AVIS DE SOUTENANCE
Mademoiselle Eva AL HAJJ SLEIMAN
Soutiendra publiquement ses travaux de thèse intitulés :
« Determination of Operating Margins Related to Multipactor Phenomena for Radio-Frequency Components of Magnetically Confined Controlled Nuclear Fusion Applications»
Soutenance prévue le jeudi 19 octobre 2023 à 10h00
Lieu : IRFM salle René Gravier
et par SKYPE (lien ci-dessous)
Composition du jury proposé :
Mr Benito Gimeno-Martínez, Université de Valencia, Espagne, Rapporteur
Mr Eric Rius, Université de Bretagne, Brest, Rapporteur
Mr Yves Elskens, Université Aix-Marseille, Examinateur
Mr Nicolas Fil, CNES Toulouse, Invité
Mr Thibault Hamelin, CEA/IRFU, Saclay, Examinateur
Mr Walid Helou, ITER, Examinateur
Mr Mohamed Belhaj, ONERA/DPHY, Toulouse, Encadrant
Mme Yolanda Gómez Martínez, LPSC/IN2P3, Grenoble, Examinatrice
Mr Julien Hillairet, CEA Cadarache, Directeur de thèse
Résumé :
High-power radio-frequency (RF) waves are commonly used in a vacuum environment in magnetically confined nuclear fusion research in experimental devices such as tokamak. The power transmission capability of the antennas can be limited by the multipactor phenomenon — an exponential increase in the number of electrons. The multipactor effect generally occurs when the energy of the electrons colliding with the surfaces of the component is high enough to release additional electrons and when the electrons’ motion is synchronised with the phase change of the RF signal. Under these conditions, an electron avalanche phenomenon is created, which can generate RF perturbations, rise in the local temperature of the components leading to a subsequent increase in the pressure due to surface particles desorption, and, if not stopped, eventually trigger a corona discharge or an electric arc in the low-pressure residual gas that can lead to a component partial or total destruction.
The multipactor initiation depends on the RF electric field’s magnitude and frequency within the RF device and the secondary electron emission properties of the materials used, emph{i.e.}, its surface composition, morphology, and history, and the presence of a magnetic field. The prediction of the multipactor is relatively well understood for simple metal geometries but remains uncertain for complex structures with multi-material and complex electric fields, such as RF feed-through composed of dielectric and conductive materials.
RF systems are used for plasma heating on the tokamak WEST, located at CEA-Cadarache in France. In particular, the following phenomenon was observed during experimental campaigns on the three Ion Cyclotron Resonance Heating (ICRH) antennas: when only one antenna is powered, the pressure increases in the antennas which are not powered. The problem is that once the pressure in one antenna exceeds a predefined threshold, the security system prohibits the application of RF power to avoid the generation of RF-induced plasma inside the antenna. The latter affects the operation of the ICRH system. The multipactor phenomenon is a possible cause explaining this pressure rise, and this hypothesis is investigated in this work.
The objective of this work is to model the multipactor phenomenon for realistic RF components subject to standing waves, such as the ICRH antennas of WEST. The study of the physical mechanisms underlying these pressure increases in the antennas aims to determine whether the multipactor is responsible for these measured pressure levels and if it provides an explanation for them.
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