Exploration of Non-Resonant Divertor Features on the Compact Toroidal Hybrid

TL;DR

This study investigates non-resonant divertor features on the Compact Toroidal Hybrid, revealing resilient strike line patterns and complex edge structures that influence plasma-wall interactions and divertor configurations.

Contribution

It provides new insights into the chaotic magnetic topology and strike line pattern resilience in non-resonant divertors on the CTH device, aiding future divertor design.

Findings

Resilient helical strike line pattern observed

Chaotic edge structures connect to the strike line pattern

High current creates a private flux region with distinct SOL channels

Abstract

Non-resonant divertors (NRDs) separate the confined plasma from the surrounding plasma facing components (PFCs). The resulting striking field line intersection pattern on these PFCs is insensitive to plasma equilibrium effects. However, a complex scrape-off layer (SOL), created by chaotic magnetic topology in the plasma edge, connects the core plasma to the PFCs through varying magnetic flux tubes. The Compact Toroidal Hybrid (CTH) serves as a test-bed to study this by scanning across its inductive current. Simulations observe a significant change of the chaotic edge structure and an effective distance between the confined plasma and the instrumented wall targets. The intersection pattern is observed to be a narrow helical band, which we claim is a resilient strike line pattern. However, signatures of finger-like structures, defined as heteroclinic tangles in chaotic domains, within the plasma edge connect the island chains to this resilient pattern. The dominant connection length field lines intersecting the targets are observed via heat flux modelling with EMC3-EIRENE. At low inductive current levels, the excursion of the field lines resembles a limited plasma wall scenario. At high currents, a private flux region is created in the area where the helical strike line pattern splits into two bands. These bands are divertor legs with distinct SOL parallel particle flow channels. The results demonstrate the NRD strike line pattern resiliency within CTH, but also show the underlying chaotic edge structure determining if the configuration is diverted or limited. This work supports future design efforts for a mechanical structure for the NRD.