  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.

Plasma Physics