The Experimental Study Of Zonal Flow And Its Role In The L-H Mode Transition In The Edge Of Tokamak Plasmas

Zonal flows (ZFs) are ubiquitous in fluid turbulence, for example, in planetary atmospheres, and the terrestrial jet stream, and are of general interest in turbulence self organization across different scales. Over the last decade, it has been recognized that spontaneously generated ZFs may also play an important role for turbulence or transport self-regulation in magnetically confined plasmas. Considerable attention has focused on their potential role as a trigger mechanism for the transition from low-to high confinement regimes (L-to H-mode transition) in toroidal fusion plasmas, reduc-ing edge plasma turbulence (and transport) possibly in combination with equilibrium E x B flow shear. Our study concentrate at the excitation, propagation and nonlinear interaction with ambient turbulence, as well as the role of the L-H mode transition. The main results are performed as following:1) Three sets of triple probe arrays (four-tip) are applied to study the transport properties in the edge of HL-2A tokamak. The Geodesic Acoustic Mode (GAM) ex-hibits the intermittency characteristics during the discharge. The radial particle flux has been studied under different phases corresponding to the variable GAM intensity. The experiment results reveal that the radial particle flux has been suppressed by13%during the GAM bursts contrasting to weak GAM cases in the frequency range of am-bient turbulence (20kHz-100kHz). Power of density fluctuations and coherence between density and potential fluctuations contribute most reducing of particle flux, while changing of cross phase between density and potential fluctuations and suppres-sion of power of potential fluctuations contribute few to it. It suggests that the GAM may regulate the turbulent transport mainly by changing the amplitude of ambient turbulence, rather than cross phase between density and potential fluctuations.2) Geodesic acoustic mode (GAM) and low frequency zonal flow (LFZF) are both observed through Langmuir probe arrays during electron cyclotron resonance heating (ECRH) on the HL-2A tokamak edge. The radial distributions of the ampli-tude and peak frequency of GAM in floating potential fluctuations are investigated through rake probe arrays under different ECRH powers. For the first time it is ob- served that the GAM frequency decreased due to the carbon impurity increasing and the experimental results are qualitatively compared with theoretical dispersion rela-tionship of GAM including the effects of impurities. It is also found that during ECRH phase besides the mean flow, both GAM and LFZF are strengthened. The total fluc-tuation power and the fraction of that power associated with zonal flows power both increase with the ECRH power, consistent with a predator-prey model. The auto-and cross-bicoherence analyses show the coupling between GAM and its second harmon-ic during ECRH phase firstly. Moreover, the results also suggest that the couplings between GAM and the components with multiple GAM frequency are strengthened during ECRH phase. These couplings may be important for the GAM saturation.3) Multiple GAMs, especially dual GAMs are studied through the Langmuir probe arrays at the edge plasmas of HT-7Tokamak with Lithium coated wall. The dual GAMs are called as low freqeuncy GAM (LFGAM) and high frequency GAM (HFGAM) respectively and it is found that the two modes both have maximum am-plitudes at the location with radial wavenumber close to zero. Within the measur-ing range, HFGAM propagates outwards while LFGAM propagates both inwards and outwards with their central frequencies nearly unchanged. These phenomena could be basically explained by the kinetic GAM theory and quantitative comparison is al-so done on the dispersion relationship of HFGAM. The nonlinear couplings between the kinetic GAMs are also analysed. It is observed that LFGAM strongly interacts with HFGAM near the location with maximum amplitude, additionally, bicoherence and amplitude correlation analyses both suggest that the second harmonic GAMs are probably generated from self interaction of fundamental GAMs.4) An intermediate stage (I-phase) between L-mode and H-mode has been ob-served on HL-2A when heating power is close to threshold power of L-H mode tran-sition. Langmuir probe arrays have performed to measure the physical parameters inside LCFS as potential Φf, electric field Er, density ne, temperature Te and pressure Pe. The experimental results reveal that an oscillation with frequency of-2kHz can be found in all those parameters which lag with amplitude of electric field|Er|by π/2, implying that the oscillations in ne Te Pe are induced by the time changing of|Er|. The amplitude of ambient turbulence|ΦAT|estimated from envelope analysis is also found lags with|Er|by π/2. The result disagrees with the predictions of "Predator- Prey" model. Moreover, magnetic hystersis loop of mean electric field Er has been found during the L-I mode transition and I-L mode back transition, which is consistent with the prediction of the transition model by S-I. Itoh.