Edge turbulence in tokamaks and the L-mode to H-mode transition

Experimental data on the transition from the low to high confinement mode i n tokamaks is briefly reviewed, concentrating on those cases where the tran sition is made by a slow change in external parameters. The first H-modes, which occurred after the sudden application of neutral beam heating, appear ed to result from a bifurcation of the edge transport. However, slow transi tions produced, for example, by ohmic heating do not have the character of a bifurcation but appear to result from a slow and reversible change in the characteristics of the edge turbulence, which becomes increasingly intermi ttent as the high confinement mode is approached. The experimental results are interpreted in the light of various models of the transition process an d of the type III or transition edge localized modes that accompany it. The evidence is mainly against the bifurcation hypothesis but nonlinear proces ses are clearly involved. The implications for the next generation of tokam aks intended to reach thermonuclear ignition are discussed.