Comparisons and physics basis of tokamak transport models and turbulence simulations

TLDR

Gyrokinetic, gyrofluid, and thermal transport models for ITG turbulence in tokamak plasmas are compared to assess how differences in physics content affect fusion‑relevant performance metrics for ITER. The study uses a simplified plasma model and geometry based on the Doublet III‑D H‑mode experiment, examining numerical convergence (mesh size, system size, particle number) to explain discrepancies among models. While most models agree on linear growth rates and frequencies, they differ significantly in predicted transport levels, especially due to equilibrium choices, impacting fusion‑performance predictions.

Abstract

The predictions of gyrokinetic and gyrofluid simulations of ion-temperature-gradient (ITG) instability and turbulence in tokamak plasmas as well as some tokamak plasma thermal transport models, which have been widely used for predicting the performance of the proposed International Thermonuclear Experimental Reactor (ITER) tokamak [Plasma Physics and Controlled Nuclear Fusion Research, 1996 (International Atomic Energy Agency, Vienna, 1997), Vol. 1, p. 3], are compared. These comparisons provide information on effects of differences in the physics content of the various models and on the fusion-relevant figures of merit of plasma performance predicted by the models. Many of the comparisons are undertaken for a simplified plasma model and geometry which is an idealization of the plasma conditions and geometry in a Doublet III-D [Plasma Physics and Controlled Nuclear Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 159] high confinement (H-mode) experiment. Most of the models show good agreements in their predictions and assumptions for the linear growth rates and frequencies. There are some differences associated with different equilibria. However, there are significant differences in the transport levels between the models. The causes of some of the differences are examined in some detail, with particular attention to numerical convergence in the turbulence simulations (with respect to simulation mesh size, system size and, for particle-based simulations, the particle number). The implications for predictions of fusion plasma performance are also discussed.