Microwave Gas and Multipactor Breakdown in Inhomogeneous Fields

Doktorsavhandling, 2012

Microwave gas and multipactor breakdown remains to be one of the limiting factors for the maximum power in microwave devices. Above a certain electric field strength, the so-called breakdown threshold, free electrons can multiply by making ionizing impacts with neutral gas molecules, or causing secondary emission upon impact with system surfaces. The free electron number will rise exponentially, and a number of problems may arise, ranging from noise and changes in the device impedance, to the melting of metal parts and possible destruction of the system. In this thesis we focus our attention at certain aspects of microwave breakdown in satellites and space related systems. This entails air breakdown during testing on ground and ascent, as well as multipactor breakdown in the vacuum of orbit. Our approach to the breakdown problem is a purely theoretical one. Starting from well known physical laws and empirical approximations we apply them to novel systems in an effort to determine the breakdown characteristics. For the case of gas breakdown, our main concern in this thesis has been on analyzing the effect of there being small regions of field enhancement or gas heating inside the microwave system. We try to answer the questions: under what circumstances can small regions of breakdown plasma expand and cause full scale breakdown, and what might be the effect of having local heating of the gas? In the case of multipactor, all our research have sprung from the analysis of a complicated quadri-filar helix antenna. The open geometry, curved surfaces, and large wire separation has lead us to explore an approximate model for the electron dynamics: where the trajectories are dictated by the geometry of the emitting surface, and the action of the ponderomotive force; and the electron impacts can be treated using statistical methods.