Concepts and prototype elements of the low-Z wall protection for the stellarator W7-X

Stationary operation, envisaged at WENDELSTEIN 7-X, demands for an active cooled wall protection system. To minimise the radiation losses of the plasma during long pulse operation the inner surface of the vacuum vessel has to be covered completely with materials having low atomic numbers. Unlike from the target plates (receiving a high heat flux of up to 10 MW/m2) that consist of Carbon Fibre Composite (CFC) brazed on a cooling structure, a more simple solution for the Plasma Facing Components (PFC) on the basis of CFC tiles clamped on a cooling structure or larger stainless steel (SS) -panels covered with B4C will be developed for the less loaded baffle areas of the divertor and the 120 m2 wall protection. The choice of the components to be installed in the various areas for wall protection will be determined mainly by the conditions of the physics of the plasma, additional loads by plasma heating (mainly neutral beam injection, NBI and electron cyclotron resonance heating, ECRH), demands by the diagnostics and the geometrical restrictions at different locations in the vacuum vessel.