Moment-Based Approach to the Flux-Tube linear Gyrokinetic Model

TL;DR

This paper introduces a moment-based gyrokinetic model for tokamak flux-tube geometry, enabling detailed velocity-space analysis and efficient collision modeling, validated against established codes and applicable across various plasma regimes.

Contribution

It develops and implements a gyro-moments hierarchy approach for linear electromagnetic gyrokinetics, capturing fine velocity-space structures and collision effects in tokamak plasmas.

Findings

The GM approach accurately reproduces known linear modes and damping.

Fewer moments are needed at higher collisionality for convergence.

Collision operator approximations vary in accuracy depending on plasma conditions.

Abstract

This work reports on the development and numerical implementation of the linear electromagnetic gyrokinetic (GK) model in a tokamak flux-tube geometry using a moment approach based on the expansion of the perturbed distribution function on a velocity-space Hermite-Laguerre polynomials basis. A hierarchy of equations of the expansion coefficients, referred to as the gyro-moments (GM), is derived. We verify the numerical implementation of the GM hierarchy in the collisionless limit by performing a comparison with the continuum GK code GENE, recovering the linear properties of the ion-temperature gradient, trapped electron, kinetic ballooning, and microtearing modes, as well as the collisionless damping of zonal flows. The present investigation reveals the ability of the GM approach to describe fine velocity-space scale structures appearing near the trapped and passing boundary and kinetic effects associated with parallel and perpendicular particle drifts. In addition, the effects of collisions are studied using advanced collision operators, including the GK Coulomb collision operator. The main findings are that the number of GMs necessary for convergence decreases with plasma collisionality and is lower for pressure gradient-driven modes, such as in H-mode pedestal regions, compared to instabilities driven by trapped particles and magnetic gradient drifts often found in the core. The accuracy of approximations often used to model collisions (relative to the GK Coulomb operator) is studied, showing differences between collision operator models that increase with collisionality and electron temperature gradient in the case of TEM. The present linear analysis demonstrates that the GM approach efficiently describes the plasma dynamics for typical parameters of the tokamak boundary, ranging from the low-collisionality banana H-mode to the high-collisionality Pfirsch-Schl\"uter conditions.