Stationary and transient heat load in the island divertor of W7-AS

The power exhaust from a burning plasma is a key problem in the design of future fusion reactors. Presently employed divertor materials restrict the stationary heat flux to 10 MW m−2. This paper deals with the power load onto the divertor target of W7-AS. The divertor experiments of W7-AS serve as a testbed for W7-X, since this is also equipped with a modular island divertor and its edge structure is similar to that of the W7-AS. The heat flux onto the divertor target is determined by means of infrared thermography. In W7-AS divertor discharges, bunches of edge localized modes (ELMs) with an overall duration of a few milliseconds expel plasma, mostly at the transition from high to low confinement regimes or during density ramps. The heat flux and energy loss they produce is comparable to strong ELMs (type I ELMs) in a tokamak. These bunches of ELMs induce further asymmetries of the heat flux deposition onto the divertor targets.