Investigation of Some MHD Events in the SUNIST Spherical Tokamak

Internal reconnection event (IRE), which is characterized by a perturbation in plasma current Ip, loop voltage Vloop, Hα radiation and magnetic perturbation dBp, was observed in the SUNIST experiment. It is found that, the fluctuation before IREs is characterized by a structure of m =2 / n =1, then changes to m =4 / n =1 during the IREs; and, after IREs, the mode number changes to m =3 / n =1. An analysis in the evolution of equilibrium parameters during IREs shows that a positive spike appears in the evolution of the plasma’s elongation and a negative spike in the poloidal beta of plasma. A collapse in the pressure profile, corresponding to the occurrence of IREs, is also found.