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Dynamics of ion-driven fishbones in tokamaks : theory and nonlinear full scale simulations

Abstract : In tokamak plasmas, fast particles generated by fusion reactions and by non-inductive heating techniques can resonantly interact with Magneto-Hydro-Dynamic (MHD) instabilities, potentially leading to their transport out of the plasma core. This topic is important in the context of burning plasmas, where the collisional relaxation of alpha particles is expected to compensate the energy losses. The resonant transport time of fast particles being much lower than their typical relaxation, these Kinetic-MHD instabilities can adversely impact the plasma energy balance, and therefore the fusion performance of future commercial reactors. In this thesis, we study the interaction of energetic ions with the internal kink mode, resulting in the onset of the fishbone instability. To this end, we use the nonlinear hybrid Kinetic-MHD code XTOR-K to simulate the nonlinear phases of the fishbone instability, during which fast particles are being transported. Firstly, the linear theory of the fishbone instability is re-derived, recovering similar expressions with Porcelli’s dispersion relation. Differences arise when considering deeply passing particles. A linear code has been implemented to solve non-perturbatively the fishbone dispersion relation obtained. Results with this code are consistent with those obtained from XTOR-K linear simulations, in terms of mode growth rates, rotations frequencies and resonant surfaces. This provides a linear verification of XTOR-K, that enables its use on complex equilibria and during nonlinear phases. Secondly, a parametric study is provided by XTOR-K regarding the alpha fishbone linear stability with equilibria relevant for the ITER 15 MA case. Our simulations show that this scenario is likely to be fishbone unstable for ITER relevant alpha particle densities. Finally, nonlinear results obtained with XTOR-K in low energy circular equilibria and in the ITER 15 MA case are presented. These results document the self-consistent dynamics of fast particles and MHD modes during one fishbone oscillation. Resonant fast particle transport is a common feature of these simulations, together with frequency chirping of the Kinetic-MHD mode. Differences between these simulations are discussed, as well as the nonlinear regime characterizing the observed mode based on existing theories. During one fishbone oscillation, in our different simulations, the overall alpha particle transport in the core plasma impacts from 5 to 10% of the initial population, which shows that the reduction of fusion performance due to the alpha-fishbone instability is limited. From these simulations, a mechanism explaining the nonlinear coupling between resonant particle transport and mode chirping is presented.
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Guillaume Brochard. Dynamics of ion-driven fishbones in tokamaks : theory and nonlinear full scale simulations. Plasma Physics [physics.plasm-ph]. Institut Polytechnique de Paris, 2019. English. ⟨NNT : 2019IPPAX002⟩. ⟨tel-02495955⟩

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