Tearing Mode Behavior in the Globus-M Spherical Tokamak Plasma

The 2/1 tearing mode instability during the plateau phase of the plasma current time evolution has been observed since the start of the Globus-M spherical tokamak operation. Because of a small amount of shots in a database, the poloidal beta dependence of the instability growth rate had been investigated with an insufficient accuracy. In this work, a more detailed analysis has been produced that proves the fact that the observed modes can be treated as neoclassical. The saturated island width as a function of poloidal beta has been obtained; a time evolution of the magnetic island has been calculated from the Mirnov magnetic coil signals and compared to the modified Rutherford theory, and the critical magnetic island width has been found for the neoclassical tearing mode (NTM) of the Globus-M tokamak.