Séminaires et colloques

[Postdoc seminar] Modelling and measurement of dielectrics electron emission and of multipactor phenomenon in presence of magnetic fields for the controlled nuclear fusion and spatial fields

by Adrien Plaçais (LPSC)

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The magnetic confinement fusion in Tokamak-like reactors and the spatial field with satellites payloads share the use of High-Frequency (HF) and high power (kilowatt to megawatt) vacuum systems. The transmissible power of such systems P_max is limited by the multipactor effect, which appears when the electrons in the system enter in resonance with the HF signal. If their energy is high enough, other electrons can be extracted from the walls and the electrons population can grow exponentially. The created electron cloud can disturb the transmitted signal, outgas the surfaces or provoke an electric arc.

Even if the multipactor is well understood in the simplest configurations (e.g. : metallic rectangular waveguides), its prediction is more complex in advanced scenarios, in particular when dielectrics and/or magnetic fields are present. These two parameters influence the electrons trajectories as well as the electron emission phenomenon at the origin of the electron population augmentation. The goal of this PhD is to model and understand the multipactor apparition in these complex configurations. In this framework, I developed the model POTOMAC (Physical simulatiOn TOol for Multipactor in Advanced Configurations).

Thanks to POTOMAC, I observed the tremendous effect of the electron emission model on the multipactor apparition, especially when dielectrics are present. In order to model realistic geometries, I proposed an extension of Dionne model to three dimensions, which is the only electron emission model that can take into account the influence of the charge deposited on the surface of dielectrics. Finally, I showed that the accumulation of charges on the surface of such materials had a tremendous effect on the multipactor apparition, by advantaging or disadvantaging some multipactor modes. This influence was partially checked experimentally, in collaboration with an industrial partner.