Retention of gaseous impurities in the divertor of DIII-D

The retention capability of a divertor configuration for recycling impurities, such as hydrocarbons produced at the divertor plate, but also of He ions originating from D,T-fusion processes in the main plasma, is one of the most important issues for future devices. This can be investigated by the injection of short gas puffs into the main plasma. The use of a heavy inert gas as a test impurity has the advantage that it can be spectroscopically investigated both in the plasma boundary and in the center without using charge exchange recombination lines, which are only available during neutral beam injection (NBI). Therefore, in the present study Ar was introduced in the main vessel of DIII-D, and its time dependent concentration balance between main plasma and divertor was studied by monitoring the Ar VII, VIII, XV, XVI lines. From this balance a divertor time constant, tD, could be obtained and its variation during NBI studied. Similar experiments have been previously performed in ASDEX and JET and allow a comparison of different divertor configurations. (author) 6 refs., 5 figs., 1 tab