The topology of non-resonant stellarator divertors

TL;DR

This paper uses topological methods to analyze magnetic field diversion in stellarators, revealing how unpaired X-points can guide plasma away from confined regions, with implications for future stellarator design.

Contribution

It introduces topological analysis of stellarator divertors, highlighting the role of unpaired X-points in magnetic field diversion, and presents new examples with potential for improved divertor configurations.

Findings

Unpaired X-points can effectively divert magnetic fields in stellarators.

Topological methods reveal novel divertor phenomena in optimized stellarators.

Examples show potential for improved plasma control in future devices.

Abstract

We apply topological methods to better understand how the magnetic field in the stellarator edge can be diverted away from the confined region. Our primary method is calculating the winding numbers of closed contours, which gives information on the number and nature of fixed points within a bounded region. We first apply this to the non-resonant divertor (NRD) Hamiltonian system, and present a simple explanation for the system's diversion: trajectories are guided away from the confined region by X-points which are "unpaired" i.e. do not have corresponding O-points and therefore do not resemble an island chain. We show how similar phenomena can occur in a similar, axisymmetric Hamiltonian system. Secondly, we find examples of neoclassically optimised stellarators in the QUASR database which divert the magnetic field via unpaired X-points. We present and discuss three examples, each containing novel phenomena which might be desirable for stellarator divertors. These findings broaden the horizons of how magnetic fields can be diverted in realistic stellarators, and may be attractive for future experiments and stellarator reactor design.