Microwave Breakdown: Physics and Applications

Doctoral thesis, 2005

Theories for microwave breakdown in gases, with emphasis on Air, are presented in this thesis. Underlying physical processes are described together with models suitable for scientific research and technical development. Application of the models, both to microwave devices where the breakdown phenomenon limits normal operation and to devices where breakdown is an inherent feature of proper operation, is discussed. Paper A gives analytical and numerical models for breakdown in resonant structures involving inhomogeneous electric fields. The theoretical predictions are verified by experiments. Paper B introduces a model for breakdown in electric fields with stochastically varying amplitude. The stochastic model together with classical deterministic models for pulsed breakdown are used to assess microwave breakdown in multi-carrier and modulated fields. Paper C presents analytical and experimental results on the breakdown properties of narrow metallic slots, with special emphasis on parameters important for protection against high power microwave (HPM) radiation. Paper D examines in detail the concept of effective diffusion length for the determination of breakdown thresholds. The consequences of inhomogeneous electric fields and pressure dependence are covered, together with a discussion on determining the diffusion length from breakdown threshold data. Paper E gives analytical estimates for the nonlinear characteristics of high-Q resonant cavities during breakdown and discharge stages. The results can be used for predicting the behaviour of filter resonators and plasma processing devices. Paper F treats the problem of breakdown around a metal wedge, presenting conditions for localised breakdown not to occur close to the wedge. Conversely, the analysis also indicates when a more thorough analysis is needed to accurately determine the microwave breakdown threshold due to localisation of the breakdown close to the wedge.