In a circular cross-section plasma bounded by a limiter, the H-mode transition is triggered by a rapid rampdown in plasma current during auxiliary heating, even in the case when the edge electron temperature gradually decreases prior to the transition. This result suggests that the transition is governed by the enhancement of the magnetic shear near the plasma edge, associated with the radial modification of the edge current-density profile.

• Received 20 March 1989

DOI:https://doi.org/10.1103/PhysRevLett.64.1895