Analysis and Improvement of Performance Instability in Extended Interaction Klystrons With Random Geometrical Perturbations

This article describes and evaluates the effect of random geometrical perturbations on resonance characteristics and output performance of extended interaction klystrons (EIKs). The perturbations, assuming a normal distribution, will result in random variation in resonance characteristics, including resonant frequency shift, quality factor shift, and field distortion. The results are demonstrated in a 220-GHz extended interaction cavity by combining theory, simulations, and experimental measurements. In addition, 3-D particle-in-cell (PIC) simulation and small-signal theory are employed to gain insights into the output performance instabilities caused by variation in the resonance characteristics. The comprehension of the factors that contribute to performance instability is fundamental to avoid excess costs in the fabrication process. Aided by the small-signal theory code with a calculation speed that is several orders faster than PIC simulation, we address the instability by applying cathode voltage adjustment technology, frequency tuning technology, and multiparameter optimization based on a genetic algorithm. Such analysis and improvement patterns are general for the EIKs operating in the terahertz regime.