A quantitative comparison is made between the tearing-type modes observed in TFTR supershot plasmas and the nonlinear, neoclassical p: ssure gradient (Vp) driven tearing mode theory. Good agreement is found on the nonlinear magnetic island evolution of a single helicity mode {mjn = 3/2, 4/3 or 5/4, where m and n are the poloidal and toroidal mode numbers, respectively). Statistical data on the island width and growth rate are also found to be consistent with this theory. The results imply that the supershot plasmas are stable to the classical current-driven tearing modes.
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Seed magnetic island formation due to a dynamically growing external source in toroidal confinement devices is modeled as an initial value, forced reconnection problem. For an external source whose amplitude grows on a time scale quickly compared to the Sweet-Parker time of resistive magnetohydrodynamics, the induced reconnection is characterized by a current sheet and a reconnected flux amplitude that lags in time the source amplitude. This suggests that neoclassical tearing modes, whose excitation requires a seed magnetic island, are more difficult to cause in high Lundquist number plasmas.
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