Experimental evidence of forward energy transfer induced by core-edge coupling in multiscale tokamak turbulence
Turbulence in magnetically confined plasmas is inherently quasi-two-dimensional and can exhibit both inverse and forward energy transfer across scales. Experimental evidence for cross-scale energy transfer in the hot plasma core has, however, remained scarce. Understanding such energy transfer is crucial because it governs turbulent transport, which directly impacts confinement quality of fusion plasmas. Here we report experimental evidence consistent with forward energy transfer associated with core-edge coupling in tokamak turbulence. We combined multichannel collective Thomson scattering (CTS) and radially localized Doppler backscattering (DBS) to resolve cross-scale fluctuation dynamics. The results reveal that a quasi-coherent mode (QCM) originating from the edge is also present in the core and resonantly couples with broadband turbulence, producing significant forward energy transfer and characteristic secondary instabilities. These combined measurements indicate that quasi-two-dimensional tokamak turbulence can support a net energy transfer from ion to electron scales through cross-scale wave-wave interactions, contrary to the inverse cascade paradigm of idealized two-dimensional turbulence. Such a plasma regime is attractive for fusion reactors, as it avoids large edge-localized modes (ELMs) that damage first-wall components and maintains high confinement quality even in the presence of high-Z impurities.