Resilient Stellarator Divertor Characteristics in the Helically Symmetric eXperiment

TL;DR

This study investigates resilient divertor features in the Helically Symmetric Experiment, revealing consistent edge structures across configurations and introducing a new metric to optimize divertor placement considering magnetic chaos.

Contribution

It introduces a new metric, min(δ_N), to analyze magnetic edge structures and guides resilient divertor design in stellarators with chaotic edge fields.

Findings

Resilient plasma wall interaction patterns are consistent across configurations.

A new metric, min(δ_N), correlates with connection length and edge chaos.

Magnetic footprint structures inform optimal divertor placement.

Abstract

Resilient divertor features connected to open chaotic edge structures in the Helically Symmetric Experiment (HSX) are investigated. For the first time, an expanded vessel wall was considered that would give space for implementation of a physical divertor target structure. The analysis was done for four different magnetic configurations with very different chaotic plasma edges. A resilient plasma wall interaction pattern was identified across all configurations. This manifests as qualitatively very similar footprint behavior across the different plasma equilibria. Overall, the resilient field lines of interest with high connection length $L_C$ lie within a helical band along the wall for all configurations. This resiliency can be used to identify the best location of a divertor. The details of the magnetic footprint's resilient helical band is subject to specific field line structures which are linked to the penetration depth of field lines into the plasma and directly influence the heat and particle flux patterns. The differences arising from these details are characterized by introducing a new metric, the minimum radial connection $\text{min}(\delta_N)$ of a field line from the last closed flux surface. The relationship, namely the deviation from a scaling law, between $\text{min}(\delta_N)$ and $L_C$ of the field lines in the plasma edge field line behavior suggests that the field lines are associated with structures such as resonant islands, cantori, and turnstiles. This helps determine the relevant magnetic flux channels based on the radial location of these chaotic edge structures and the divertor target footprint. These details will need to be taken into account for resilient divertor design.