Global fluid simulation of plasma turbulence in a stellarator with an island divertor

TL;DR

This paper presents a comprehensive 3D simulation of plasma turbulence in a stellarator with an island divertor, revealing the dominant transport mechanisms and the absence of blob-like intermittent events.

Contribution

It extends the GBS code to simulate plasma turbulence in non-axisymmetric magnetic fields, specifically in stellarator configurations with magnetic islands.

Findings

Radial transport mainly driven by a field-aligned mode with poloidal number m=4

Transport is ballooning in nature, based on linear theory analysis

No intermittent blob-like transport events observed in the simulation

Abstract

Results of a three-dimensional, flux-driven, electrostatic, global, two-fluid turbulence simulation for a 5-field period stellarator with an island divertor are presented. The numerical simulation is carried out with the GBS code, recently extended to simulate plasma turbulence in non-axisymmetric magnetic equilibria. The vacuum magnetic field used in the simulation is generated with the theory of Dommaschk potentials, and describes a configuration with a central region of nested flux surfaces, surrounded by a chain of magnetic islands, similarly to the diverted configurations of W7-X. The heat outflowing from the core reaches the island region and is transported along the magnetic islands, striking the vessel walls, which correspond to the boundary of the simulation domain. The radial transport of particles and heat is found to be mainly driven by a field-aligned coherent mode with poloidal number $m=4$. The analysis of this mode, based on non-local linear theory considerations, shows its ballooning nature. In contrast to tokamak simulations and experiments, where blobs often contribute to transport, we do not observe the presence of intermittent transport events.