Dynamique des particules tests dans des simulations de magnétohydrodynamique non-linéaire 3D et application à la formation des électrons découplés pendant les disruptions plasma

Abstract : Tokamks are toroidal devices where a hot plasma is magnetically confined in order to produce energy from nuclear fusion reactions. However, several issues have to be resolved before achieving commercial capabilities. Among them are disruptions, which are undesired fast plasma terminations leading to large heat fluxes on plasma facing components (PFC) as well as electromagnetic forces on the machine. In addition disruptions might lead to the formation of Runaway Electrons (RE) which are electrons having an energy high enough that the collision drag does not counterbalance the electric (E) field acceleration. These electrons can therefore reach large kinetic energies (∼10MeV) and tend to form a beam, capable to carry a considerable fraction of the plasma current and, if strikes PFC, to cause substantial melting or sputtering. Therefore, the understanding of RE generation mechanisms is desirable for avoiding their formation. In this perspective, this thesis, investigates the electron dynamics during the disruption thermal quench (TQ) phase (phase characterised by a sudden loss of plasma confinement). In this respect, a relativistic particle tracker is introduced in the JOREK 3D non-linear magnetohydrodynamics (MHD) code and applied to analyse the dynamics of test electrons in a simulation of a JET disruption triggered by massive gas injection. This tracker integrates relativistic particle trajectories, with either a Full Orbit or a Guiding Center approach, in JOREK numerical 3D time-varying fields and it was successfully tested and benchmarked. We firstly study the electron transport caused by the TQ chaotic magnetic field. For doing so, test electron populations are initialised in the pre-TQ with different energies and radial positions, and evolved until the CQ onset. Collisions and inductive E-field are neglected in these simulations. Results show that at least a few percent of the initial population remain confined throughout the TQ, whatever the initial energy. This is due to the fact that closed magnetic flux surfaces reappear promptly after the completely stochastic magnetic field characterising the simulated TQ. These findings seem to support the possibility of the mechanism. Secondly, a possible RE generation during the TQ are investigated. Indeed, the strong TQ MHD activity generates (at least in the simulation) an intense parallel electric field capable of creating RE. This study required the introduction of a drag force modelling the effects of Coulomb collisions. An analysis of the parallel (to the magnetic field) E and of the drag force fields shows that the drag dominates the pre-TQ and CQ phases for 1keV electron populations, i.e. typical pre-TQ thermal electrons, while the E field is predominant during the TQ for energies >∼1keV and during the whole disruptions for energies >10keV. Test particle simulations show, for a population initialised at 1keV, that the TQ E-field accelerates a small number of electrons to energies high enough for later becoming RE due to the CQ E-field, for those of them which reconfined by the reformation of closed flux surfaces. However, results are not in quantitative agreement with the experimental measurements, possibly due to differences between simulated and experimental plasma parameters. For example, higher than realistic JOREK plasma resistivity is found to increase the RE production in these simulations. Finally, first experimental data analyses of the interactions between CQ MHD activity and the RE beam current in ASDEX Upgrade suggest that the RE current decreases with some indicators of the intensity of MHD activity. A possible interpretation is that the MHD activity deconfines the electrons and thereby reduces RE formation.
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Cristian Sommariva. Dynamique des particules tests dans des simulations de magnétohydrodynamique non-linéaire 3D et application à la formation des électrons découplés pendant les disruptions plasma . Plasma Physics [physics.plasm-ph]. Aix Marseille Université; CEA IRFM, 2017. English. ⟨tel-01813986⟩

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