Interpretation of density limits and the H-mode operational diagram through similarity parameters for edge transport mechanisms

ELMy H-modes are currently a promising scenario envisaged for the operation of a future fusion reactor. The reliable extrapolation of the present experimental data to a reactor requires a model which is capable of explaining the variety of experimental phenomena observed in high-density H-modes. This paper attempts to construct a model based on the assumption that the behaviour of high-density ELMy H-modes can be explained through the similarity of edge transport mechanisms. We have identified three dimensionless parameters as the most representative for the high-density H-mode operation: (a) F-beta = q(2) R del beta/f(s) representing the ideal ballooning limit, (b) collisionality nu(e)* = Z(eff)nqR/T-e(2) which is postulated to be responsible for the transition from type I to type III edge localized modes (ELMs) and (c) the L-H transition boundary represented by FL-H = T-e(3)/((BL)-L-2 (perpendicular to) Z(eff)/root m(i)). Fixing any two out of these three parameters allows one to find scalings for the main operational points in the edge n(e)-T-e diagram and reproduce the Greenwald/Hugill dependences: (n) over bar(e) similar to B/qR for density limits. More detailed scalings for the type I to type III ELM transition point, which may be of particular interest for a reactor, show that the critical separatrix density should scale as n(e) similar to B-alpha/q(beta) R-gamma, where alpha approximate to 1, beta > 1, gamma < 1 (but being close to unity) and q is assumed to be the safety factor at 95% of the flux, q(95). Good agreement is found between experimental results on JET for the density at the top of the pedestal and the scaling (n) over bar(e) similar to B/R(3/4)q(5/4) for the critical separatrix density at the transition, in the conditions where the two densities are expected to be proportional to each other.