Global fluid turbulence simulations in the scrape-off layer of a stellarator island divertor

Abstract

Isothermal fluid turbulence simulations have been performed in the edge and scrape-off layer (SOL) of an analytic stellarator configuration with an island divertor, thereby providing numerical insight into edge turbulence in regions around islands in a stellarator. The steady-state transport follows the a curvature drive that is inverse to the major radius ( $1/R$ ) toward the outboard side, but large fluctuations are present throughout the island divertor region, with the average wavelength of similar size to the island width. The system exhibits a prominent $m=2$ , $n=5$ mode, where m is the poloidal mode number and n is the toroidal mode number, although other modes are present. The amplitude and radial extent of the density fluctuations are similar throughout the edge and SOL, but can decrease near island O-points. The fluctuations exhibit a predominantly positive skewness on the outboard midplane, indicating blob-like perturbations for the transport into the outer SOL. It is determined that a point on the separatrix is generally more correlated with regions outside of the SOL than a nearby reference point which does not lie on the separatrix.

Filament simulations for stellarators; a review of recent progress

“…Rather, the system is dominated by a single coherent mode, whose fluctuation amplitude is highest on the inboard ("good" curvature) side of the configuration. This geometry is again studied in [26] with a more simplified model and a broader-range of turbulent fluctuations are found, although again no distinct filaments are observed. Rather, broadband turbulence is observed with increased fluctuation amplitudes on the outboard side of the torus, see Figure 1.…”

“…As such, other methods must be employed; either a non-aligned coordinate system [23][24][25]28], or a locally-aligned approach, such as the Flux-Coordinate-Independent method for parallel derivatives [14,16,18,19,21]. Global fluid turbulence simulations have been performed in analytic [25,26] and experimentally-relevant geometries [28], although a consistent picture of the dynamics within these systems is as yet undetermined.…”

“…Fluctuation amplitudes increase on the outboard side of the torus. Reproduced under the Creative Commons CC BY 4.0 license from [26].…”

“…Global simulations of fluid turbulence experimentally-relevant geometries with an island divertor should enable a thorough examination of these phenomena. The recent development of SOL turbulence codes for complicated magnetic geometries [21,25,26,28], including nonaxisymmetric island chains, has in principle enabled such studies, although global simulations in Wendelstein 7-X geometries remain as yet unrealized due to the considerable numerical expense of such simulations. As such, simulations of smaller-scale experiments, like that which is done in [26,28] or in [45], could also ascertain such dynamics.…”

“…As such, the investigation of filaments in stellarators has until recently been limited to experimental analysis [3][4][5][6][7][8][9][10][11][12][13]. The utilization of alternative numerical approaches, such as the Flux-Coordinate-Independent (FCI) approach [14][15][16][17][18][19][20][21][22] or direct approach [23,24] to parallel derivatives has facilitated the global simulation of turbulent structures in the stellarator SOL [21,25,26]. Simultaneously, simulations in simplified geometries have allowed for a more detailed analysis of the relevant mechanisms.…”