Predicting twystrode output circuit performance

Summary form only given, as follows. The twystrode employs a helix slow-wave structure together with a pre-bunched beam to generate microwaves. Because the beam can be tightly bunched as it enters the helix region, the conversion of energy to microwaves does not occur in the same manner as for conventional helix TWT´s, since in particular, the linear growth regime is completely absent. MRCs electromagnetic PIC simulation code, MAGIC, has been used to model the twystrode device with several new state-of-the-art techniques. Most important is the Helix Polarizer Model, which permits 2-D simulation of the helical element, based on a sheath helix approximation. Other new simulation techniques include enhanced boundary modeling, improvements to better model three-dimensional structures such as dielectric supports and vanes, and new diagnostics which reveal the evolution of efficiency, harmonic content, backward- and forward-wave components, and beam quality throughout the twystrode. Additionally, an on-line collector-analysis capability provides an estimate of the recoverable energy in the spent beam and suggests optimum collector stage voltages. MAGIC has previously demonstrated good agreement with experimental measurements, and has been instrumental in the design of EGA-IIC, a high-efficiency twystrode experiment incorporating a highly tapered helix (pitch ratio of 3-to-1). Such a device is predicted to permit the slow-wave interaction of the pre-bunched electron beam to proceed into the 50% efficiency range. This paper will present simulation results for the 487 MHz EGA-IIB (an operating experiment using an untapered helix), EGA-IIC (an experiment underway using a tapered helix), and a hypothetical 10-GHz FEA-driven device (motivated by Microwave Power Module (MPM) requirements).