Critical parameters for turbulent transport in the SOL: mechanism for the L-H transition and its impact on the H-mode power threshold and density limits

An explanation for the GH 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 GH transition physics: rho* and beta/nu*rho*(2) (the latter scales as aT(2) in physical parameters). The maximum of the diffusion coefficient, corresponding to the L-H transition, is reached when the collisional skin depth Delta(coll skin) = root c(2)/8 sigma nu (nu is the drift frequency) equals the characteristic radial displacement Delta of the drift turbulence. The same criterion can also be presented as: D-perpendicular to = Delta(2) nu = c(2)/8 sigma-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 Tc-13/8(T-e + T-i)(3/8)/B(qR)(1/4)z(eff)(3/4) (scales as [beta/nu*rho*(2/3)](3/4) in dimensionless parameters) as a critical parameter for the L-H transition, for the case of the k perpendicular to rho(s) = constant 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 beta for the LH transition. The proposed mechanism for the L-H transition clarifies the R dependence of the H-mode power threshold: P-thres similar to R-1.75 scaling is predicted, with P-thres < 70 MW for (n) over bar(e)=5x10(19) m(-3) 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 n(e.sep)/n(GW) 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 (similar to R-1/4) but favourable dependence of n(e,sep)/n(GW) 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 R-1/4 dependence, an excess over the Greenwald limit, (n) over bar(e)/n(GW), by about 30% higher in ITER compared with JET for 'similar' conditions (q, n(e.sep)/(n) over bar(e) separatrix z(eff) and the T-e/T-i 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.