Efficiency optimization in a CARM amplifier

Computer simulations were used to examine the detailed physics of magnetic field tapering and frequency detuning in a cyclotron autoresonance maser (CARM) amplifier and the dependence of its efficiency on electron beam current and input wave intensity. It was found that, depending on the initial frequency mismatch, tapering the magnetic field with either a negative or a positive gradient can enhance the efficiency over that of the resonantly tuned CARM. The ability of tapering to enhance the efficiency depends on the initial frequency mismatch. For a given set of parameters, the efficiency increases with beam current up to a critical current beyond which the efficiency decreases with increasing current. The decrease in efficiency results from the dominance of force bunching over inertial bunching in the electron-wave interaction. In the force-bunching regime, neither magnetic field tapering nor frequency detuning is effective in enhancing the efficiency.<>