Role of stable eigenmodes in saturated local plasma turbulence

Abstract

The excitation of stable eigenmodes in unstable plasma turbulence, previously documented in collisionless trapped electron mode turbulence, is shown to be a generic behavior of local (quasihomogeneous) systems. A condition is derived to indicate when such excited eigenmodes achieve a sufficient level in saturation to affect the turbulence, and produce changes in saturation levels, instability drive, and transport. The condition is shown to be consistent with the results of collisionless and dissipative trapped electron turbulence, and is further illustrated by an entirely different model describing simple ion turbulence driven by the ion temperature gradient. The condition suggests that all eigenmodes of the ion model affect saturation, but none dominates. This is consistent with the results of simulations, which show nonlinear modifications to eigenmode structure, growth rate, and transport that occur intermittently in time, despite fixed driving gradients.