F. Hurd

A proposed scraper element to protect the end of the W7-X divertor target elements

The three main areas of the W7-X divertor target are: the horizontal target, the vertical target and the high iota tail, with a pumping gap between the horizontal and vertical targets. For each of the standard operational scenarios the target has been designed so that, based on vacuum field calculations, the plasma strike points lie away from the ends of the target. Each target consists of CFC armoured water cooled poloidally running target elements capable of operating reliably in these central regions at up to 10MW/m2 in steady state operation, as necessary for the standard configurations. Due to the U-turn shape of the cooling channel at the pumping gap end of the elements the cooling is not optimal and a reduced performance is expected. However, recent studies of experiment scenarios, also now taking into account the influence of bootstrap currents, showed that the strike point drifts over the ends of the target elements adjacent to the pumping gap on the L/R time scale; potentially leading to damage of the elements in these scenarios. Various operational methods have been studied to satisfactorily avoid this situation without success. The paper describes design changes being made to improve the ability of the end of the elements to accept reasonable power levels, however it is not expected that the elements in these regions will meet the full power requirements. Hence, this paper also describes the design of a protection element; the purpose of this so-called “scraper element” is to intercept the power to the targets before the full power reaches the sensitive end of the elements. This is done without reducing significantly the pumping at the pumping gap and addresses issues such as how to control local impurity flows. The programme of work concerning design and technology qualification still needed to realise this scraper element is described.