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Tokamak plasmas are confined by a magnetic field that limits the particle andheat transport perpendicular to the field. Parallel to the field the ionisedparticles can move freely, so to obtain confinement the field lines are"closed" (ie. form closed surfaces of constant poloidal flux) in the core of atokamak. Towards, the edge, however, the field lines begin to intersectphysical surfaces, leading to interaction between neutral and ionisedparticles, and the potential melting of the material surface. Simulation ofthis interaction is important for predicting the performance and lifetime offuture tokamak devices such as ITER. Field-aligned coordinates are commonlyused in the simulation of tokamak plasmas due to the geometry and magnetictopology of the system. However, these coordinates are limited in the geometrythey allow in the poloidal plane due to orthogonality requirements. A novel 3Dcoordinate system is proposed herein that relaxes this constraint so that anyarbitrary, smoothly varying geometry can be matched in the poloidal plane whilemaintaining a field-aligned coordinate. This system is implemented in BOUT++and tested for accuracy using the method of manufactured solutions. A MAST edgecross-section is simulated using a fluid plasma model and the results showexpected behaviour for density, temperature, and velocity. Finally, simulationsof an isolated divertor leg are conducted with and without neutrals todemonstrate the ion-neutral interaction near the divertor plate and thecorresponding beneficial decrease in plasma temperature.

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