Theory of the quasioptical electron cyclotron maser

Abstract

In this work a promising new electron cyclotron maser oscillator is proposed and analyzed. The configuration utilizes an open resonator cavity containing a gyrating electron beam which translates along an external magnetic field. The magnetic field is directed perpendicular to the axis of symmetry of the open resonator. Because the wave-particle interaction volume is extremely large, the total input electron beam power can be high and the power density low. This configuration has a number of potentially very attractive features. Among them are (i) high operating radiaion power levels (>MWs); (ii) high efficiency operation (>45%); (iii) naturally suited to short wavelength operation ($\lambda <2\mathrm{}$ mm); (iv) operates efficiently at low electron beam voltage (10-100 keV); (v) natural transverse mode selectivity; (vi) moderate insensitivity to beam temperature effects. The nonlinear interaction between the electrons and resonator field have been analyzed and an expression for the steady-state efficiency obtained. The expression for efficiency has a rather simple analytic form when the amplitude of the resonator fields are small, i.e., small-signal regime. In this case, the efficiency is an essentially odd, nonoscillating function of the frequency mismatch (difference between resonator frequency and relativistic cyclotron frequency). For a uniform external magnetic field, total efficiencies in excess of 30% can be realized. We have considered enhancing the efficiencies by spatially contouring the magnetic field. Appropriately contouring the magnetic field across the resonator by ∼5% increases the efficiency from ∼35 to ∼45%.