Magneto-Fluid Dynamics Seminar

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Zonally dominated dynamics and Dimits threshold in curvature-driven ITG turbulence

Speaker: Plamen Ivanov, University of Oxford

Location: Warren Weaver Hall 905

Date: Monday, November 9, 2020, 10 a.m.

Synopsis:

The saturated state of turbulence driven by the ion-temperature-gradient (ITG) 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). 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, zonal-flow-dominated state to a high-transport state seen in gyrokinetic numerical simulations. In the near-marginal Dimits state, turbulence is suppressed by a quasi-static "zonal staircase" arrangement of the zonal flows and zonal temperature. This structure is reminiscent of the "E × B staircase" observed in global GK simulations. The breakup of the zonal staircase, and, thus, of the low-transport Dimits regime, is linked to a competition between the two different sources of poloidal momentum — the Reynolds stress of the E × B flow and the diamagnetic advection of the poloidal E × B flow. By analysing the linear ITG modes, we obtain a semi-analytic model for the Dimits threshold at large collisionality.