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

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.

Figures (5)

• Figure 1: Characterization of the Zonal Flow (ZF) and its coupling with turbulence. (a) Radial profiles of the cross-correlation at three time delays, $\tau = -0.1\,\mathrm{ms}$, $0\,\mathrm{ms}$, and $+0.1\,\mathrm{ms}$, obtained using the toroidal correlation technique between separated probes. (b) Radial profiles of two key metrics: The effective ZF amplitude, evaluated from the fluctuation amplitude and coherence (black), and the anti-correlation between the ZF and the turbulence amplitude (blue).
• Figure 2: Isotope dependence of the ZF activity. Amplitude and coherence (long-range correlation) are plotted as a function of the H/D ratio, which was varied from $\sim 0.1$ (D-dominant) to $\sim 0.8$ (H-dominant). The ZF amplitude and coherence are also represented by the plot size and color, respectively. The inset figures on the right contain the same information as the three-dimensional figure but from different perspectives. Both the coherence and amplitude increase ($\sim$0.1–0.3 to $\sim$0.3–0.5 and $\sim$0.1–0.2 to $\sim$0.4–0.5, respectively) as the deuterium fraction increases.
• Figure 3: Isotope dependence of turbulence characteristics ($\mathrm{H}/\mathrm{D}$ ratio varied from $\sim 0.1$(D dominant) to $\sim 0.8$(H dominant). (a) Frequency spectra for potential fluctuation and (b) for density fluctuation. (c) Two-point cross-correlation of floating potential and (d) ion saturation current (density fluctuation), measured between adjacent probes (5 mm separation). (e) Cross-correlation between potential and density fluctuations. (f) Joint Probability Density Function in D plasma, and (g) H plasma, $PDF_{D}$ and $PDF_{H}$, showing a round/elliptic shape indicating decoupled/correlated quantities(Is vs Vf).(h) Difference between the two PDF distributions ($PDF_{D} - PDF_{H}$), highlighting the reduction of the correlated, asymmetric component at large fluctuation amplitudes in D ( $>$$\sigma$). Note that the axes are normalized by the standard deviation of fluctuation level to visualize the difference in the statistical properties of PDFs, and the H plasma has a broader distribution due to its larger fluctuation level without the normalization.
• Figure 4: Bicoherence analysis results for potential fluctuation in (a) D-dominant and (b) H-dominant discharges. (c) Spectra of summed bicoherence in the H and D dominant discharges.
• Figure 5: Turbulence-induced transport properties versus H/D ratio varied from $\sim$ 0.1 (D-dominant) to $\sim$ 0.8 (H-dominant). (a)fluctuation-induced particle flux in D(red) and H(black), (b)one-dimensional PDF, isotope dependence of (c)average particle flux <$\Gamma$>, (d)fluctuation amplitude of particle flux $\delta\Gamma$