Radiation guiding and efficiency enhancement in the cyclotron autoresonance maser

The effect of the electron beam on the propagation of the radiation beam in a cyclotron autoresonance maser (CARM) is investigated. It is shown that the natural tendency of the radiation beam to spread diffractively can be overcome and the radiation beam can be focused because of its interaction with the electron beam. The radiation beam can be transported without defocusing following wave saturation only if the Doppler-shifted wave frequency is slightly larger than the gyrofrequency. Efficiencies in excess of 70% can be attained through the use of a tapered magnetic field. The effect of thermal spreads in the pitch angle and energy of the electrons on the guiding of the radiation beam and the efficiency of the CARM is investigated. The CARM is much more sensitive to spreads in pitch angle than to spreads in energy. If the spread in pitch angle becomes too large, typically a few percent, then the instability is quenched.