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Spontaneous m = 1 instability in the JET sawtooth collapse

The collapse phase of sawtooth oscillations in tokamaks is believed to be due to the rapid growth of an m = 1 instability. We show here that the onset of the rapid growth is spontaneous and cannot be described by the increase of the linear growth rate arising from development of the plasma equilibrium. This result calls for a reappraisal of the sawtooth problem.