Three-dimension spectral characteristics of low-frequency Zonal Flow in the edge plasma of HL-2A tokamak

The low-frequency Zonal Flow (LFZF), coexisting with the geodesic acoustic mode (GAM) was observed at the edge of HL-2A tokamak plasmas using four Langmuir probe arrays. It is identified as a low-frequency coherent mode peaking near zero frequency and broadening several kilohertz on the power spectral density (PSD) of the floating potential fluctuations. Besides the poloidal and toroidal axisymmetry, the radial wavenumber-frequency spectra of the LFZF was firstly estimated. The threedimensional wavenumber-frequency spectral characteristics: kθ = kφ ≃ 0, k̄rρi = 3.3 × 10 with the full width at half maximum ∆krρi = 0.19, are almost consistence with the theoretical predications and simulation results. The radial spectra showing that the LFZF packages propagate both outwards and inwards with a small net value outward, resulting in smaller phase shifts between two probe tips separated radially at the LFZF frequency than the GAM frequency. The envelope and the bispectrum analysis of the poloidal velocity fluctuations both reveal that the interaction between the LFZF and the ambient turbulence (AT) is as similar as that between the GAM and the AT, suggesting the same generation mechanism. But the differences are: the coherent coefficients between the LFZF and the envelope of the AT are smaller than that between the GAM and the AT, and the intensity of the tri-wave interactions including the LFZF is also much smaller than that including the GAM. These phenomena may both caused by the small amplitude of the LFZF in the edge region.