Simulation of a Crossed-Field Amplifier Using a Modulated Distributed Cathode

Current crossed-field amplifiers (CFAs) use a uniformly distributed electron beam, and in this work, the effects of using a spatially and temporally controlled electron source are simulated and studied. Spatial and temporal modulation of the electron source in other microwave vacuum electron devices have shown an increase in gain and efficiency over a continuous current source, and it is expected that similar progress will be made with CFAs. Experimentally, for accurate control over the electron emission profile, integration of gated field emitter arrays (GFEAs) as the distributed electron source in a crossed-field amplifier (CFA) is proposed. Two linear format, 600 and 900 MHz CFAs, which use GFEAs in conjunction with hop funnels as an electron source, were designed, modeled in VSim, and built at BSU. The hop funnels provide a way to control the energy of the electron beam separately from the sole potential and to protect the GFEA cathode. The dispersion of the meandering microstrip line slow wave circuit used in the device and the electron beam characteristics were measured and validated the simulation model, but experiments failed to show electron beam interaction with the electromagnetic wave due to insufficient current from the available cathode. To complete the research, a working CFA built at Northeastern University (NU) was modeled. The NU CFA was a linear format, device operating at 150 MHz, with 10 W of RF input power, and typically 150 mA of injected beam current. The electrically short device (6 slow wave wavelengths long) achieved 7 dB of gain. After validating the Vsim model against the experimental results, an electrically longer version (9 wavelengths) was simulated with both an injected beam and distributed cathode. To model the distributed cathode computationally efficiently, where the emitted electron energy can be controlled separately from the sole potential, a new electron injection method was developed, using a divergence-free region. Static electron emission profiles showed no improvement over the injected beam model but the temporally modulated cathode was found to significantly improve the performance. It was found that the temporal modulation could improve the small-signal-gain from 13 dB for an unmodulated source to 25 dB with an injected current of 150 mA and 0.1 W of RF drive power. This improvement is only likely to be observed for higher power devices (\u3e10 kW) because of the additional RF drive power required by the GFEA, however. For larger RF drive powers, the improvements to gain become much smaller. With an RF drive power of 10 W, the modulated cathode showed 9 dB of gain, and the injected beam variant showed 8 dB. The signal-to-noise ratio (SNR) using the modulated cathode was consistently at least 15 dB higher than the SNR of the unmodulated cathode. This reduces the likelihood of excitation of unwanted modes. Even though this device showed small improvements to gain at large RF drive powers, it is proposed here that improvements to maximum power in higher power devices are likely, due to the inherent mode-locking mechanism of the modulated cathode, but this still needs to be confirmed. Previous research studying the effects of a modulated cathode in a magnetron and the improvements to the SNR shown here, show promise in this regard