Stabilization of tearing modes to suppress major disruptions in tokamaks

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Abstract
It is shown, for q-profiles which lead to a disruption, that the control of the amplitude of the 2/1 tearing mode avoids the disruption, q-profiles measured in T–4 and PLT before a major disruption have been studied. Two methods of controlling the 2/1 mode amplitude have been considered: 1) Feedback stabilization with the feedback signal locked in phase with the 2/1 mode. The major disruption is suppressed if the feedback parameters (time delay and gain) are properly chosen. Otherwise, only a delay in the disruption is observed. 2) Heating slightly outside the q = 2 surface. Modifying the current density profile can decrease, and even eliminate, the 2/1 –3/2 interaction. In this way, the disruption is avoided. –In both cases it is only necessary to decrease the 2/1 mode amplitude to suppress the disruption. It is not always necessary to stabilize the unstable modes fully.