Predicting the Z-pinch Dimits shift through gyrokinetic tertiary instability analysis of the entropy mode

TL;DR

This paper uses gyrokinetic simulations and tertiary instability analysis to predict the Dimits shift in plasma turbulence, confirming features from fluid models and improving predictive accuracy with kinetic effects.

Contribution

It extends the tertiary instability framework to fully gyrokinetic simulations in Z-pinch geometry, enhancing the understanding and prediction of the Dimits shift.

Findings

Confirmed key features of tertiary instability in gyrokinetic context

Validated reduced-mode tertiary model for predicting the Dimits shift

Achieved accurate predictions with minimal modifications for kinetic effects

Abstract

The Dimits shift, an upshift in the onset of turbulence from the linear instability threshold, caused by self-generated zonal flows, can greatly enhance the performance of magnetic confinement plasma devices. Except in simple cases, using fluid approximations and model magnetic geometries, this phenomenon has proved difficult to understand and quantitatively predict. To bridge the large gap in complexity between simple models and realistic treatment in toroidal magnetic geometries (e.g. tokamaks or stellarators), the present work uses fully gyrokinetic simulations in Z-pinch geometry to investigate the Dimits shift through the lens of tertiary instability analysis, which describes the emergence of drift waves from a zonally dominated state. Several features of the tertiary instability, previously observed in fluid models, are confirmed to remain. Most significantly, an efficient reduced-mode tertiary model, which previously proved successful in predicting the Dimits shift in a gyrofluid limit (Hallenbert & Plunk, J. Plasma Phys., vol.87, issue 05, 2021, 905870508), is found to be accurate here, with only slight modifications to account for kinetic effects.