Study Of Magnetohydrodynamic Behaviors During Minor Disruptions In SUNIST Plasmas

Owing to its unique magnetic configuration, Spherical Tokamak(ST) has a natural advantage of good magnetohydrodynamic stability. There are seldom major disruptions,except for minor disruptions such as internal reconnection events(IREs) occurring. The study of MHD behaviors during these gentle disruptions is of scientific and practical value in understanding the physical mechanism of disruption and, furthermore, in avoiding major disruptions in future fusion devices. This dissertation is devoted to a systematic research on the MHD behaviors during IREs and minor disruptions in the SUNIST spherical tokamak. Some physics processes involved in IREs and minor disruptions are examined,as well as the toroidal Alfvén eigenmode(TAE) excited by runaway electrons during minor disruptions has been identified for the first time in SUNIST.Initially, the dissertation introduces the design and installation of a system of highfrequency magnetic probes with operation frequency up to 2MHz in the SUNIST device.The lumped-circuit model and the transmission line model are used to calibrate the frequency response of high-frequency magnetic probe(HFMP) accurately, which provide a solid foundation for studying MHD behaviors in SUNIST plasmas. Furthermore, some novel data processing methods, especially an improved Hilbert-Huang transform method,are developed in the dissertation, which contribute to an accurate analysis of frequency,mode number and other features of high-frequency magnetic perturbation signals.The evolution of MHD behaviors before and during IRE is examined in the dissertation. During the precursor process, an n =-1 mode grows with a large growth rate firstly, and then grows slowly and saturates eventually. Meanwhile, there appears a nonlinear couplings between m/n =-3/- 1 mode and m/n =-6/- 2 mode, which confirms the process predicted by previous nonlinear MHD simulation. However, during the current increase stage of the IRE, there is a burst of high-frequency(HF) MHD with main mode number m/n =-4/-1. The HF MHD begins to decay after plasma current reaches its local peak. In addition, according to the stepwise change in plasma current, the IREs in SUNIST plasma are classified into three types, namely negative stepwise change, nearly zero stepwise change and positive stepwise change, respectively. Among them, the type of positive stepwise change has not been reported before.The evolution of MHD behaviors during minor disruptions is investigated, which has some quite different characters with IREs. During the precursor stage of minor disruptions it is found that m/n =-3/-1 mode grow rapidly in the early stage and subsequently there exists mode locking which lead to minor disruptions eventually. In contrast, there is no mode locking during IRE. Furthermore, different parameter ranges between IREs and minor disruptions in SUNIST plasmas are given. Especially, it is observed that each minor disruption is accompanying with a runaway plateau, where a kind of HF MHD instability occurs. The characteristics of this HF MHD mode mainly include that: the frequency is between 150-300 kHz; a mode with toroidal mode number n =-1 propagates in the direction of the electron diamagnetic drift direction; the poloidal magnetic field perturbation exhibits a ballooning mode feature; there is a strong nonlinear coupling between m = 3 mode and m =-4 mode; the growth rate and the damping rate of MHD mode are about 5.2% and 5.1%, respectively. By scanning the toroidal field and electron density, this HF MHD mode might be identified as TAE, which is driven by runaway electrons generated during minor disruptions. From the wave-precession drift resonance mechanism, the excitation condition of TAE is calculated, which can be met in SUNIST.The potential application of mitigating runaway electrons by exciting TAE is discussed as well in the dissertation.