OBSERVATION OF DISRUPTIONS IN TOKAMAK PLASMA UNDER THE INFLUENCE OF RESONANT HELICAL MAGNETIC-FIELDS

Disruptive instabilities were investigated in the small-tokamak TBR-1 during the application of resonant helical magnetic fields created by external helical windings. Indications were found that the main trigge ring mechanism of the disruptions was the rapid increase of the m = 2/ n = 1 mode which, apparently after reaching a certain amplitude, inter acts with other resistive modes: the internal 1/1 mode in the case of a major disruption, or with higher m components, as the 3/1 or 4/1, in the case of minor disruptions. After the coupling, the growth of the associated islands would create a chaotic field line distribution in t he region between the corresponding rational magnetic surfaces which c aused the gross particle transport and, finally, destroyed the confine ment. In addition, investigations on higher Z(eff) discharges in which a mixture of helium and hydrogen was used resulted in much more unsta ble plasmas but apparently did not alter the basic characteristics of the disruptions.