Profile Staircases and Edge Shear Layers Near the Density Limit

Edge shear layers, generated by drift waves, collapse when the adiabaticity parameter α = k∥² V²_T / ων drops below α_crit ∼ 1. Evolution of α <α_crit is thought to be relevant to density limit phenomenology. We have investigated the role of this parameter and the initial density contrast in the formation and dynamics of profiles and shear layers. We use the Hasegawa-Wakatani model, assuming the starting density profile formed after particle injection. Gradient-driven drift waves spontaneously generate a shear flow. The density profile then relaxes under the competition between the turbulent transport and its suppression by the shear flow. The relaxation dynamics generically go through a staircase phase, wherein the initial density step splits into smaller, smoother steps separated by quasi-plateaus. Zonal flows and vortical motion self-regulating the turbulent transport accompany this process. The final extent of the confinement zone depends on the adiabaticity α and the initial density contrast, Δn. Turbulent spreading fronts propagate in both directions from the initial localization of the transport barrier at approximately the same speed, contingent upon the density contrast and adiabaticity parameter. The spreading fronts are consistent with recent experimental findings¹.

¹T. Long et al 2021 Nucl. Fusion 61 126066