Infoscience

Thesis

Interaction Between Fast Ions and Microturbulence in Thermonuclear Devices: Theory and Modelling

The work carried out in this thesis focuses on the interaction between fast ions and turbulence. The aim of the project is to explore this phenomenon and develop the numerical framework required for investigations on present day machines and predictions for burning plasmas. The analysis of the background plasma turbulence and the resulting fast ion diffusivities is carried out with the gyrokinetic code GENE. A set of kinetic transport quantities are defined in order to discriminate the transport of ions with different energies. Gyroaveraging effects are studied. It is observed that only at large values of the E/Te ratio is the particle transport efficiently suppressed (E is the energy of a fast particle and Te the electron temperature). For smaller values, E/Te < 15, larger fast ion transport is observed due to resonant interactions between the particle motion and the phase velocity of the underlying turbulent waves. The transport of fusion generated alpha particles induced by electrostatic fluctuations is lower than collisional expectations, due to their large energies. Magnetic turbulence has an even smaller effect. To verify whether similar conclusions can be drawn for neutral beam ions, substantial upgrades to the VENUS code have been implemented. The results of numerical simulations of the beam ion transport in ITER, DEMO and TCV, with the inclusion of collisional and turbulent effects, are discussed. It is demonstrated that the transport of the 1 MeV ions generated by the neutral beam injector of ITER is only marginally affected by microturbulence and it is concluded that fast ion confinement is not compromised. Given the large plasma temperatures foreseen for DEMO, anomalous transport of beam ions is significant, and in particular collisional models fail to estimate the correct heat deposited on the ions and the electrons. Given the low energy of the planned TCV NBI injector, even stronger anomalies are expected. The effect, however, can be regulated with auxiliary ECRH heating, which would allow for new studies of the fast ion turbulent transport.

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