Modelling of magnetic null points using BOUT++

One of the major challenges to viable fusion energy is the exhaust of hot plasma, as future magnetic fusion devices will have unacceptably high heat fluxes on the plasma facing components. Recent research into advanced divertor designs and alternative magnetic configurations attempts to alleviate this issue, however the effectiveness of these configurations relies on cross field transport in the poloidal magnetic null region, which is currently poorly understood. Simulations of instabilities and turbulence in X-point configurations are challenging due to the limitations of field-aligned coordinate systems: X-point dynamics are often interpolated based on nearby flux surfaces, which could exclude relevant physics. Here we present the results of turbulence and transport simulations relevant to tokamak X-points in various magnetic geometries using coordinate systems which are not aligned to the magnetic field.

First, we present results as part of a feasibility study of a university-scale linear plasma device capable of producing azimuthal X-points. The turbulent characteristics of this system are explored and measurements using synthetic diagnostics are proposed. These studies are then extended to toroidal geometries by simulating filament propagation in TORPEX poloidal magnetic null point scenarios and comparison to experiment. It is determined that the null region can cause an acceleration of filaments due to increasing connection length, but this acceleration is small relative to other effects, which we quantify. Experimental measurements are reproduced, and the dominant acceleration mechanism is identified as that of a developing dipole in a moving background. Finally, the implementation of the Flux Coordinate Independent method for parallel derivatives into BOUT++ is investigated by simulating transport and diffusion in nonaxisymmetric geometries. The potential for BOUT++ to be used as a stellarator turbulence and transport code is also discussed.