Three-dimensional computation of drift Alfvén turbulence

TLDR

Drift Alfvén modeling is essential for first‑principles calculations of tokamak edge turbulence. The study uses a transcollisional electromagnetic fluid model with parallel heat flux to simulate electron drift turbulence at the tokamak L‑H transition. The simulations show that electron drift turbulence at the L‑H transition is drift Alfvén turbulence, highly sensitive to plasma beta, weakly affected by magnetic flutter, curvature, or collision frequency, with transport dominated by flow eddies, and scaling strongly with magnetic shear while gradients drive turbulence without passive scalar behavior.

Abstract

A transcollisional, electromagnetic fluid model, incorporating the parallel heat flux as a dependent variable, is constructed to treat electron drift turbulence in the regime of tokamak edge plasmas at the L - H transition. The resulting turbulence is very sensitive to the plasma beta throughout this regime, with the scaling with rising beta produced by the effect of magnetic induction to slow the Alfvénic parallel electron dynamics and thereby leave the turbulence in a more robust, non-adiabatic state. Magnetic flutter and curvature have a minor qualitative effect on the turbulence mode structure and on the beta scaling, even when their quantitative effect is strong. Transport by magnetic flutter is small compared to that by the flow eddies. Fluctuation statistics show that while the turbulence shows no coherent structure, it is coupled strongly enough so that neither density nor temperature fluctuations behave as passive scalars. Both profile gradients drive the turbulence, with the total thermal energy transport varying only weakly with the gradient ratio, . Scaling with magnetic shear is pronounced, with stronger shear leading to lower drive levels. Scaling with either collision frequency or magnetic curvature is weak, consistent with their weak qualitative effect. The result is that electron drift turbulence at L - H transition edge parameters is drift Alfvén turbulence, with both ballooning and resistivity in a clear secondary role. The contents of the drift Alfvén model will form a significant part of any useful first-principles computation of tokamak edge turbulence.