Disruptive instability mode structure in the LT-3 Tokamak

The mode structure of helical perturbations was studied in a LT-3 Tokamak machine with separate capacitor banks supplying the toroidal field and primary circuit. The LT-3 was operated in its unstable mode, and the cyclically occurring disruptive instability exhibited an oscillatory phase in the wake of each disruption event with oscillations damped as the contraction phase set in prior to the subsequent disruption event. Modes growing exponentially, with time constants not greater than 10 microsec, prior to the disruption include: m = 2, n = 1 and m = 6, n = 2, and less frequently m = 3, n = 1. All of these modes collapse at the disruption. The disruption may accompany destruction of magnetic surfaces via interaction of helical perturbation modes.