Fast transport changes and power degradation in the W7-AS stellarator

The turbulent heat transport in high-temperature fusion plasmas is not understood. It increases with power heating. The physical mechanism of this process is investigated in the W7-AS stellarator. The fundamental question addressed here is whether transport is governed by local plasma parameters or by a global quantity. Three different types of experiments with critical sensitivity are carried out. They can consistently be described on the basis of a non-local dependence of the transport coefficient on the global heating power. (author)