The Eulerian variational formulation of the gyrokinetic system in general spatial coordinates

TL;DR

This paper develops a variational formulation of the gyrokinetic system in general coordinates, deriving a symmetric pressure tensor and momentum balance equations useful for advanced plasma turbulence simulations.

Contribution

It extends previous work by deriving a coordinate-invariant variational formulation that yields a symmetric pressure tensor and accounts for turbulence, collisions, and external sources.

Findings

Derived a coordinate-invariant local momentum balance equation.

Obtained a symmetric pressure tensor including turbulence effects.

Analyzed modifications due to collisions and external sources.

Abstract

The Eulerian variational formulation of the gyrokinetic system with electrostatic turbulence is presented in general spatial coordinates by extending our previous work [H. Sugama, {\it et al}., Phys.\ Plasmas {\bf 25}, 102506 (2018)]. The invariance of the Lagrangian of the system under an arbitrary spatial coordinate transformation is used to derive the local momentum balance equation satisfied by the gyrocenter distribution functions and the turbulent potential which are given as solutions of the governing equations. This derivation is in contrast with the conventional method using the spatial translation in which the asymmetric canonical pressure tensor generally enters the momentum balance equation. In the present study, the variation of the Lagrangian density with respect to the metric tensor is taken to directly obtain the symmetric pressure tensor which includes the effect of turbulence on the momentum transport. In addition, it is shown in this work how the momentum balance is modified when the collision and/or external source terms are added to the gyrokinetic equation. The results obtained here are considered useful for global gyrokinetic simulations investigating both neoclassical and turbulent transport processes even in general non-axisymmetric toroidal systems.