Relativistic quantum kinetic theory of stimulated bremsstrahlung of an axial vacuum mode by an electron beam travelling in a uniform magnetic field
Stimulated emission of ordinary plane-polarized electromagnetic waves propagating in the axial direction by a relativistic or non-relativistic electron beam is investigated using a new quantum kinetics based on the Dirac equation. The momentum and energy conservation laws and the time-reversal invariance of the transition probability are used explicitly to calculate the gain without expanding the transition rate as a power series in ħ. It is found that electroncyclotron masing occurs when the longitudinal temperature differs greatly from the transverse temperature for any beam energy. This electron-cyclotron masing is an individual-electron effect, which is not related at all to the phase bunching.