On a number of tokamaks, locked modes are believed to grow to larger amplitude and to disrupt more readily than rotating modes.[1,2] When a magnetic island is rotating rapidly past an error field, the growth of the island isn't strongly affected by the error field, induced wall currents can slow the mode rotation and mode lock occur. In this work an analytical model assuming tokamak ordering and narrow island clarifies the conditions under which error field are amplified. A small, resonant magnetic field instabilities affect the plasma significantly only in a narrow region about the rational surface, allowing us to test understanding of the physics of magnetic islands. Characteristics of several disruptions in the Iran Tokamak1 with different triggering are described.

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