Radiation Focusing in the Cyclotron Autoresonance Maser

In the cyclotron autoresonance maser, an electromagnetic wave is amplified by interaction with an electron beam. Because of the copropagating electron beam, the refractive index seen by the wave is modified from the vacuum index and the dielectric properties of the electron beam can alter the propagation of the radiation beam. This phenomenon is studied through the use of a source‐dependent modal expansion of the fully three‐dimensional radiation field. In the exponential gain regime, the natural tendency of the radiation beam to spread diffractively is overcome and the beam is focused. The nature of the focusing is found to depend on the relative magnitude of the Doppler‐shifted wave frequency and the gyrofrequency.