Experimental Study Of Low-frequency Zonal Flows In Tokamak Edge Plasma

The zonal ?ows in tokamak plasma are predicted to play an important role inregulating the turbulence and enhancing the confinement. In other words, the role ofthe mutual interaction between zonal ?ows and turbulence are emphasized as the pro-cess of determining turbulence saturation and transport level. There are two branchesfor the zonal ?ows in toroidal plasmas: the low-frequency zonal ?ows (LFZF) andGeodesic Acoustic Mode (GAM). The experimental study of LFZF is the main sub-ject of this thesis.The LFZF, coexisting with the GAM is observed at the edge of HL-2A tokamakplasmas using four Langmuir probe arrays. It is identified as a low-frequency coher-ent mode peaking close to zero frequency and broadening about 3kHz on the powerspectral density of the ?oating potential ?uctuations. Besides the poloidal and toroidalaxisymmetry, the radial wavenumber and correlation length of the LFZF are also esti-mated. The averaged radial wavenumber is kr = 0.15～0.5cm?1 with the full widthat half maximum ?kr = 3～4 cm?1. The spectral for the LFZF radial wavenumberappear to have two peaks at approximately symmetric positive and negative values,implying the LFZF packages propagate both inwards and outwards. This is the firstcomplete measurement of three-dimension spectral structure of the LFZF in the edgeplasma.The envelope and the bispectrum analysis both reveal that the interaction betweenthe LFZF and the ambient turbulence (AT) is similar as that between the GAM and theAT, suggesting the same generation mechanism. One difference is that the intensity ofthe wave triad interactions including the LFZF is much smaller than that including theGAM and the other difference is the coherence coefficients between the LFZF and theenvelope of the AT is also smaller that that between the GAM and the AT envelope,both of which could be explained by the smaller amplitude of the LFZF in the edgeregion.The trispectrum analysis is firstly applied to the ?oating potential ?uctuations.The tricoherence of GAM is also larger than that of LFZF. It is also found that the averaged intensity of four-wave interactions is much smaller than that of three-waveinteractions, implying that the three-wave coupling dominates the nonlinear interac-tion process.