Analysis of multipactor effect in partially dielectric-loaded rectangular waveguides

Resumen

The multipactor effect is a physical phenomenon that is triggered when free electrons, which may be generated in space-borne equipment by cosmic radiation, are subject to electromagnetic fields strong enough to lift electrons from the surface walls of physical devices. Given vacuum conditions, the free electrons can be accelerated rapidly from a surface wall of the device to another because there are no gas particles for the electrons to collide with and be slowed down. Depending on their energy, angle of impact, and the secondary emission characteristics of the wall surface, these impacting electrons can cause secondary electrons to be emitted from the wall. If the radio-frequency field changes phase at the time of collision, it can accelerate these secondary electrons towards the opposite device wall, generating more secondary electrons and enabling exponential growth in the electron population. This build-up, known as a radio-frequency discharge, or the multipactor effect, can be sufficiently large to reflect the incident power, and even damage the device or system involved. The multipactor phenomenon has been studied within several fields in the past. Thus, particle physicists have studied the phenomenon in relation to plasma science and particle beam dynamics, while engineers have used the phenomenon to amplify signals in vacuum tubes and klystrons. Therefore, depending on the situation, the multipactor phenomenon can be viewed either as a valuable tool or an undesirable effect. The current research work focuses on the effects of multipactor breakdown in radio-frequency devices on board space vehicles (e.g. satellites), where it is viewed as an undesirable phenomenon because it can cause irreparable damage to these devices, rendering them unusable. In the context of this research, typical radio-frequency devices used in space applications include waveguides, filters, and multiplexers. While the power handling of these devices has increased in recent years, so has their geometric complexity. Because most multipactor models make some simplifying assumptions, not least about the geometries involved, current models of multipactor are becoming increasingly inaccurate and inefficient as devices become more complex. In the first part of this thesis, a rigorous study of the multipactor effect in a partially dielectric-loaded rectangular waveguide is presented. An efficient multipactor model is developed that could, eventually, analyse the increasingly complex structures used in the space industry. To produce the simulations presented in this thesis, a detailed analysis of the electron dynamics inside this form of waveguide has been performed, taking into account the radio-frequency electromagnetic fields propagating in the waveguide and the electrostatic field that appears because of the charging of the dielectric layer therein. The characterization of this electrostatic field is obtained by computing the electric potential produced by an arbitrary charge distribution on the dielectric layer in a dielectric-loaded waveguide; by numerically solving the equations of motion, the electron trajectories are obtained. Another important element of the electron emission process is the secondary emission yield; defined as the number of secondary electrons emitted per incident electron and being material-specific, it is one of the main drivers of the multipactor effect. A number of different alternatives for modelling the secondary emission yield have also been studied in this thesis. The second part of the thesis has involved detailed numerical simulations of multipactor discharges inside several configurations of rectangular waveguide containing dielectric materials. These simulations have been carried out using bespoke code specifically developed for the purpose in the course of the research work presented here. The results obtained have been validated with real measurements, carried out in the laboratory, on devices manufactured for this purpose.