In this paper we show the self-consistent evolution of an isolated density perturbation in models of tokamak scrape-off layer turbulence. Our purpose is to explain the possible mechanisms responsible for radial propagation of density perturbations observed in the scrape-off layer. Results of both two-dimensional numerical simulations and one-dimensional quasilinear modeling of the propagative events are presented, and shown to be consistent with many experimental observations. The role of sheath dissipation for front propagation and turbulent mixing is also addressed.

Transport and confinement within the resistive-g paradigm are investigated by means of two-dimensional numerical simulations. The system is driven by a constant incoming heat flux at the inner radial boundary. Different confinement and transport states are identified, involving self-sustained sheared poloidal flows. At the onset of turbulent convection the probability distribution functions of pressure and radial velocity fluctuations measured in the centre of the plasma layer have a nearly Gaussian form. Further increasing the heat flux drive these distributions become increasingly non-Gaussian, developing exponential tails. This large-scale intermittency is ascribed to the presence of bursting in the domain averaged convective transport and the fluctuation energy integrals. The quasi-periodic bursts are separated by shear-dominated quiescent periods in which the mean flow energy decreases and the confined heat increases on diffusive timescales. The time-averaged thermal energy confined within the plasma layer shows a power law dependence and significant increase with the injected power over the range of turbulent convection investigated.

In the present contribution, the quasilinear dynamics of convective turbulence is studied. In essence, and contrary to the “frozen gradient” assumption, the quasilinear approach takes into account the back-reaction of the convective flux on the mean gradient driving the instability. The dynamical regulation of convective transport by a sheared mean flows is also included. Close to the instability threshold it naturally gives rise to a transition from low to high confinement modes. Further away, regular relaxation oscillations are sustained. In this time-dependent state, each transient maximum of the convective flux activity triggers a ballistic transport event observed on the mean profile. The period of the oscillations is not controlled by the nonlinearity but by the dissipation on the mean flow. A “Dimits-shift” regime is thus identified in the limit of zero damping on the mean flow. This infinite period cycle corresponds to a single ballistic transport event triggered before the system settles into its diffusive state. Far away from the threshold, relaxation oscillations are still sustained in the presence of mean flow dissipation, but are superimposed on high-frequency fluctuations. This particular behavior makes the convective transport to follow exponential statistics when measured at a local probe.

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