Local Gyrokinetic Collisional Theory of the Ion-Temperature Gradient Mode

TL;DR

This paper develops a gyrokinetic collisional theory for ion-temperature-gradient modes, analyzing their linear properties with various collision operators, and highlights the importance of finite Larmor radius effects and energy diffusion at different collisionalities.

Contribution

It introduces a gyro-moment hierarchy approach to study collisional effects on ITG modes, comparing multiple collision operators and emphasizing the role of finite Larmor radius effects.

Findings

Strong damping of ITG modes at small scales with increased collisionality.

Finite Larmor radius effects suppress short wavelength ITG branches.

The Sugama collision operator closely matches Coulomb collision results.

Abstract

We present a study of the linear properties of ion temperature gradient (ITG) modes with collisions modelled by the linearized gyrokinetic (GK) Coulomb collision operator (Frei et al. 2021) in the local limit. The study is based on a Hermite-Laguerre polynomial expansion of the perturbed ion distribution function applied to the linearized GK Boltzmann equation, yielding a hierarchy of coupled equations for the expansion coefficients, referred to as gyro-moments. We explore analytically the collisionless and high-collisional limits of the gyro-moment hierarchy. Parameter scans revealing the dependence of the ITG growth rate on the collisionality are reported, showing strong damping at small scales as the collisionality increases. These properties are compared with the predictions based on the Sugama, the momentum-conserving pitch-angle scattering, the Hirshman- Sigmar-Clarke, and the Daugherty collision operators. The importance of finite Larmor radius (FLR) terms in the collision operators is pointed out by the appearance of a short wavelength (SW) ITG branch when collisional FLR terms are neglected, this branch being completely suppressed by collisional FLR effects. We demonstrate that energy diffusion is important at high collisionality and small scale lengths and that, among the collision operators considered in this work, the GK Sugama collision operator yields, in general, the smallest deviation on the ITG growth rate compared to the GK Coulomb collision operator. Convergence studies of the gyro-moment method are reported.