MHD activity in torus system and helical field

Summary form only given, as follows. The understanding of the collapse phenomena in toroidal plasmas is an essential subject, because such events determine the boundaries of the plasma parameters and the stationary operation. One of the possible candidate of the mechanisms that causes the collapse is the Mirnov instability. If the rotational transform at the boundary of low shear stellarator plasmas, such as CLEO and W7-AS, with and without a net toroidal current approaches low rational values (1/2 and 1/3), Mirnov data indicate the presence of modes with the expected poloidal harmonic m. Moreover, in tokamaks, the disruptive instability is often preceded by strong Mirnov oscillation (usually m=2 or m=3) in the outer regions of the plasma. This article presents an experimental study on the disruptive instability occurring in the Iran-Tokamak 1 (IR-T1) in comparison with stellarator experiments, particularly CLEO machine. It is found that the fluctuations of the poloidal mode numbers m=2 and 3 at the rational surfaces q=2 and 3 are responsible for most of global magnetohydrodynamic (MHD) activities in low shear stellarators and tokamaks. Diagnostics and operational region with and without l=2 and l=3 helical fields of the IR-T1 tokamak have already been presented.