X-point modelling in linear configurations using BOUT++

Abstract

Magnetic X-point configurations in tokamak geometries are critical in determining edge and scrape off layer (SOL) dynamics, and hence particle and heat flux onto plasma facing components. Alternative configurations have been proposed which aim to reduce fluxes to material surfaces, but their performance depends on cross-field transport in the region of the null point which is currently poorly understood. There is therefore a need for theoretical and experimental studies of turbulence in X-point magnetic configurations. In conventional 3D turbulence simulations of tokamaks, a field-aligned coordinate system is used, which introduces numerical instabilities at the null point due to zero volume elements. As a result, X-point dynamics are often interpolated based on nearby flux surfaces, which could exclude relevant physics. Here we simulate X-point configurations in linear geometries using a non-field-aligned coordinate system, and present results of 3D drift-wave turbulence and flow simulations in X- point configurations using an isothermal model which evolves density, vorticity, parallel velocity and parallel current density. Simulations have been performed to explore the feasibility of experimentally studying X-point configurations in linear plasma devices which indicate that even a modest coil set carrying 300A should produce a measurable effect on the driftwave turbulence and plasma profiles near the null region. The energy dynamics of the system are also explored and indicate that an X-point causes ohmic dissipation in higher mode number turbulence.