Study of the detachment phases in the Wendelstein 7-AS stellarator

An experimental overview of the detachment process in the W7-AS stellarator with island divertors is given. Results of different spectroscopic methods are combined with those of Langmuir probes and neutral pressure measurements. Two phases of the detachment process are identified: the detachment transition due to the unstable range of the radiative loss function of carbon and a volume recombination phase for densities well above those needed for the onset of detachment. The first phase is characterized by a sudden drop in the temperature at the plasma edge and in the divertor. As a consequence, the particle fluxes at the divertor targets on top and at the bottom are reduced to a different degree, depending on the region. In the second phase, the plasma between the strike lines in the upper divertor is strongly recombining. Simultaneously, a strong carbon radiation belt is observed in the inboard scrape-off layer. The physical mechanisms of both phases are discussed. (author)