Sustainability and Dynamics of Transport Barriers Near the Density Limit

Numerous studies have demonstrated that an edge shear layer generated by drift waves collapses when the adiabaticity parameter α = k² V²_th /ων is decreased below α_crit ∼ 1 . We have investigated the role of this parameter and the initial density contrast in the formation and dynamics of transport barriers. This study is relevant to the fate of shear layer and transport barriers near the density limit. We use the Hasegawa-Wakatani model, assuming the starting density profile formed after, e.g., a pellet- or supersonic neutral beam injection, or other means of build-up. In this setting, no shear flow is initially imposed but created by the drift waves generated by the density gradient. 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 two smoother steps separated by a flat density region. This process is accompanied by the shear flow formation and suppression of the turbulent transport. The final spread of the transport barrier depends on the adiabaticity α and the initial density contrast, Δn. Turbulent fronts propagate at approximately the same speed in both directions from the initial localization of the transport barrier, depending on the density contrast Δn and adiabaticity α.