A new model of $L$- to $H$-mode transition in tokamak plasmas is presented. Nonambipolar particle losses determine the consistent radial electric field near plasma periphery. A "cusp-type catastrophe" among the radial electric field, particle flux, and edge gradients is found. At the transition, plasma loss can take multiple values for one profile of density and temperature near the edge. A critical edge condition for the transition is obtained.

• Received 28 October 1987

DOI:https://doi.org/10.1103/PhysRevLett.60.2276