Divertor retention for recycling impurities

A quick and simple experimental method of comparing the global retention capabilities of different divertor configurations for gaseous impurities is presented. Short gas puffs into the main chamber and analysis of the time dependence of central impurity lines are used to determine the retention capability in terms of time constants, which characterize the impurity leakage rate from the divertor chamber to the main plasma and to the pumping system. A simple two-reservoir model is used to define these basic parameters of divertor retention. For a more detailed evaluation of the divertor retention parameters from spectroscopic data, an impurity transport code is used to simulate the impurity radiation measured in various experiments. The dependence of the retention capability on the divertor plasma density and temperature as well as on the distance along the field lines back to the main plasma is discussed. The dependence on the impurity species is also investigated. On the basis of these results, the consequences of the divertor retention for helium exhaust in future fusion devices are analysed by numerical simulation. (author). 32 refs, 7 figs, 4 tabs