Spoke Characterization in Re-Entrant Backward Wave Crossed-Field Amplifiers via Simulation

The electron spokes for stable operation in the L-4953 crossed-field amplifier (CFA) from Stellant Systems are characterized via particle-in-cell (PIC) code simulations in VSim (TechX). The CFA is a pulsed, re-entrant, backward wave device with a peak output power {P}_{\text {out}}^{\text {peak}}\approx {5}~\text {MW} , gain {G}\approx {11.2}~\text {dB} , and a bandwidth from {f}={1.28}~\text {to}~{1.35}~\text {GHz} . Spatial, spatial-temporal, spatial-spectral, and spatial-tonal visualizations of the electron density in both the lab and rotating frames of reference were developed to characterize the spokes. The spatial visualizations for simulations at the operation frequency {f}_{\text {op}}={1.3}~\text {GHz} identify the Brillouin and cycloidal electron flow within the spoke for space-charge limited emission (SCLE) and non-SCLE, respectively. The spatial-temporal/spectral visualizations identify an oscillation (transit wobble) within the spoke at a third of the operation frequency \text {(}{f}_{\text {transit}}={f}_{\text {op}}/{3}={0.43}~\text {GHz}\text {)} caused by the spoke transiting 360° in the re-entrant design. This fundamental spoke oscillation, though, does not directly appear on the output. However, a much lower amplitude oscillation at {f}_{\text {sideband}}={0.18}~\text {GHz} identified on the front of the spoke directly appears as sidebands on the output. While the mechanisms that initiate sideband oscillations within the spoke and generate noise on the output are relatively unknown, spatial-tonal visualizations show that the phase difference of the oscillations between spokes is critical to this process. The ability of the visualizations to directly connect oscillations within the spoke to the output is demonstrated, but strategies are still needed to interpret the visualizations to identify and characterize noise generation mechanisms.