Analysis of Finite-β Ion Temperature Gradient Turbulence Saturation

Gyrokinetic simulations of finite-β ion temperature gradient (ITG) turbulence for Cyclone-base-case conditions show strong dependence of the ion heat flux on the triplet correlation time of the interaction of unstable modes, zonal flows and stable modes¹. Analysis of a simpler fluid model dominated by the same saturation channel shows that the β scaling of the correlation time arises when the detuning of the three-wave resonance by finite Larmor radius induced dispersion uncovers the β dependence of the complex mode frequency. Consequently, the triplet correlation time has strong dependence on both β and inverse perpendicular and parallel wavenumbers. A five-field finite-β ITG model is derived and studied in low β and strong ballooning limits to further investigate the triplet correlation time and specifically, the role of parallel mode structure, whether the ion parallel flow can decouple as it does at zero β, and whether flutter nonlinearities can compete with electrostatic zonal flow catalyzed transfer from unstable to stable modes. The role of marginally stable modes, particularly in lower β regimes, is investigated, and parameter space is searched for regions with low fluxes.

1. G.G. Whelan et al., PRL 120, 175002 (2018).