Tokamak Edge Plasma Turbulence Interaction with Magnetic X-Point in 3D Global Simulations

Davide Galassi,Nicolas Nace,Hugo Bufferand,Guido Ciraolo,Philippe Ghendrih,Eric Serre,Patrick Tamain

doi:10.3390/fluids4010050

Abstract

Turbulence in the edge plasma of a tokamak is a key actor in the determination of the confinement properties. The divertor configuration seems to be beneficial for confinement, suggesting an effect on turbulence of the particular magnetic geometry introduced by the X-point. Simulations with the 3D fluid turbulence code TOKAM3X are performed here to evaluate the impact of a diverted configuration on turbulence in the edge plasma, in an isothermal framework. The presence of the X-point is found, locally, to affect both the shape of turbulent structures and the amplitude of fluctuations, in qualitative agreement with recent experimental observations. In particular, a quiescent region is found in the divertor scrape-off layer (SOL), close to the separatrix. Globally, a mild transport barrier spontaneously forms in the closed flux surfaces region near the separatrix, differently from simulations in limiter configuration. The effect of turbulence-driven Reynolds stress on the formation of the barrier is found to be weak by dedicated simulations, while turbulence damping around the X-point seems to globally reduce turbulent transport on the whole flux surface. The magnetic shear is thus pointed out as a possible element that contributes to the formation of edge transport barriers.

Highlights

In the Deuterium-Tritium phase, ITER is expected to operate in the so-called High-confinement (H) mode
At the first order, a structure perfectly aligned to a flux tube, one can study the spatial variation of the structure in the poloidal plane by knowing the magnetic field shape
In [25] we have verified that turbulent structures lying on the flux surface ψ at the poloidal position θ are characterized by poloidal wavenumber mp k θ (ψ, θ ) ' k θ (ψ) θ,mp ( ψ )

Summary

Introduction

In the Deuterium-Tritium phase, ITER is expected to operate in the so-called High-confinement (H) mode. The H-mode regime, in opposition to the Low-confinement mode (L), is characterized by a visible steepening of the profiles of plasma density in the edge region that leads to a higher plasma pressure at the magnetic axis. This phenomenon is associated with a reduction in turbulent transport, which is the dominant particle and heat transport mechanism in the edge plasma. The transition from L to H-mode is experimentally obtained when the power injected into the plasma exceeds a certain threshold.

Results

Discussion

Conclusion