Gyrokinetic turbulence: a nonlinear route to dissipation through phase space

TL;DR

This paper introduces a phase space cascade framework for gyrokinetic turbulence, highlighting how small-scale structures in velocity space enable energy dissipation through collisions, with implications for space and fusion plasmas.

Contribution

It presents a novel conceptual model of kinetic plasma turbulence as a phase space cascade, including derived scaling laws and estimates for collisional effects.

Findings

Identification of nonlinear perpendicular phase mixing as a turbulence mechanism

Derivation of fluctuation spectra scaling relations

Estimation of collisional cutoff scales

Abstract

This paper describes a conceptual framework for understanding kinetic plasma turbulence as a generalized form of energy cascade in phase space. It is emphasized that conversion of turbulent energy into thermodynamic heat is only achievable in the presence of some (however small) degree of collisionality. The smallness of the collision rate is compensated by the emergence of small-scale structure in the velocity space. For gyrokinetic turbulence, a nonlinear perpendicular phase mixing mechanism is identified and described as a turbulent cascade of entropy fluctuations simultaneously occurring at spatial scales smaller than the ion gyroscale and in velocity space. Scaling relations for the resulting fluctuation spectra are derived. An estimate for the collisional cutoff is provided. The importance of adequately modeling and resolving collisions in gyrokinetic simulations is biefly discussed, as well as the relevance of these results to understanding the dissipation-range turbulence in the solar wind and the electrostatic microturbulence in fusion plasmas.