Magnetohydrodynamic Equilibrium Reconstruction And Instability Analysis In The SUNIST Spherical Tokamak

Magnetohydrodynamic (MHD) equilibrium reconstruction and instability analysis is of great importance in device operating,understanding of basic plasma behaviors and further physics researching in fusion experimental devices. In this dissertation, equilibrium reconstruction and analysis of MHD instabilities are achieved for the SUNST spherical tokamak, based on a newly designed magnetic diagnostics system. The evolution of equilibrium parameters and MHD behaviors are obtained for the ohmic discharge in the SUNIST and the so-called internal reconnection events (IREs) are preliminarily investigated.Designing, manufacturing and installing the magnetic diagnostics system is the basis for the equilibrium reconstruction and instability analysis in the SUNIST. In this dissertation, based on the plasma equilibrium reconstruction with the current filament method, a magnetic diagnostics system is successfully designed with constraints of physical size of components, narrow central space in chamber, vacuum compatibility, protection from plasma damage and engineering availability. This new magnetic diagnostics system consists of 15 poloidal magnetic coils, 6 toroidal magnetic coils, 13 poloidal flux loops, 1 diamagnetic flux loop and 1Rogowski loop. Calibration results show the system meets the requirement of the equilibrium reconstruction and instability analysis in the SUNIST.Using the idea of EFIT, an MHD equilibrium code, named SU-EFIT, is developed for the SUNIST. Then, the evolution of the plasma equilibrium in the SUNIST is analyzed. It is found that the SUNIST plasma is characterized by large elongation (κ~1.7) and strong paramagetism (βp~0.08). However, due to the strong coupling of plasma current to vertical field current and also the lack of vertical field control, in the typical ohmic discharge, the plasma is generated outside the vacuum vessel at the beginning, then moves to inner side and finally disappears near the central column of the vacuum vessel. The results from the equilibrium fit are consistent with the observation from high-speed CCD camera. It is noted that, in this dissertation, the influence of eddy currents in the vacuum vessel on the equilibrium is evaluated. It is found that, although the existence of dielectric break in the SUNIST vacuum vessel causes the effect of eddy currents much weaker than that without the break, this effect still should be considered in the equilibrium reconstruction.Mode structures of MHD activities are revealed in the SUNIST by means of singular value decomposition. It is found that, during the typical ohmic discharge, the poloidal mode decreases from m=5 to m=2 in turn but the toroidal mode keeps n =1.The IREs, which is characterized by a perturbation of the plasma current Ip, the loop voltage Vloop, the Hαradiation and the magnetic perturbation dBp, have been confirmed in the SUNIST experiments. It is found that, the fluctuation before the IREs is characterized by the structure of m =2 / n =1 and the frequency about 10 kHz; then it changes to m =4 / n =1 and less than 1 kHz during the IREs; and, after the IREs, the mode number decreases to m =3 / n =1 and the frequency remains less than 1 kHz. The analysis of equilibrium parameters evolution during the IREs shows that a positive spike appears in the elongation of the plasma and a negative spike in the plasma poloidal beta. A collapse in the pressure profile is also found corresponding to the occurrence of IREs, which suggests the IREs in the SUNIST are driven by the pressure gradient possibly.