Description
Identifying the elusive 'mediator' structures that facilitate nonlocal transport, in which thermodynamic fluxes respond to forces in distant regions, remains a fundamental challenge across turbulent fluids, geophysical flows, and disordered materials. In magnetically confined plasmas, while nonlocal behaviors like hysteresis in flux-gradient relations are frequently observed, direct detection of the mediating structures has been hindered by their transient nature and the limited spatiotemporal resolution of conventional diagnostics. This study presents the direct experimental identification of coexisting local and nonlocal turbulence in the Large Helical Device (LHD), utilizing high-resolution spatiotemporal diagnostics to overcome these limitations.
We conducted Modulated Electron Cyclotron Heating (MECH) experiments with systematically varied pulse durations (s) ranging from 4 ms to 1280 ms to probe the plasma's response across different timescales[1,2]. Using microwave Doppler reflectometry and W-band millimeter-wave backscattering, we simultaneously resolved ion- ($k_{\perp}\rho_s\sim 1.7$) and electron-scale ($k_{\perp}\rho_s\sim7$) turbulence. Spectral analysis successfully isolated two distinct regimes: a high-frequency "local" component (50–100 kHz) and a low-frequency "nonlocal" component (10–20 kHz). The local component exhibits a strong linear correlation with the local electron temperature gradient and carries bulk heat flux. Conversely, the nonlocal component appears nearly simultaneously (within $\sim$2 ms) across macroscopic distances and shows no correlation with local gradients. Cross-correlation analysis confirms that this low-frequency mode facilitates rapid spatial coupling on timescales of $\sim 100\ \mu$s, effectively acting as a mediator.
A critical finding is the discovery of a power-law scaling between the heat-pulse propagation speed $v$ and the pulse duration: $v\propto s^{-1.06}$. This relationship demonstrates that shorter heating pulses, which induce stronger deviations from the steady state, result in significantly faster propagation speeds, consistent with the trend extrapolated to avalanche events observed during transport barrier collapse[3]. Furthermore, the intensity ratio of nonlocal-to-local turbulence increases as the pulse duration shortens, confirming that the nonlocal mediator becomes dominant in conditions strongly deviating from the steady state. These results provide the direct evidence of mediator structures enabling nonlocal pathways, offering a comprehensive framework for analyzing multiscale transport dynamics in complex non-equilibrium systems.
References
[1] N. Kenmochi et al., Scientific Reports 14 (2024) 13006
[2] N. Kenmochi et al., Communications Physics 8 (2025) 492
[3] N. Kenmochi et al., Scientific Reports 12 (2022) 6979