April 4-6

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Author: Plamen G. Ivanov
Requested Type: Consider for Invited
Submitted: 2022-03-04 02:36:16

Co-authors: A.A.Schekochihin, W.Dorland

Contact Info:
University of Oxford
Rudolf Peierls Centre for Theo
Oxford, OX1 3JP, Oxfordshir   0000

Abstract Text:
We present analytical and numerical results on the nonlinear saturation of ion-scale electrostatic turbulence driven by ion-temperature-gradient (ITG) instabilities in slab geometry with constant magnetic curvature. Our work is based on a 3D extension of our 2D, long-wavelength, cold-ion fluid model. We identify two qualitatively distinct routes to saturation: a Dimits state dominated by strong zonal flows (ZFs), and a strongly turbulent state whose saturation is aided by `parasitic' small-scale ITG modes.
The 3D Dimits state is governed by the same underlying physics as that in the 2D model. Turbulence is suppressed by a quasi-static zonal-staircase arrangement of the ZFs and zonal temperature. This structure is reminiscent of the ExB staircase observed in global GK simulations. The zonal staircase consists of interleaved regions of strong zonal shear that suppresses the ITG turbulence in those regions, and localised turbulent patches at the turning points of the ZF velocity.
The distinctive feature of 3D cold-ion ITG turbulence, as opposed to its 2D counterpart, is the existence of a `parasitic' small-scale slab-ITG instability driven predominantly by the gradients of large-scale perturbations, rather than by the equilibrium gradients. We demonstrate analytically and numerically that the parasitic modes extract energy from the large-scale perturbations and provide an effective enhancement of large-scale thermal diffusion, thus aiding the energy transfer from large injection scales to small dissipative ones. Furthermore, these modes always favour a ZF-dominated state. In fact, a Dimits state with a zonal staircase is achieved regardless of the strength of the linear drive provided the system is sufficiently extended along the magnetic field and sufficient parallel resolution is provided. If a Dimits state cannot be maintained, the system enters a strongly turbulent regime where energy transport from large to small scales is dominated by the parasitic instability.