Quenching of the quiescent H-phase in ASDEX

Highest energy confinement during neutral-beam heating is obtained when the discharge is in the quiescent, ELM-free H-mode. The improved energy confinement is linked to higher particle and also impurity confinement. In the original ASDEX DV-I divertor configuration and with stainless-steel vessel walls, this resulted in an accumulation of metallic impurities. The excessive radiation power losses terminated the H-phase after about 100 ms. After carbonization of the walls aimed at reducing the metallic impurity content and, secondly, the prolongation of the ASDEX NI-heating pulse in the new DV-II divertor configuration, much longer quiescent H-phases could be expected. This, however, was not achieved. After a quiescent phase of less than 150 ms, the H-mode converts back into the L-mode. (author) 6 refs., 5 figs