Experimental Research On The Magnetohydrodanamic Instabilities With Low Modes On J-TEXT Tokamak

In tokamak plasma research, magnetohydrodynamic theory is the most simple and applicable method to describe the plasma behavior. In this system, when the plasma is under non-thermal equilibrium, there is free energy which could be converted into perturbation, and such perturbation would become the collective motion in the right conditions. This collective motion in plasmas is called magnetohydrodanamic (MHD) instability. In tokamak plasmas, there are different kinds of instabilities, such as ideal MHD and non-ideal MHD considering the resistivity effect. The most common instability on J-TEXT is tearing mode, which is a kind of non-ideal MHD. The tearing mode could change the topology of the magnetic field in tokamak, lead to a structure with helical magnetic island, increase the radial particle transport and radial energy transport, flatten the plasma density, temperature inside the region of island, and deteriorate the confinement. Especially, the tearing modes with low modes are the most dangerous, which could lead to disruption and influence the operation of the tokamak. The more complicated is the interaction between different tearing modes and other MHD activities, which would produce different effects on the stable operation. Therefore, the thesis introduces the experimental study on different kinds of MHD activities with low modes on J-TEXT tokamak.In the thesis, the theories of tearing mode and sawtooth instability are briefly reviewed firstly. The diagnostics used to study the MHD activities on J-TEXT, such as soft x-ray arrays, magnetic sensors, electron cyclotron emission (ECE) system and polarimeter-interferometer, and various digital signal processing method to analyze the experimental results of MHD activities are introduced secondly. In addition, the principle, construction and experimental results of a2mm millimeter wave interferometer which is used to measure the electron density, are described in detail in this part. Finally, the last chapters give the experimental results of the MHD activities with low mode.In low qa discharge, the compound sawtooth is observed, and it is comparatively studied with the previous theoretical models. In impurity injection experiment, it’s observed that the sawteeth activities in central soft x-ray alter to the inverse sawteeth, while the sawteeth in central density and temperature signals don’t change. A self-consistent model is proposed to successfully interpret the observations of the inverse sawtooth shape on the central SXR chords.On J-TEXT, the bifurcation of tearing modes due to supersonic molecular beam injection (SMBI) has been observed. the relaxation could bifurcate into two different types of tearing modes’ behaviors. One is the tearing mode with single m/n=2/1helicity (m and n are the poloidal and toroidal mode numbers, respectively), the other one is the tearing modes with different helicities (m/n=2/1,3/2), which lasts about tens milliseconds and decreases to noise level finally. It is proposed that the mechanism responsible for the bifurcation is closely related to changes of the pressure, and lately the plasma current density profile, due to the gas injection.The correlation between sawtooth and tearing mode is investigated in the final part of the thesis. The mode change and coexistence between sawtooth and tearing mode are all observed. We verify that the current profile plays a decisive role during the mode change and coexistence, by analyzing the evolution of the various parameters combined with numerical simulation.