Generalized universal instability: Transient linear amplification and subcritical turbulence

TL;DR

This paper demonstrates that a kinetic plasma system without unstable eigenmodes can still exhibit significant transient linear amplification and sustain nonlinear turbulence, challenging traditional stability assumptions.

Contribution

It shows that non-modal growth and subcritical turbulence can occur in plasma systems lacking unstable eigenmodes, revealing new dynamics in plasma stability analysis.

Findings

Transient linear amplification occurs under weak magnetic shear and collisionality.

Nonlinear turbulence can be sustained at sufficient initial amplitudes.

Turbulent states exhibit transient linear amplification of gyrokinetic free energy.

Abstract

In this work we numerically demonstrate both significant transient (i.e. non-modal) linear amplification and sustained nonlinear turbulence in a kinetic plasma system with no unstable eigenmodes. The particular system considered is an electrostatic plasma slab with magnetic shear, kinetic electrons and ions, weak collisions, and a density gradient, but with no temperature gradient. In contrast to hydrodynamic examples of non-modal growth and subcritical turbulence, here there is no sheared flow in the equilibrium. Significant transient linear amplification is found when the magnetic shear and collisionality are weak. It is also demonstrated that nonlinear turbulence can be sustained if initialized at sufficient amplitude. We prove these two phenomena are related: when sustained turbulence occurs without unstable eigenmodes, states that are typical of the turbulence must yield transient linear amplification of the gyrokinetic free energy.