Victoria Winters (IPP Greifswald): Plasma edge transport in the stellarator island scrape-off layer

Physikalisches Kolloquium

The success of the Wendelstein 7-X (W7-X) experiment in Greifswald, Germany has renewed global interest in the stellarator as a viable candidate for a magnetically-confined fusion power plant. However, there remain critical research questions to be addressed prior to the construction of such a reactor. One extremely important point is the accurate prediction of the conditions at the divertor target plate and the region of open field lines that intersect this plate at the plasma boundary, known as the scrape-off layer (SOL). In W7-X, this plasma boundary is formed via a magnetic island chain that surrounds the last closed flux surface (LCFS), which is then intersected by discontinuous divertor targets. The geometry of the magnetic island together with the divertor shape then determines the dominant plasma and neutral transport mechanisms.

To date, no model has been able to quantitatively reproduce or predict the 3D plasma boundary and target conditions in a stellarator. However, simplified models exist that explain qualitative behavior observed both in experiment and in our most sophisticated simulation toolsets. These models are helpful to frame the physics of the island divertor in relation to its simpler counterpart, the 2D SOL of a tokamak. To this end, a simplified 1D model of the island SOL, known as the stellarator two-point model (2PM), is used as a common basis for comparing the two devices. This talk will introduce the W7-X island divertor and utilize the stellarator 2PM, in combination with associated experimental results at W7-X, to discuss differences between this divertor configuration and that of tokamaks in physics areas such as plasma density build-up at the divertor target, heat transport/detachment and impurity transport.