Study On Turbulence Characteristics Of Electron Modes In The Early Stage Of Plasma Disruption In EAST And Prediction Of Plasma Disruption

Tokamak is one of the devices to realize magnetic confinement fusion,and its research has made great progress,but it still faces many challenges.Predicting and alleviating plasma disruption is a key problem for the safe operation of tokamak.Instability evolution driven by free energy can lead to disruption,in which overlapping tearing modes and nonlinear evolution of neoclassical tearing modes often involve multi-scale physical processes,while electron mode turbulence is very sensitive to the change of free energy and closely related to the development of large-scale instability,so this paper tries to explore the prediction effect of electron mode turbulence on disruption.Based on the plasma experiment of disruption on the EAST device,combined with the diagnostic systems such as CO2 laser coherent scattering,soft X-ray(SXR)and electron cyclotron emission(ECE),experiments were carried out to study the electron mode turbulence(k=12 cm-1,k=22 cm-1)in the core region(ρ=0～0.4)and gradient region(ρ=0.4～0.8)caused by tearing mode.Firstly,the time point when the disruption entered the precursor phase and the evolution of large-scale instability were determined.Then,before this time point,the evolution characteristics of the turbulence characteristics of the electron mode are observed,including the duration of the obvious change in the pre-precursor phase of the turbulence,which is intended as the prediction time,and the characteristics of the spatial correlation evolution of the turbulence are summarized.Finally,the evolution of turbulence signals in different scales and regions is compared,as well as the differences between the characteristics of turbulence signals in different shots.At the same time,the relationship between turbulence and nonlinear evolution of macro instability is analyzed in combination with the evolution of turbulence characteristics.Through the analysis of experimental data,it is found that the spatial correlation of turbulence in ρ=0～0.4 and ρ=0.4～0.8 is enhanced in the pre-precusor phase of major disruption caused by tearing mode(n=1),and the repeatable experimental results show that the spatial correlation of turbulence begins to change obviously 40～170 ms before entering the precursor phase.The turbulence with the scale of k=22 cm-1 is about 110 ms earlier than that with the scale of k=12 cm-1.In addition,the turbulence intensity began to increase obviously 40～80 ms earlier than the precursor phase,and the turbulence intensity at the scale of k=22 cm-1 increased about 20 ms earlier than that at the scale of k=12 cm-1.It is also found that before the rigidity of the electron temperature profile is broken(followed by the nonlinear physical phenomenon of MHD instability),the correlation of turbulence signals reaches the strongest almost at the same time.The spatial correlation of turbulence decreases before the minor disruption caused by tearing mode(n=1)occurs,and the duration exceeds 30 ms;Turbulence intensity is obviously enhanced 30～45 ms before the magnetic fluctuation greatly increases,the enhancement of turbulence intensity with the scale of k=22 cm-1 is about 15 ms earlier than that with the scale of k=12 cm-1,and a sudden change of turbulence power spectrum is also observed during this period.From the experimental results,it can be seen that the intensity and spatial correlation of electron mode turbulence have changed obviously in the pre-precusor phase,and the duration is not less than 30 ms,which reflects that electron mode turbulence is highly sensitive to free energy;The correlation of turbulence in different regions changes in the opposite trend between major disruption and minor disruption,which can distinguish the global major disruption from the local minor disruption;The turbulence with the scale of k=22 cm-1 generally changes earlier than the turbulence with the scale of k=12 cm-1;The sensitivity of electron mode turbulence to the change of free energy is further verified by the change of electron temperature profile.In this paper,the evolution of turbulence characteristics in the early stage of disruption caused by tearing mode is demonstrated through experiments,and the feasibility of predicting disruption by electron mode turbulence in major and minor disruptions is evaluated.Compared with ITER,the shortest warning time required for successfully mitigating disruption is about 30 ms,which meets the needs of disruption prediction of ITER and other future devices.The research results of this paper provide a new idea for the improvement and promotion of disruption prediction method,and provide scientific basis and experimental support for the safe and stable operation of tokamak device.