A novel feature of the H-mode induced by compact torus injection on the STOR-M tokamak is observed. There is almost no change in the radial electric field profiles during and after the L-H transition. The usual hypothesis of the $\mathbf{E}\times \mathbf{B}$ shear stabilization mechanism is therefore unlikely to play a role in this transition. A new mechanism of the stabilization of microinstabilities by parallel flow is suggested as the plausible cause for the transition to this improved regime.

• Received 26 October 1999

DOI:https://doi.org/10.1103/PhysRevLett.88.185001