Zonally dominated dynamics and Dimits threshold in curvature-driven ITG turbulence
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 $\boldsymbol {E}× \boldsymbol {B}$ flow. By analysing the linear ion-temperature-gradient modes, we obtain a semi-analytic model for the Dimits threshold at large collisionality.
- Publication:
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Journal of Plasma Physics
- Pub Date:
- October 2020
- DOI:
- 10.1017/S0022377820000938
- arXiv:
- arXiv:2004.04047
- Bibcode:
- 2020JPlPh..86e8502I
- Keywords:
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- fusion plasma;
- plasma nonlinear phenomena;
- plasma instabilities;
- Physics - Plasma Physics
- E-Print:
- 63 pages, 30 figures, to be submitted to J. Plasma Phys