Abstract of TOPOX Project

Turbulence in fusion plasmas is known to be affected by the shaping of the magnetic flux surfaces. Modern tokamaks, including the ITER experiment under construction, have a diverted magnetic structure in which the last closed flux surface is a separatrix with an X-point. The effect of the magnetic shear in this region on the drift-wave turbulence which is believed to cause heat/particle transport is thought to be severe but is at present unresolved because present-day computational models encounter insurmountable difficulties with coordinate-cell deformation there. We propose to work on coordinate mappings which do the same job as actual transformations in taking parallel derivatives within the computational scheme. Mappings can be divorced from the mathematical constraints of pure Clebsch representations of divergence-free vector fields which are used to construct field-aligned coordinates. Both grid-node topology and dynamical capture of a slowly-varying, self-consistent magnetic equilibrium are involved. Success of the project will enable well-resolved gyrofluid computations of the core-edge-sol closed-to-open field-lines transition region. This is a necessary step for direct simulation of the complete tokamak edge in a diverted plasma, and therefore for first-principles predictions of performance of the ITER pedestal and H-mode confinement regime.