Chris Smiet: Turnstiles and topological index in fusion reactor divertors (March 21, 2025)

Описание: A divertor often exploits the topology of ‘x’-points –– hyperbolic fixed points in the Poincaré map of the magnetic field line trajectories –– to create diversion of field lines and increase the separation between the plasma and wall. Around these x-points the magnetic field can appear chaotic, and trajectories in the Poincaré map do not trace out neat surfaces, but appear randomly distributed. Despite appearing chaotic, field line flow is deterministic, and the amount of field lines that pass through a given boundary can be precisely quantified. In particular, an important quantity for analyzing chaotic transport in Hamiltonian systems is the so-called turnstile, which dictates the exchange of magnetic flux between two well separated regions of space, and whose area quantifies the efficiency with which this exchange happens.

In this talk, we present a numerical algorithm that can evaluate the turnstile area directly from a stellarator coil solution, and which is efficiently calculated using an action principle by Meiss. We also explore how the magnetic topology (i.e. the location and interactions between fixed points) and the magnetic chaos can be controlled in the edge of fusion reactors.

Using this tool, we explore select configurations from the QUASR database which exhibit novel divertor topologies where transport to the wall is mediated through interacting turnstiles. Furthermore, we show that in a low-iota configuration of W7-X, the turnstile mechanism causes intricate features in connection length plots used to analyze divertor heat loads. Finally, we present the first results of optimizations on W7-X and other configurations where fixed-point topology and stochasticity in the divertor region is controlled.