Integrated Observation of Isotope-Dependent Turbulence, Zonal Flow, and Turbulence-Driven Transport
To address the long-standing unresolved issue of "isotope effect" in plasma transport, this study investigates the hydrogen/deuterium (H/D) isotope dependence of a nonlinear turbulence system. The analysis focuses on the zonal flow (ZF) activity, turbulence properties, their nonlinear interaction, and resulting turbulent transport in a torus plasma. ZF activity, observed in low-density electron cyclotron heating plasmas in Heliotron J, is enhanced with increasing D gas fraction from 10 to 80 percent. While the turbulence scale size in the edge region (rho approx 0.8) is larger in D plasmas, the reduction and decoupling of fluctuations, associated with an enhanced ZF, results in beneficial impacts on turbulent transport, driven by an enhanced nonlinear coupling between the ZF and turbulence in D plasmas. These differences in the turbulence nature lead to the significant reduction of turbulence-induced transport observed in the D plasma. These comprehensive observations suggest that the isotope dependence on the turbulence system is essential for explaining the isotope effect on confinement improvement and is vital in predicting the performance of future fusion reactors.