetic field influences weakly on the efficiency of a planar FEL with an axial (guide) magnetic field if the operating parameters correspond to the magnetostatic resonance (undulator frequency is close to cyclotron one), while far away from it the influence is strong. Our analysis also reveals that the maximal output power is attained at substantially higher beam currents than the efficiency.