Nonlinear energy transfer between multi-scale fluctuations in HL-2A tokamak edge plasmas

Abstract Plasma turbulence is a key factor in determining plasma transport behavior. Based on HL-2A edge plasma turbulence experiments, this study investigates nonlinear energy transfers among multi-scale fluctuations by observing phenomena such as inverse energy cascade, energy cascade, and dual cascade within turbulent systems. This is a comprehensive picture of the nonlinear energy transfer in tokamak edge-plasma turbulence. The turbulence spectral data were measured experimentally using the two-point method, and bispectral analysis was applied for data analysis. In edge plasma, a typical case is that the energy cascades from small-scale (in the high-frequency region) to large-scale structures (in the low-frequency region). The so-called inverse energy transfer can trigger or strengthen large-scale fluctuations. In another case, a certain portion (approximately 25%) of the energy in low-frequency turbulence cascades to higher-frequency turbulence, indicating that broadband free energy is transmitted to smaller scales through the coupling of E × B nonlinearity and large-scale potential fluctuations, causing the vortices to decrease and transform into higher-frequency turbulence. In the third case, in the two frequency regions, the energy in the relatively low-frequency and relatively high-frequency turbulence cascades and inversely cascades to medium-frequency turbulence, respectively, promotes the development of the latter. A concentrated three-wave coupling region is observed and it is distributed diagonally around the f 1 − f 2 plane, where f 1 and f 2 represent the mode frequencies. Finally, some brief discussions were held on the relevant results.