Abstract
The tertiary instability is believed to be important for governing magnetised plasma turbulence under conditions of strong zonal flow generation, near marginal stability. In this work, we investigate its role for a collisionless strongly driven fluid model, self-consistently derived as a limit of gyrokinetics. It is found that a region of absolute stability above the linear threshold exists, beyond which significant nonlinear transport rapidly develops. Characteristically, this range exhibits a complex pattern of transient zonal evolution before a stable profile can arise. Nevertheless, the Dimits transition itself is found to coincide with a tertiary instability threshold, so long as linear effects are included. Through a simple and readily extendable procedure, tracing its origin to St-Onge (J. Plasma Phys., vol. 83, issue 05, 2017, 905830504), the stabilising effect of the typical zonal profile can be approximated, and the accompanying reduced mode estimate is found to be in good agreement with nonlinear simulations.
Highlights
Experimental fusion devices exhibit significantly higher transport than neoclassical predictions
St-Onge (2017) and Zhu, Zhou & Dodin (2020a), for example, based accurate predictions upon it, while Li & Diamond (2018) and Ivanov et al (2020) on the contrary reported finding it unimportant. To help rectify this confusion, in this paper we will attempt to shed some light on the tertiary mode in the Dimits regime, investigating its relevance for the Dimits transition in a strongly driven fluid system directly derived from gyrokinetics
We will here present another self-consistently closed gyrofluid system in two spatial dimensions, in the hope that it may prove yet another useful stepping stone to solidify and clarify the emerging picture of the Dimits shift when proceeding towards the full gyrokinetic problem
Summary
Experimental fusion devices exhibit significantly higher transport than neoclassical predictions. St-Onge (2017) and Zhu, Zhou & Dodin (2020a), for example, based accurate predictions upon it, while Li & Diamond (2018) and Ivanov et al (2020) on the contrary reported finding it unimportant To help rectify this confusion, in this paper we will attempt to shed some light on the tertiary mode in the Dimits regime, investigating its relevance for the Dimits transition in a strongly driven fluid system directly derived from gyrokinetics. The tertiary instability alone seems sufficient to encapsulate the Dimits transition for the system under consideration This is despite the fact that this system is ostensibly similar to the one recently studied by Ivanov et al (2020), where the opposite case was found to hold, a discrepancy arising from the present absence of collisional zonal flow damping.
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