Experimental Studies Of Self-Regulation Dynamics Of Drift Wave-Zonal Flow Turbulence In A Linear Magnetized Plasma

Zonal flows, which are potential structures and driven by plasma turbulence spon-taneously, are studied in a linear magnetized plasma. Theoretical and numerical results predict that zonal flows have symmetric characteristics in both poloidal and toroidal directions and finite radial wavenumbers. Zonal flows have two branches:Geodesic Acoustic Mode (shortened as GAM) and the Low-Frequency Zonal Flow (shortened as LFZF), while the former is coupled in a torus and only the latter can be observed in a linear device.From theory and simulation results, zonal flows gain energy from the ambient tur-bulence and also regulate the turbulence by E×B shearing. As a result, the amplitude of the turbulence is reduced and the radial coherent length is shortened, leading up to the control of turbulence and the enhancement of confinement. The drift wave-zonal flow system, which is constituted by both drift-wave turbulence and zonal flows, has already been widely accepted. Therefore, zonal flows play a very crucial role in the magnetized fusion plasmas researches.Linear magnetic plasma devices are normally smaller comparing to tokamaks and stellarators. However, they have many advantages such as relatively simple topology of magnetic field, easier operation and more comprehensive diagnosis of plasma pa-rameters. Therefore many basic problems can be studied in experiment more deeply. Some devices of this kind are still being operated today.The Linear Magnetic Plasma Device (shortened as LMPD) is designed and built by the Chinese Academy of Sciencers (shortened as CAS) Key Laboratory of Basic Plasma Physics. We find it that a wide spectrum with several drift-wave harmonics driven by density gradient is captured by increasing the axial magnetic field and LFZF emerges. The Langmuir probe arrays are arranged so that the mode characteristics of three directions are estimated to be consistent with the theory and simulation predic-tions, i.e., the azimuthal mode number m=0, the axial wavenumber k||=0, a finite radial wavenumber of kr (?)0.15cm-1,△kr(?)1.14cm-1, and a very near zero mode frequency. The Lorentzian distribution fitting of the cross power spectra of ZF with the az-imuthal angle of π from B=480-940G give very near values of the half-width, suggesting that the linear damping rate of ZF remains nearly constant. Considering the plasma and the neutral gas parameters, the ZF’s damping rate by ion viscosity is cal-culated to be much smaller than the one by ion-neutral collision. So, the latter, which is nearly constant along with the change of B, should be the primary mechanism in ZF’s damping.There are significant wave-wave couplings between ZFs and the high-frequency fluctuations of ambient turbulence through auto-bispectral analysis. Through envelope analysis, the couplings include two processes:the direct regulation of ZFs during their generation in the energy-conserving triad interaction, i.e., the parametric-modulational instability, and the shearing effect by ZFs, i.e., the Doppler-shearing which transfers the turbulent energy from low to high frequencies.The self-regulation dynamics of the DW-ZF system are studied in experiment by changing the axial magnetic field. It is clear that the linear growth rate and the total energy increase along with B, while ZFs are excited above some threshold of magnetic field and then develops. The ratios of relative energies, are qualitatively consistent with the self-regulation dynamics of the DW-ZF turbulence predicted by the generalized predator-prey model.In order to understand the shearing physics of DW by ZF, the evolutions of the radial and azimuthal wavenumber spectrum with the effective shearing rate induced by ZF are studied. With the increase of shearing rate, both the averaged radial wavenum-ber and its width increase, suggesting the decrease of the step size of turbulent diffu-sion and thus the turbulent transport level, while both the average azimuthal wavenum-ber and its width decrease. These results imply that the averaged eddies of plasma tur-bulence are narrowed in the radial direction and elongated in the azimuthal direction by the ZF shearing effect, in agreement with theory and simulation predictions.Through conditional analysis, the suppression of the radial flux by ZF’s shearing effect is also studied in detail, showing that the suppression of cross phase of potential and density fluctuations plays the dominant role in such suppression. Such results are consistent with some analytical and numerical predictions, and also in agreement with several observations which were performed on other devices.Furthermore, the nonlinear energy transform is also studied through the estima-tion of the nonlinear energy coupling function, the calculation of the transition flow, and amplitude correlation analysis.Lastly, some experiments on coherent structures and blobs are also carried on. We find that in the edge area i.e. shadow region of the limiter, the Ne profile is flat and fluctuations of Is show strong intermittent feature, indicating the existence of blobs.