Numerical simulations of coherent blob-like filaments in magnetized plasmas

Abstract

In a world more and more dependent on new and effective energy sources, the introduction of fusion power could prove invaluable. Magnetic confinement of plasma has proved to be difficult, and the curvature of the magnetic field lines in tokamaks give rise to charge polarization at the boundary of the fusion devices leading to cross-field transport. Experiments has shown that the dominating transport is due to 3D filament structures which can be considered as isolated field-aligned 2D blob structures in the plane perpendicular to the magnetic field. The center of mass motion of such blob structures are the main focus of this thesis. By deriving the reduced two-fluid model while relaxing and retaining the Boussinesq approximation, and by incorporating parallel closure with particle loss and sheath dissipation, a set of invariants has been obtained. Using center of mass diagnostics, we examine the behavior of blobs with varying initial amplitudes and particle loss parameter derived from the parallel closure. Additionally, the impact of the Boussinesq approximation is analyzed both with and without parallel closure. Utilizing numerical simulations, we verify the scaling found with initial amplitude. Further we obtain results indicating blob stagnation in the limit of larger values of the particle loss parameter, accompanied with behavior equal to the dynamics shown for the sheath dissipation parameter in the case of larger values of the particle loss term. The limitations of the Boussinesq approximation have been verified, and by incorporating the particle loss and the sheath dissipation terms, a transition region has been identified. This region separates the dominance of the Boussinesq approximation in the larger amplitude regime from the scaling behavior determined by sheath dissipation. We also demonstrated an apparent dependence of the initial amplitude on the acceleration of the blobs in the intermediate initial amplitude regime.