Description
Turbulence in magnetically confined plasmas exhibits features of quasi-two-dimensional flows, an intermediate state between three-dimensional (3D) and two-dimensional (2D) turbulence[1]. In 3D, energy cascades forward from large to small scales until viscous dissipation, whereas in 2D, it transfers inversely to form large-scale structures like zonal flows[2], saturating turbulence. In fusion plasmas, both forward and inverse transfers can coexist. While the inverse cascade is known to generate zonal flows and improve confinement, experimental observation of the forward energy transfer, especially in the hot plasma core, remains limited due to diagnostic challenges in resolving multi-scale fluctuations.
Here we report direct evidence of forward energy transfer induced by core-edge interactions in tokamak density fluctuations, observed via collective Thomson scattering (CTS) and Doppler backscattering (DBS) diagnostics. A quasi-coherent mode (QCM) from the edge penetrates into the core and interacts with broad-band turbulence, producing significant forward energy transfer and secondary instabilities. These results reveal a pathway where multi-scale modes couple via resonant wave–wave interactions to drive forward energy transfer. Such a plasma regime is attractive for fusion reactors, as it avoids large ELMs that damage the first walls and maintains high confinement quality in the presence of high-Z impurities.
[1] Alexandros Alexakis, Reviews of Modern Plasma Physics, 2023 7.31.
[2] Z. Lin, T. S. Hahm, W. W. Lee et al., Science, 1998 281.1835.