Comparative analysis of electron beam quality on the operation of a FEM with axial guide magnetic field and a CARM

A theoretical analysis of influence of electron beam quality on the operation of a Free-Electron Masers (FEM) with axial guide magnetic field and a Cyclotron Autoresonance Masers (CARM), is presented. For both FEM and CARM transverse velocity component achieved by transversely driving a rectilinear electron beam. The quality of electron beam depends on amplitude of initial random transverse oscillations of the particles. It is shown, that in a FEM operating in the regime far from cyclotron resonance (including the reversed-guide-field regime) the spread in the averaged axial particle velocities is equal to the spread in the initial beam and proportional to the square of the amplitude of parasitic initial oscillations. However, in a CARM this spread is linearly proportional to the initial transverse velocity and, thus, essentially exceeds the spread in the FEM for the same quality of electron beam. As a result, a drop in CARM efficiency with decline of beam quality is much more dramatic than in the FEM.