DIFFER Seminar: Error-field-induced homoclinic tangles in a tokamak divertor

Abstract: In tokamaks imperfections in the placement of the coils can result in ``error fields`` of the order of milliteslas that break axisymmetry. We show that on the TCV tokamak such a minute asymmetry (a few thousandths of a tesla) is enough to tear the edge magnetic surface into a chaotic ``homoclinic tangle,'' a structure predicted by dynamical-systems theory but rarely seen directly. The tangle's transport channels coincide with bright regions imaged by a multispectral imaging system in the divertor region.
Such structures can be seen in experiments with low poloidal field strength and multiple x-points in the divertor region. These results link abstract chaos theory to directly photographed phenomena and can provide a sensitive probe to milli-tesla level fields in the divertor region. Understanding these phenomena in multi-x-point discharges is important for creating advanced divertor scenarios such as the x-point target divertor.