Characterization and modelling of the secondary electron emission properties under magnetic field for high power RF systems subject to multipactor effect

Space communication payload as well as magnetic confinement fusion devices, among other applications, are affected by multipactor effect. This undesirable phenomenon can appear inside high frequency (HF) components under vacuum and lead to increase the electron density in the vacuum within the system. Multipactor effect can thus disturb the wave signal and trigger local temperature increases or breakdowns, which can cause components deteriorations or even their destruction. This PhD research aims to improve our understanding and the prediction of the multipactor effect. The multipactor phenomenon is a resonant process which can appear above a certain RF power threshold. To determine this power threshold, experimental tests or/and simulations are commonly used. The power threshold is closely related to the electron emission properties of the RF component materials, in particular, to the Total Electron Emission Yield (TEEY). Therefore, we have made a study to evaluate the multipactor power threshold sensitivity to the TEEY. Two particular critical parameters have been found: first cross-over energy and the energies between the first cross-over and the maximum energies. Six TEEY models frequently used in multipactor simulations have been benchmarked with respect to their relevance in modelling these two previous regions of interest of the TEEY. it was found that only one TEEY model is suited to predict accurate multipactor power threshold. In some situations, the HF components are submitted to DC magnetic fields which might affect the electron emission properties and hence the multipactor power threshold. Magnetic fields are generated by the coils and the plasma in a tokamak while for space application we use isolators and circulators which are built with permanent magnets. Current multipactor simulation codes don't take into account the effect of the magnetic field on the TEEY. A new experimental setup specially designed to investigate this effect was developed during this work. We aimed to have experimental conditions closed to RF components actual use. Our new experimental setup and the associated TEEY measurement technique were analysed and optimized thanks to measurements and SPIS simulations. SPIS, for Spacecraft Plasma interaction System, is a PIC code which allows us to simulate plasma-matter interactions and in particular to analyse electron trajectories under uniform magnetic field. We used the setup to study the influence of magnetic field perpendicular to the sample surface on the TEEY of copper. We have demonstrated that the magnetic field affects the copper TEEY, and hence multipactor power threshold. it is clearly established that the magnetic field has to be taken into account on both electron emission properties and multipactor simulation. (author)