在SUNIST上实现磁面平衡反演和开展磁流体力学研究对SUNIST等离子体物理研究具有重要意义。论文在SUNIST上建立了一套磁诊断系统，并以此为基础实现了磁面平衡反演和磁扰动模式分析，对SUNIST欧姆放电的平衡及磁流体不稳定性特性进行了研究，并探讨了内部磁重连事件（IRE）的物理机制。设计、制作和安装SUNIST磁诊断系统是本文工作的第一部分，也是其余研究工作的基础。设计以等离子体平衡反演为基础，并考虑了SUNIST真空室特点、探针尺寸、真空室真空度及等离子体可能造成的损害等因素。SUNIST磁诊断系统包括15枚极向磁探针、6枚环向磁探针、13个单匝环、1个罗科线圈和1个逆磁线圈。标定结果显示，磁诊断信号能够满足SUNIST相关实验需求。基于EFIT算法，论文发展了适用于SUNIST装置的平衡反演程序SU-EFIT，研究了SUNIST放电中平衡磁面及相关参数的演化。平衡的SUNIST等离子体具有大拉长形变（~1.7）和顺磁性（p~0.08），但由于垂直场与等离子体电流的强耦合及对垂直场控制能力不足，在SUNIST放电中，等离子体在真空室外侧产生，并随时间一直向内侧移动，最终在中心柱附近熄灭。特别地，在反演过程中考虑了真空室涡流对等离子体平衡的影响。SUNIST真空室由于存在环向和极向隔缝，感应的涡流对等离子体影响比不存在隔缝时小，但仍然不可忽视。利用奇异值分解方法，论文还发展了SUNIST上的磁扰动模式分析程序SU-MHD，并对SUNIST放电过程中磁扰动模式的演化进行了研究。结果显示，极向磁扰动模数从高到低逐步依次减小，分别为m =5, m =4, m =3和m =2，环向磁扰动模数均为n =1模。IRE是SUNIST放电中观察到的一种实验现象。IRE发生时，等离子体电流出现5%的正尖峰，磁扰动信号明显增强。IRE发生前，等离子体扰动模式为m =2 / n =1，扰动频率为10kHz；IRE发生过程中，扰动模式增加到m =4 / n =1，扰动频率低于1kHz；IRE结束以后，扰动模式减少到m =3 / n =1, 扰动频率低于1kHz。磁面反演结果显示，在IRE发生过程中，等离子体拉长比增加，极向比压减小，等离子体压强分布经历了一个峰化—平坦—再峰化的过程，表明SUNIST中的IRE现象体现出等离子体压强梯度驱动的迹象。

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 (~1.7) and strong paramagetism (p~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 Hα radiation 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.