Impact of magnetic islands on turbulent transport in the WEGA stellarator.

The role of the magnetic field topology for the complex interplay between turbulence and zonal flows (ZFs) leading to the formation of transport barriers is a recent topic in fusion research. Gaining knowledge on their relation in experiments requires a flexible magnetic configuration and a good diagnostic access with high temporal and spatial resolution. WEGA as a small classical stellarator with moderate plasma parameters is well suited for such experiments. Stationary n = 1 islands can be modified in size by external error field compensation coils. Configurations with m/n = 5/1 islands at the plasma edge are compared with configurations showing no major resonances (i.e. no islands). The spatio-temporal structure of fluctuations was studied with Langmuir-probe arrays at different toroidal and poloidal positions. In low density discharges ZF-like patterns were observed in potential fluctuations. A characteristic feature of these oscillations is a long range correlation (LRC) with fluctuations being symmetric on a flux surface. Itwas shown for the first time that the presence of magnetic islands alters the structure of these low frequency fluctuations. At r <risland the LRCs were still observed. Inside the islands the fluctuations showed up more like meso-scale structures with finite poloidal wave number. The poloidal correlation length was shown to be decreased with increasing islands width. Frequency resolved measurements of the turbulent transport across the island's X-point showed that these structures do not significantly contribute to the transport due to their nearly symmetric character. However, their decorrelation goes together with an increase in transport in the frequency range of the ambient turbulence. [ABSTRACT FROM AUTHOR]