Integrated Observation of Isotope-Dependent Turbulence, Zonal Flow, and Turbulence-Driven Transport

TL;DR

This study investigates how isotope differences between hydrogen and deuterium affect plasma turbulence, zonal flow activity, and transport, revealing that deuterium enhances zonal flows and reduces turbulence-driven transport, which is crucial for fusion reactor performance.

Contribution

It provides the first comprehensive observation linking isotope-dependent turbulence, zonal flow activity, and transport in plasma, highlighting the importance of isotope effects in fusion confinement.

Findings

Deuterium increases zonal flow activity in plasma.

Turbulence scale size is larger in D plasmas at the edge.

Deuterium reduces turbulence-induced transport significantly.

Abstract

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.