An explanation for the L - H transition through the electromagnetic mechanism (suppression of drift wave turbulence by the skin effect) is offered. In dimensional space, the bifurcation is attributed to the involvement of two parameters in L - H transition physics: and (the latter scales as in physical parameters). The maximum of the diffusion coefficient, corresponding to the L - H transition, is reached when the collisional skin depth ( is the drift frequency) equals the characteristic radial displacement of the drift turbulence. The same criterion can also be presented as: - the condition for equal rates of the plasma diffusion into the magnetic field and the diffusion of the magnetic field into the plasma. The analysis yields the combination (scales as in dimensionless parameters) as a critical parameter for the L - H transition, for the case of the scaling for the wavevector of the drift turbulence. This threshold parameter should be applied near the separatrix position. At low densities, the requirement that the collisionless skin depth must be smaller than the radial displacement of the drift fluctuations in the L-mode, which is necessary for turbulence suppression, determines the threshold for the L - H transition. The proposed mechanism for the L - H transition clarifies the R dependence of the H-mode power threshold: scaling is predicted, with for in ITER. The critical parameter for the L - H transition, together with dimensionless parameters characterizing pressure gradient and resistivity, create the set of similarity parameters describing ELM behaviour. The scaling for the separatrix density, normalized to the Greenwald density limit with the machine size and toroidal field which ensures `similar' ELM behaviour, can thus be obtained. For the fixed similarity parameters, the analysis yields weak but favourable dependence of on the major radius. In recent experiments on JET and other machines, the degradation in the edge confinement associated with increased ELM frequency was found to be responsible for the density limit in high-power H-modes. Owing to the approximately dependence, an excess over the Greenwald limit, , by about 30% higher in ITER compared with JET for `similar' conditions (q, , separatrix and the ratio, wall conditions, the use of pellets etc) in ELMy H-modes is predicted. This is with the provision that a limit on the central density, related to mechanisms in the plasma core, is not encountered.
H-mode transitions and limit cycle oscillations from mean field transport equations