Three-dimensional computation of drift Alfvén turbulence

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 Alfvenic parallel electron dynamics and thereby leave the turbulence in a more robust, nonadiabatic 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 ExB flow eddies. Fluctuation statistics show that while the turbulence shows no coherent structure, it is strongly enough coupled 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, d log T/d log n. 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 Alfven turbulence, with both ballooning and resistivity in a clear secondary role. The contents of the drift Alfven model will form a significant part of any useful first-principles computation of tokamak edge turbulence. (orig.)