The decisive role ofm= 1 and 2 perturbations in density limit disruptions on TEXTOR

The behavior of magnetohydrodynamic phenomena associated with disruptions of ohmically heated density limit discharges (qa = 3.9) in the TEXTOR tokamak has been investigated in detail. A minor disruption is demonstrated to be only due to an m = 2 mode instability, possibly due to the magnetic reconnection of an m = 2 island. When the m = 2 mode oscillations cease, the internal m = 1 kink mode instability is drastically enhanced at once, twisting the plasma core considerably. If the twisting is too strong, the twisted plasma core is inevitably displaced to the outside and brings cold plasma to the central region, leading to the energy quench at a major disruption.