A novel flexible field-aligned coordinate system for tokamak edge plasma simulation

TL;DR

This paper introduces a new 3D coordinate system for tokamak edge plasma simulation that allows arbitrary geometries while maintaining field alignment, improving modeling accuracy of plasma-wall interactions.

Contribution

A novel flexible field-aligned coordinate system is developed, relaxing orthogonality constraints to better match complex tokamak edge geometries.

Findings

Successfully implemented in BOUT++, tested with manufactured solutions.

Simulated MAST edge cross-section showing expected plasma behavior.

Demonstrated improved modeling of ion-neutral interactions near divertor plates.

Abstract

Tokamak plasmas are confined by a magnetic field that limits the particle and heat transport perpendicular to the field. Parallel to the field the ionised particles can move freely, so to obtain confinement the field lines are "closed" (ie. form closed surfaces of constant poloidal flux) in the core of a tokamak. Towards, the edge, however, the field lines begin to intersect physical surfaces, leading to interaction between neutral and ionised particles, and the potential melting of the material surface. Simulation of this interaction is important for predicting the performance and lifetime of future tokamak devices such as ITER. Field-aligned coordinates are commonly used in the simulation of tokamak plasmas due to the geometry and magnetic topology of the system. However, these coordinates are limited in the geometry they allow in the poloidal plane due to orthogonality requirements. A novel 3D coordinate system is proposed herein that relaxes this constraint so that any arbitrary, smoothly varying geometry can be matched in the poloidal plane while maintaining a field-aligned coordinate. This system is implemented in BOUT++ and tested for accuracy using the method of manufactured solutions. A MAST edge cross-section is simulated using a fluid plasma model and the results show expected behaviour for density, temperature, and velocity. Finally, simulations of an isolated divertor leg are conducted with and without neutrals to demonstrate the ion-neutral interaction near the divertor plate and the corresponding beneficial decrease in plasma temperature.