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

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.

Validation of GBS plasma turbulence simulation of the TJ-K stellarator

“…The equilibrium density profile agrees well with the experiment, but in the case of the electron temperature the value on axis is underestimated. As experimentally observed in TJ-K and confirmed by the GBS simulations presented here, the turbulent transport is mainly due to a low-m mode, something that was also recently observed in an GBS simulation of a stellarator with an island divertor [8]. This contrasts with the typical plasma turbulence in tokamaks, where more broadband turbulence is observed.…”

“…an extension of the BOUT++ code [3], simulated seeded filaments in a rotating ellipse [4]. More recently, the first global flux-driven simulations of a stellarator, performed by using the GBS code [5][6][7], considered a vacuum magnetic field generated with the Dommaschk potentials, reporting important differences with respect to tokamak simulations, namely the existence of a low-m mode, where m is the poloidal mode number, dominating the turbulent transport [8]. Such surprising result calls for the validation of turbulence simulations in stellarators.…”

Effect of triangularity on plasma turbulence and the SOL-width scaling in L-mode diverted tokamak configurations

“…Initially developed to study turbulence in basic plasma physics experiments [37] and limited tokamak configurations, the GBS code [38][39][40] solves the drift-reduced Braginskii equations to evolve plasma turbulence at the tokamak boundary. The implementation of a spatial discretization algorithm independent of the magnetic field [41] allows GBS to simulate diverted configurations with an arbitrary magnetic equilibrium [42,43], as well as non-axisymmetric configurations, such as the stellarators [44]. While the plasma model implemented in GBS was developed in recent years to include the neutral dynamics [45], we do not include it in the simulations presented here, therefore focusing on the sheath-limited regime, where only pure plasma composed of ions and electrons, without radiative impurities, is considered in the present study.…”

Effects of drifts on scrape-off layer transport in W7-X

“…Two-dimensional codes applicable to the tokamak SOL such as UEDGE [16] and SOLPS [17] include drifts, but they are not always turned on in simulations due to computational cost, especially when drift effects are not the primary focus of investigation. Drift modeling in stellarators is in its infancy: there are currently no 3D SOL codes with plasmaneutral coupling that self-consistently include drifts, although recently a two-fluid edge turbulence code that includes drifts but not neutrals has been developed [18]. Efforts are underway to add drifts to EMC3-Eirene [19], the primary SOL code used to model W7-X, but a functional implementation is not complete.…”