A comparison of kinetic and fluid approaches to ITG turbulence using the gyrokinetic code GX

Following optimizations to reduce neoclassical losses, turbulent transport will be the dominant mode of transport in modern stellarator designs. The geometric properties of the three-dimensional confining magnetic field that influence the behavior of turbulence are not well-understood. In an analytic model developed by Hegna, Terry, & Faber (2018), specific geometric quantities of the confining magnetic field that influence the turbulence associated with the Ion Temperature Gradient (ITG) instability have been identified. This analytic model is constructed using a three-field fluid model. To gain an understanding of the limitations and shortcomings that this may impose on the rest of their model, we compare a three-field fluid model against a kinetic model in gyrokinetic calculations of quantities associated with ITG turbulence. We do this by using the unique gyrokinetic solver GX, which employs an algorithm that reduces to a three-field fluid model at low-velocity resolutions and converges to an Eulerian approach at high-velocity resolutions. We demonstrate that a fluid approach used for turbulence-related calculations will suffer from inaccuracies in low collisionality environments, which are typical in modern tokamaks and stellarators.