Bistable turbulence in strongly magnetised plasmas with a sheared mean flow

TL;DR

This paper reports the discovery of bistable turbulence in strongly magnetised plasmas, where two distinct turbulent states coexist under identical conditions, challenging the traditional view of a unique turbulent state.

Contribution

It reveals the existence of two stable turbulence states in magnetised plasmas and explains their regulation by mean flow shear and zonal flows, based on first-principle gyrokinetic simulations.

Findings

Low-transport state matches experimental observations.

High-transport state can coexist with low-transport state.

Bistability arises from the interplay between mean shear and zonal flows.

Abstract

The prevailing paradigm for plasma turbulence associates a unique stationary state to given equilibrium parameters. We report the discovery of bistable turbulence in a strongly magnetised plasma. Two distinct states, obtained with identical equilibrium parameters in first-principle gyrokinetic simulations, have turbulent fluxes of particles, momentum and energy that differ by an order of magnitude, with the low-transport state agreeing with experimental observations. Occurrences of the two states are regulated by the competition between an externally imposed mean flow shear and "zonal" flows generated by the plasma. With small turbulent amplitudes, zonal flows have little impact, and the mean shear causes turbulence to saturate in a low-transport state. With larger amplitudes, the zonal shear can (partially) oppose the effect of the mean shear, allowing the system to sustain a high-transport state. This poses a new challenge for research that has so far assumed a uniquely defined turbulent state.