Gyrokinetic turbulence: between idealized estimates and a detailed analysis of nonlinear energy transfers

TL;DR

This paper uses high-resolution simulations to analyze nonlinear energy transfers in gyrokinetic turbulence, revealing the existence of asymptotic locality and clarifying the role of non-local interactions within a plasma's energy cascade.

Contribution

It demonstrates for the first time that asymptotic locality exists in gyrokinetic turbulence and explains non-local interactions as local transfers within dyadic scales.

Findings

Asymptotic locality is confirmed in GK turbulence.

Non-local interactions are explained as local transfers within dyadic scales.

Locality exponents are related to energy exchanges between scales.

Abstract

Using large resolution numerical simulations of GK turbulence, spanning an interval ranging from the end of the fluid scales to the electron gyroradius, we study the energy transfers in the perpendicular direction for a proton-electron plasma in a slab magnetic geometry. In addition, to aid our understanding of the nonlinear cascade, we use an idealized test representation for the energy transfers between two scales, mimicking the dynamics of turbulence in an infinite inertial range. For GK turbulence, a detailed analysis of nonlinear energy transfers that account for the separation of energy exchanging scales is performed. We show that locality functions associated with the energy cascade across dyadic (i.e. multiple of two) separated scales achieve an asymptotic state, recovering clear values for the locality exponents. We relate these exponents to the energy exchange between two scales, diagnostics that are less computationally intensive than the locality functions. It is the first time asymptotic locality is shown to exist for GK turbulence and the contributions made by highly non-local interactions, previously reported in the literature, are explained as very local transfers of energy that occur between wavenumbers within the same dyadic signal. The results presented here and their implications are discussed from the perspective of previous findings reported in the literature and the idea of universality of GK turbulence.