Modeling non-stationary, non-axisymmetric heat patterns in DIII-D tokamak

Abstract

Non-axisymmetric stationary magnetic perturbations lead to the formation of homoclinic tangles near the divertor magnetic saddle in tokamak discharges. These tangles intersect the divertor plates in static helical structures that delimit the regions reached by open magnetic field lines reaching the plasma column and leading the charged particles to the strike surfaces by parallel transport. In this article we introduce a non-axisymmetric rotating magnetic perturbation to model the time development of the three-dimensional magnetic field of a single-null DIII-D tokamak discharge developing a rotating tearing mode. The stable and unstable manifolds of the asymmetric magnetic saddle are calculated through an adaptive method providing the manifold cuts at a given poloidal plane and the strike surfaces. For the modeled shot, the experimental heat pattern and its time development are well described by the rotating unstable manifold, indicating the emergence of homoclinic lobes in a rotating frame due to the plasma instabilities. In the model it is assumed that the magnetic field is created by a stationary axisymmetric plasma current and a set of rotating internal helical filamentary currents. The currents in the filaments are adjusted to match the waveforms of the magnetic probes at the mid-plane and the rotating magnetic field is introduced as a perturbation to the axisymmetric field obtained from a Grad-Shafranov equilibrium reconstruction code.