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

This is the authors' abstract. We don't add key points for this paper.

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 = q 2 R / f ( s ) representing the ideal ballooning limit, (b) collisionality * e = Z eff nqR / T 2 e 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 F L - H = T 3 e /( B 2 L Z eff /( m i) 1/2 ). 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: e ~ 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 ~ B / q R , where 1, >1, <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 e ~ 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.