Contribution of kinetic electrons in Tokamak plasmas

Abstract : Instabilities, within fusion plasmas by magnetic confinement, develop turbulent structures with milli-centimetric scales. The resulting transport impacts the energy confinement time and, ultimately, the energy performance.In unimproved confinement regimes, ion-scale turbulence generally dominates this transport. This turbulence is carried by the ions, but also by a certain class of electrons, those trapped in the local mirrors of the magnetic field. Take into account their dynamics is important, especially since they are also responsible for particle transport.The aim of this thesis is to study the impact of electrons on the damping of "Geodesic Acoustic Modes" (GAM) on the one hand and the linear growth of the turbulence modes "Ion Temperature Gradients" (ITG) and "Trapped Electron Modes" (TEM) on the other hand.GAMs are oscillations at the acoustic frequency of the average electric potential on magnetic surfaces. They interact critically with micro-turbulence, particularly through their coupling to the motion of energetic particles in the plasma. ITG and TEM represent the 2 classes of dominant electrostatic instabilities in tokamak core plasmas. As such, they are supposed to control turbulent transport in the core.This study is therefore a preliminary step for the prediction of turbulent transport while taking into account the influence of electrons.The appropriate framework for describing this turbulence is the so-called "gyrokinetic" theory, which proceeds from a 6 dimensions (6D) to 4D + 1 phase space reduction invariant by an average of the fast cyclotron motion. The self-consistent problem couples the gyrokinetic equation for each species (ions and electrons) to the Maxwell equations.The development of this kinetic model, built as an autonomous extension of the extsc{Gysela} code whose basic version gives an adiabatic response to electrons, consists in adding the treatment of the electron distribution function. Taking kinetic electrons into account is costly numerically. Three strategies are envisaged to reduce this cost: (i) consider "heavy electrons", (ii) filter electrons so as to keep only the trapped ones, and (iii) adapt the coordinates to decouple the parallel dynamics (fast) and the transverse one (slow) to the magnetic field.After a presentation of both the gyrokinetic model and some characteristics of the extsc{Gysela} code, we expose the adiabatic electrons model as it is implemented in extsc{Gysela} and introduce two new models: the "Full Kinetic Electrons" model in which electrons are treated kinetically in the same way as the ions and the "Trapped Kinetic Electrons" model in which only the trapped electrons are kinetic, the passing electrons receiving an adiabatic treatment. It is found that electrons generate an over-damping of the GAM explained by a resonant interaction between the bounce frequency of some trapped electrons and that of the GAMs.This damping depends on the electron-ion mass ratio and evolves as $(m_i/m_e)^{-1/2}$. For linear simulations on interchange instability, we find that the ITG modes are dominant over the TEM modes for large ion temperature gradients and vice versa, at finite electron temperature gradient. A satisfying agreement is obtained with the gyrokinetic code GT5D whose results have just been published. Finally, we give some suggestions for future simulations to build non linear cases that could enable to study the influence of kinetic electrons on turbulent transport.
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Charles Ehrlacher. Contribution of kinetic electrons in Tokamak plasmas. Plasma Physics [physics.plasm-ph]. Université Paris-Saclay, 2018. English. ⟨NNT : 2018SACLX033⟩. ⟨tel-01891837⟩

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