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

Experimental data on the transition from the low to high confinement mode in tokamaks is briefly reviewed, concentrating on those cases where the transition is made by a slow change in external parameters. The first H-modes, which occurred after the sudden application of neutral beam heating, appeared to result from a bifurcation of the edge transport. However, slow transitions 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 intermittent as the high confinement mode is approached. The experimental results are interpreted in the light of various models of the transition process and of the type III or transition edge localized modes that accompany it. The evidence is mainly against the bifurcation hypothesis but nonlinear processes are clearly involved. The implications for the next generation of tokamaks intended to reach thermonuclear ignition are discussed.