X-point modelling in linear configurations using BOUT++

Abstract

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 Xpoint 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.

Towards nonaxisymmetry; initial results using the Flux Coordinate Independent method in BOUT++

“…This system is inaccurate in stochastic regions and regions near magnetic X-and O-points; two coordinates (toroidal, field aligned) are parallel at X-points, causing numerical instability in the form of zero-volume elements. Recent work has sought to exploit techniques such as nonaligned coordinate systems which has allowed for X-point simulation in BOUT++ [7,8,9]. We will show here that it is possible to simulate stellarator geometries in BOUT++ using a non-field-aligned grid through the implementation of the Flux Coordinate Independent (FCI) method for parallel derivatives.…”

Dirichlet boundary conditions for arbitrary-shaped boundaries in stellarator-like magnetic fields for the Flux-Coordinate Independent method

“…BOUT++ [5][6][7] is a free and open source framework designed to solve partial differential equations, with an emphasis on models of magnetically confined plasmas. It has been used for a variety of applications, from edge 8-10 and scrape-off layer 11,12 physics in tokamaks, to turbulence in linear devices 13,14 .…”

Blob dynamics in TORPEX poloidal null configurations

“…As the model and numerical methods described in the previous section were originally tested in linear geometries [20], simulations were performed here to validate the extension of these methods to toroidal geometries and to determine the characteristics of blob propagation within the TORPEX magnetic null point scenarios. Experimental comparison was conducted to investigate the filament acceleration mechanism seen in experiment.…”

“…An isothermal cold-ion fluid model initially constructed for plasma blob studies [3,19] has been extended for use in X-point scenarios [20]. The model is electrostatic and inviscid; the isothermal electron temperature T e0 is set to 2.5 eV, as this is approximately the measured temperature in the region of filament propagation within TORPEX X-point scenarios [10].…”