Resonant electromagnetic structures are vitally important in engineering and scientific applications, ranging from devices as ubiquitous as antennas and microwave ovens, to devices as demanding as high-power microwave sources and particle accelerator components. As we push the limits on the design and operation of such structures, one of the physical limitations that we must contend with is electrical breakdown, which becomes increasingly likely as we increase field strength and reduce structure sizes. Multipactor is a type of breakdown in which electromagnetic fields accelerate free electrons into a material, which then ejects secondary electrons which are re-accelerated back into the material, and which sustains or grows the breakdown current over time. We are interested in understanding multipactor better because it is one of the common design constraints for high-power resonant structures around microwave frequencies, such as klystrons, couplers, waveguides, and accelerating cavities used in particle accelerators. Besides being a design constraint, we could also potentially employ the non-linear nature of multipactor to intentionally attenuate sporadic harmful power levels which may affect certain sensitive equipment, such as for the protection of front-end electronics on radio receivers in space-borne applications. This dissertation details the results of numerical study of two-surface multipactor driven by time-harmonic fields, with a specific focus upon how secondary electron emission models can affect the resulting multipactor predictions, and how multipactor susceptibility and trajectories can be affected by the presence of additional modes within a resonant structure. The primary focus is on multipactor occurring between the inner and outer conductors of coaxial geometries, but some parallel plate geometries are also considered. The scope of investigation is limited to the multipactor regime in which space charge effects can be neglected. In practice this means the early-time evolution of multipactor, since it takes some time before space charge effects become significant. Despite this simplifying assumption not being applicable to the late-time behavior of multipactor, this approach still allows for much practical benefit in the understanding of multipactor genesis and controllability, which is frequently the most significant concern of engineering interest.
Read
