Gyrokinetic simulation of microtearing turbulence

Abstract

In modern fusion experiments, plasma turbulence is responsible for the radial heat transport and thus determines the plasma confinement within the magnetic field of tokamak devices. Deeper theoretical understanding is still needed and the detailed physical picture continues to evolve. In the context of electron thermal transport, early analytical work indicates a role of magnetic perturbations due to the microtearing instability, which is supported by more recent numerical results. In this thesis, the linear properties of this microtearing instability and its nonlinear, turbulent behavior is investigated by means of numerical simulations with the gyrokinetic turbulence code GENE. The underlying gyrokinetic equations are also suited to describe neoclassical transport, which is very sensitive to collisions. Since collisions are of special importance for the results presented in this thesis, successful benchmarks of neoclassical results between GENE and other well-established codes are presented. Despite their extremely challenging inherent multiscale features, nonlinear microtearing simulations succeeded for the first time in the course of this work. Realistic parameter ranges for present-day and future experiments are covered. An outstanding feature is that the radial transport of (electron) heat is well described by a simple diffusivity model, as long as the magnetic field fluctuations exceed a certain threshold. Since the resulting transport level is found to be experimentally relevant, these simulations establish microtearing turbulence as an additional candidate to explain enhanced electron thermal transport in standard tokamaks.