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Microtearing modes (MTMs) are unstable in the shallow gradient region justinside the top of the pedestal in the spherical tokamak experiment MAST, andmay play an important role in the pedestal evolution. The linear properties ofthese instabilities are compared with MTMs deeper inside the core, and furtherdetailed investigations expose the basic drive mechanism. MTMs near the MASTpedestal top are not well described by existing theories. In particular thegrowth rate of the dominant edge MTM does not peak at a finite collisionfrequency, as frequently reported for MTMs further into the core. Our studysuggests that the edge MTM is driven by a collisionless trapped particlemechanism that is sensitive to magnetic drifts. This drive is enhanced in theouter region of MAST by a high magnetic shear and a high trapped particlefraction. Observations of similar modes on conventional aspect ratio devicessuggests this drive mechanism may be somewhat ubiquitous towards the edge ofcurrent day and future hot tokamaks.

Microtearing modes at the top of the pedestal