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

Part of: Focus on Fusion Invited Contributions from the 18th European Fusion Theory Conference

Published online by Cambridge University Press:  27 October 2020

Plamen G. Ivanov Open the ORCID record for Plamen G. Ivanov [Opens in a new window] ,
A. A. Schekochihin Open the ORCID record for A. A. Schekochihin [Opens in a new window] ,
W. Dorland Open the ORCID record for W. Dorland [Opens in a new window] ,
A. R. Field  and
F. I. Parra
Plamen G. Ivanov*
Affiliation:
Rudolf Peierls Centre for Theoretical Physics, University of Oxford, OxfordOX1 3PU, UK St John's College, OxfordOX1 3JP, UK EURATOM/UKAEA Fusion Association, Culham Science Centre, AbingdonOX14 3DB, UK
A. A. Schekochihin
Affiliation:
Rudolf Peierls Centre for Theoretical Physics, University of Oxford, OxfordOX1 3PU, UK Merton College, OxfordOX1 4JD, UK
W. Dorland
Affiliation:
Rudolf Peierls Centre for Theoretical Physics, University of Oxford, OxfordOX1 3PU, UK Department of Physics, University of Maryland, College Park, MD20740, USA
A. R. Field
Affiliation:
EURATOM/UKAEA Fusion Association, Culham Science Centre, AbingdonOX14 3DB, UK
F. I. Parra
Affiliation:
Rudolf Peierls Centre for Theoretical Physics, University of Oxford, OxfordOX1 3PU, UK Worcester College, OxfordOX1 2HB, UK
*
†Email address for correspondence: plamen.ivanov@physics.ox.ac.uk
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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.

Keywords

fusion plasmaplasma nonlinear phenomenaplasma instabilities
Type
Research Article
Information
Journal of Plasma Physics , Volume 86 , Issue 5 , October 2020 , 855860502
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Copyright © The Author(s), 2020. Published by Cambridge University Press

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