Zonally dominated dynamics and Dimits threshold in curvature-driven ITG turbulence. (October 2020)

• Record Type: Journal Article
• Authors: Ivanov, Plamen G.; Schekochihin, A. A.; Dorland, W.; Field, A. R.; Parra, F. I.
• Is Part Of: Journal of plasma physics. Volume 86:Number 5(2020)
• Publication Date: 2020-10
• Subjects: fusion plasma, -- plasma nonlinear phenomena, -- plasma instabilities; Plasma (Ionized gases) -- Periodicals; 530.4405
• DOI: 10.1017/S0022377820000938
• Languages: English
• ISSNs: 0022-3778

Abstract

Abstract : The saturated state of turbulence driven by the ion-temperature-gradient instability is investigated using a two-dimensional long-wavelength fluid model that describes the perturbed electrostatic potential and perturbed ion temperature in a magnetic field with constant curvature (a $Z$ -pinch) and an equilibrium temperature gradient. Numerical simulations reveal a well-defined transition between a finite-amplitude saturated state dominated by strong zonal-flow and zonal temperature perturbations, and a blow-up state that fails to saturate on a box-independent scale. We argue that this transition is equivalent to the Dimits transition from a low-transport to a high-transport state seen in gyrokinetic numerical simulations (Dimits et al., Phys. Plasmas, vol. 7, 2000, 969). A quasi-static staircase-like structure of the temperature gradient intertwined with zonal flows, which have patch-wise constant shear, emerges near the Dimits threshold. The turbulent heat flux in the low-collisionality near-marginal state is dominated by turbulent bursts, triggered by coherent long-lived structures closely resembling those found in gyrokinetic simulations with imposed equilibrium flow shear (van Wyk et al., J. Plasma Phys., vol. 82, 2016, 905820609). The breakup of the low-transport Dimits regime is linked to a competition between the two different sources of poloidal momentum in the system – the Reynolds stress and the advection of the diamagnetic flow by the $\boldsymbol {E}\times \boldsymbol {B}$ flow. By analysing the linear ion-temperature-gradient modes, we obtain a semi-analytic model for the Dimits threshold at large collisionality.