Gyrokinetic simulation of isotope effect on zonal flow response in tokamaks

The isotope effect on zonal flow is a critical issue for turbulent transport in burning plasmas. In this work, we conduct global simulations of the isotope effects of hydrogen isotopes (H, D, and T) on the zonal flow across arbitrary wavelengths using the Gyrokinetic Toroidal Code. Our results reveal that the residual zonal flow (RZF) level exhibits a significant isotope effect primarily at intermediate wavelengths (i.e., krρi∼1). The strength of this isotope effect and the wavelength at which it is strongest are mainly influenced by isotope mass Mi and safety factor q, with a lesser impact from the inverse aspect ratio ε. The isotope effect intensifies as the isotope mass increases from H to T or as the safety factor q increases. Simulations incorporating kinetic electrons show that the contribution of electron polarization to the RZF level is minimal compared to that of thermal ions at intermediate wavelengths, indicating that electron polarization has a negligible effect on the zonal flow isotope effect. Additionally, our simulation results for the isotope effect on RZF, as well as on geodesic acoustic mode frequency and damping rate, align with analytic theories. However, we suggest that earlier analytic theory may overestimate the isotope effect on RZF in this crucial intermediate wavelength range. Overall, our findings indicate that the isotope effect on zonal flow may contribute to isotope turbulent transport, particularly in trapped electron turbulence and turbulence in high-q region.