Noise Generation Mechanisms in Re-Entrant Backward-Wave Crossed-Field Amplifiers

Crossed-field amplifiers are known for being noisy; however, the mechanism of noise generation remains poorly understood. This article identifies the spurious emission noise generation mechanism by using simulation and spatial–temporal, spatial–spectral, and spatial–tonal visualizations of the electron population. The VSim simulation model is based on the L-4953 crossed-field amplifier (CFA) from Stellant Systems, a ≈ 5-MW peak power, ≈ 11.2-dB, L-band, re-entrant, backward-wave device. The electron spokes are characterized for low-noise, typical stable, and unstable operation, where the spurious emission amplitudes are -47,-37, and -15 dB, respectively. The simulation spectrum for typical stable operation matches the available physical device spectrum data, demonstrating the ability of simulation to model the spurious emission mechanism. Simulations show high sensitivity of the spectrum to operation parameters, where a relatively small shift in dc voltage from 91.6 to 93.1 kV causes a transition from typical stable to low-noise operation. The dispersion of the circuit was calculated and demonstrates the “hard” limit to bandwidth where mode competition occurs. Newly developed electron population visualizations identify noise generation mechanisms well within the band. Using these electron population visualizations, three conclusions are made: phase/frequency modulations (PMs/FMs) of the spokes cause many of the spurious emission sidebands on the output spectrum; the modulations of the spokes only couple to the output if the modulations in each spoke are in phase with each other; and the spurious emission amplitude is higher when the modulations in the spokes are a harmonic of a subharmonic of the transit frequency.