| Spherical tokamak is different from traditional toakamak as its aspect ratio A (A=R/a, R and a are the major and minor radius of device) is closed to 1. In last 20 years, it has attracted a more and more attention and has been widely studied owing to its advantage of compact construction, easily accessed high P, better magnetohydrodynamic (MHD) stabilities. Results of experiments show that MHD instabilities still happen in spherical tokamak, such as vertical displacement event (VDE), sawtooth, kink mode, neoclassical tearing mode (NTM), ELM, internal reconnection events (IRE) and microscope instabilities. It is primary and significant that to study the conditions and mechanism of MHD instabilities, explore the the possible method to avoid or suppress the occurrence of the MHD instabilities and improve the operation parameters of spherical tokamak..SUNIST is the first spherical tokamak build in China. Various MHD instabilities events have been observed on it since the first discharging in 2002, such as Mirov fluctuations and IRE. However, the detail investigation of the instabilities features, such as the mode numbers and the frequency is lacke up to now. The Mirnov fluctuations are measured and analysed by magnetic measurement system on SUNIST Spherical Tokamak. The toroidal and poloidal numbers of Mirnov fluctuations are distinguished by using singular value decomposition (SVD), and the time evolution of the mode numbers during a discharge is presented.The theoretical and experimental research indicated that proper plasma rotation and shear flow will improve MHD instabilities on spherical tokamak. The change of plasma movements is realized by a biased electrode. The electrode produces a electric filed through the radius direction Er, and changes the movements of the grain by ExB drift. As the electric filed can penetrate only a limit length in the high temperature plasma, it is found that the biased electrode has a better result for fluctuations close to plasma boundary. |
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