Quantified causality dependence of dynamical relation between zonal flow and heat transport on isotope mass in tokamak edge plasmas

The isotope effect on zonal flows (ZFs) and turbulence remains a key issue that is not completely solved in fusion plasmas. This paper presents the first experimental results of the ab initio prediction of causal relation between geodesic acoustic mode (GAM) and ambient turbulence at different isotope masses in the edge of HL-2A tokamak, where transfer entropy method based on information-theoretical approach is utilized as a quantified indicator of causality. Analysis shows that GAM is more pronounced in deuterium plasmas than in hydrogen, leading to a lower heat transport as well as more peaked profiles in the former situation. The causal impact of GAM on conductive heat flux component is stronger than on the convective component, which is resulted from a larger causal influence of zonal flow on temperature fluctuation. While a stronger GAM in deuterium plasmas has larger influence on all flux components, the relative change in temperature fluctuation and coefficient is more obvious when the ion mass varies. These findings not only offer an in-depth understanding of the real causality between zonal flow and turbulence in the present isotope experiments, but also provide useful ways for the physical understandings of transport and zonal flow dynamics in future deuterium–tritium fusion plasmas.