Experimental observations of sawtooth periods in tokamaks are consistent with a model in which the magnetic field after sawtooth collapse is given by nablatimesunderbar"B" = muunderbar"B" with mu a constant. One picture of sawtooth activity is of a magnetic field configuration evolving towards q = 1 as current penetrates into the plasma core. The plasma becomes unstable to an m = 1 mode causing a change in magnetic configuration in the core, raising q above unity. The instability causes a reduction in the central current density, hence field energy is converted to plasma energy increasing the entropy of the core. The two main factors which determine the sawtooth period in such a model are the magnitude of the change in magnetic configuration during the collapse and the size of the sawtooth region. The first of these is considered here. Kadomtsev's model of sawtooth activity assumed a rigid displacement of the plasma core leading to field line reconnection. Wesson proposed that the collapse should proceed through two convective cells as subsequently observed on JET. In that case Kadomtsev's prescription for the change in magnetic configuration during the collapse cannot apply. A new prescription is required.
Bibliographic Reference: REPORT JET-P(87)51, 1987, AVAILABILITY: JET, ABINGDON, UK
Record Number: 1989126088500 / Last updated on: 1989-05-01
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