Study Of The Magnetic Fluctuation On J-TEXT Tokamak

MHD (magnetohydrodynamics) instabilities are prevalent and important phenomenain tokamaks,they would seriously limit the tokamak plasma performance, and even lead todisruption. Identification of coherent waves from magnetic fluctuating in tokamak plasmasis important for the understanding of MHD behaviour of the plasma and its control.The J-TEXT tokamak has developed a system of Mirnov coils already and obtainedvarious Mirnov oscillation signals since the first discharging in2007. However, the detailinvestigation of the MHD instabilities features, such as the mode numbers, frequency andmodes propagation is lacked up to now. The Mirnov coils system has been remolded andthe Mirnov fluctuations are measured and analysed. The basic characteristics of the MHDinstabilities will be given in this paper. The poloidal and toroidal numbers of Mirnovfluctuations are distinguished by the method of singular value decomposition (SVD), andthe time evolution of the mode numbers and frequencies during the discharge are presented.The basic characteristics of the MHD instabilities on J-TEXT tokamak can besummarized as follows. The mode frequency is about1～10kHz and the amplitude is lessthan30Gs. The rotation of toroidal mode is usually in contrast with the plasma currentdirection. Before the disruption, the change rate of the Bθ is always larger than40Gs.While the most significant mode in the flottop is always m/n=2/1.The theoretical and experimental research have indicated that the interaction of theRMP and plasma would affect the MHD instabilities, such as suppress the MHD and lockmodes. The preliminary experimental results of the RMP affected on J-TEXT MHDinstabilities are presented.