Practical Design of Emittance Dominated Linear Beams for RF Amplifiers

Abstract

As the frequency of linear beam RF amplifiers increases, the effect of electron beam emittance starts to play a more important role in the design of real world devices. Accurate analytic models and quantitative predictive capability are of particular value when determining the operating parameter regime of a new device or when performing tradeoffs based on specific design criteria. Several considerations relating to emittance effects in the RF amplifiers are presented, and are seen to play a role for thermionic devices operating at millimeter wave frequencies and above, and for field emitter devices as low as microwave frequencies. Existing theory originally derived for particle beams in high-energy physics research is applied specifically to methods and the parameter regimes found in RF amplifier development. Practical enhancements to some expressions allow for accurate prediction of beam behavior without the need for time consuming numerical analysis and optimization. In this vein, beam emittance effects on RF defocusing in the high power energy extraction sections of RF circuits are also analyzed. The general formulation developed here has been validated with numerical optics simulations across a wide range of beam properties typical of those found in devices operating from the microwave to terahertz frequency regimes.