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

Journal Article · · Journal of Plasma Physics

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 (Dimitset 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 Wyket 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.

Research Organization:
Univ. of Maryland, College Park, MD (United States); Univ. of Texas, Austin, TX (United States)
Sponsoring Organization:
USDOE Office of Science (SC)
DOE Contract Number:
FG02-93ER54197; SC0018429
OSTI ID:
1850064
Journal Information:
Journal of Plasma Physics, Journal Name: Journal of Plasma Physics Journal Issue: 5 Vol. 86; ISSN 0022-3778
Publisher:
Cambridge University Press
Country of Publication:
United States
Language:
English

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