Study On Tearing Modes And Reversion Of Magnetic Islands

The studies of MHD instabilities are considered to play an important role in plasma confinement. Disruptions pose a serious problem for tokamak development. This is firstly because they limit the range of operation in current and density, and secondly because their occurrence leads to large mechanical stresses and to intense heat loads. Disruptions are always leaded by other instabilities, such as the tearing mode instability. The tearing mode includes classical tearing modes(CTM) and neoclassical tearing modes(NTM).In tokamak plasmas tearing modes take the form of magnetic islands. In HL-2A, magnetohydrodynamics (MHD) instabilities are investigated by means of the Mirnov probes. The mode number can be determined by the methods of phase comparison analysis or correlation analysis from the experimental data of Mirnov probes, but the analysis of complicated mode structures is difficult. An identification and analysis method of magnetic islands is presented basing on simulation of the perturbation current and magnetic field in plasmas. The cylindrical is adopted, while the effect of toroidal geometry on plasma equilibrium is also included. This method has been used in HL-2A tokamak for analyzing the magnetic island structures.The fundamental state about plasma MHD instabilities is described in the first chapter, including the analysis method of MHD on overseas tokamak, the mode number's determination, the new identification method of magnetic islands. The detailed state about tearing modes is presented in the second chapter.After a brief description of the experimental set-up and the determination of mode numbers by the methods of phase comparison analysis, correlation analysis and spatial Fourier analysis is presented in the third chapter.The new method for detailed mode analysis is present in the fourth chapter, in which chapter, the cylindrical approximation is employed firstly, then the toroidal effect is taken into account. The application of this method on HL-2A is also demonstrated in this chapter. In fifth chapter, the summaries and discussions are given.