A method to enhance the efficiency of coaxial-cavity gyrotron

To enhance the efficiency of a coaxial-cavity free-electron maser,a method by employing a tapered magnetic field is proposed. Nonlinear simulation indicates that the efficiency of 26.7% measured in the FZK experiment might be enhanced up to 31.5% by a proper taper of the axial magnetic field. The mechanism of the efficiency enhancement is that the tapered magnetic field changes the condition of the electrons' motion and improves the bunching of the electrons in the interaction between the electron beam and the wave.