Zonally dominated dynamics and Dimits threshold in curvature-driven ITG turbulence
| Autor: | Ivanov, P. G., Schekochihin, A. A., Dorland, W., Field, A. R., Parra, F. I. |
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| Rok vydání: | 2020 |
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| Druh dokumentu: | Working Paper |
| DOI: | 10.1017/S0022377820000938 |
| Popis: | 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. 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. The break up 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 ITG modes, we obtain a semi-analytic model for the Dimits threshold at large collisionality. Comment: 63 pages, 30 figures, to be submitted to J. Plasma Phys |
| Databáze: | arXiv |
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