Experimental Study On The Interaction Between Turbulence And Geodesic Acoustic Modes In Magnetically Confined Plasma

The realization of high confinement plasma is one of the key scientific problems in magnetic confinement fusion.Therefore,studying the physical mechanism on the confinement improvement is an important topic in modern fusion plasma research.Theoretical and experimental studies show that the geodesic acoustic modes(GAM)excited by turbulence nonlinear interactions can reduce turbulent transport and consequently improve plasma confinement.So,it is very important to study the physical mechanism on interaction of turbulence and GAM for predicting the scaling law of high-confinement(H-mode)power threshold.In HL-2A tokamak,a combined Langmuir probe was used to measure the edge plasma density,temperature and floating potential fluctuations to study the spatial characteristics of GAM.The experimental data show that the GAM is axisymmetric in the toroidal direction.The power spectrum analysis of the floating potential fluctuation shows that a correlation peak appears at f ≈ 10 k Hz,which is consistent with the theory prediction.However,there is no relevant activity at this frequency in the magnetic fluctuation,indicating that the GAM is an electrostatic fluctuation.The bicoherence analysis demonstrate that the nonlinear interaction of high-frequency turbulence(f ≥150k Hz)is the main reason for the excitation of the GAM-zonal flow.In our experiment,we use external particle injection to modulate turbulence and particle transport,focusing on the dynamic of the interaction between turbulence and GAM zonal flow in the plasma recovery phase after a short pulse particle injection.For the sake of comparison,time series is divided into two slices,i.e.,strong GAM and weak GAM.The experimental analysis indicates that the turbulent transport in the strong GAM case is lower than that in the weak GAM.Further analysis shows that nonlinear energy transfer is the main reason for the amplification of the GAM without induce outward particle transport,which in turn suppresses turbulent transport.In addition,the effect of isotope effects on the interaction of turbulence and GAM were investigated as well.Under the similar discharge parameters including plasma current,line-averaged density and magnetic field,it is found that:(i)the deuterium(D)plasmas has better confinement than that in hydrogen(H)plasmas;(ii)the amplitude level of the GAM zonal flow in the D plasmas is remarkably higher than that in the H plasmas;(iii)the more energy is transformed from turbulence to GAM zonal flow in D plasmas compare against that in H plasmas.These results revel that the critical role played by the GAM zonal flow in reducing turbulent transport in D plasmas.