Comparison of ergodic and axisymmetric diverters

Ergodic and axisymmetric divertor configurations are shown to follow identi cal physics insofar as parallel transport is a dominant feature. As a conse quence a unique 1-D model is used to model the divertor density regimes. Th is model indicates qualitatively that impurity radiation will lower the thr eshold to detachment and that the Mach number exhibits a maximum close to t he ionization front. Experimental data corresponding to the density regimes , thermal bifurcation on a given flux tube and Mach number profile illustra te the behaviour of the divertor states. It is shown that closed diverters should exhibit sharp transitions to detachment, as reported on DIII-D, whil e open diverters are characterized by a gradual detachment. Evidence of thi s behaviour is given for Tore Supra. Discussion of the relationship between the upstream SOL density of the model and the volume average density are d iscussed in the frame of a comparison between open and closed diverters. De pending on the tightness of the divertor with respect to neutral recirculat ion, it is shown that the effective control parameter, volume average densi ty or gas injection, will change. (C) 1999 Elsevier Science B.V. All rights reserved.