Comparison of ergodic and axisymmetric diverters

Ergodic and axisymmetric divertor configurations are shown to follow identical physics insofar as parallel transport is a dominant feature. As a consequence a unique 1-D model is used to model the divertor density regimes. This model indicates qualitatively that impurity radiation will lower the threshold to detachment and that the Mach number exhibits a maximum close to the ionization front. Experimental data corresponding to the density regimes, thermal bifurcation on a given flux tube and Mach number profile illustrate the behaviour of the divertor states. It is shown that closed diverters should exhibit sharp transitions to detachment, as reported on DIII-D, while open diverters are characterized by a gradual detachment. Evidence of this behaviour is given for Tore Supra. Discussion of the relationship between the upstream SOL density of the model and the volume average density are discussed in the frame of a comparison between open and closed diverters. Depending on the tightness of the divertor with respect to neutral recirculation, it is shown that the effective control parameter, volume average density or gas injection, will change. (C) 1999 Elsevier Science B.V. All rights reserved.