Development of an actively cooled periscope head suitable for divertor observation during quasi-continuous operation of the W7-X stellarator

High power quasi-continuous operation, as foreseen, e.g. in the stellarator W7-X, makes it necessary to actively cool all in-vessel components and thus also the front end of any periscopes used to observe the plasma in the ultravoilet (UV) to infrared (IR) spectral range. In this environment, not only the shutters which are required to prevent the windows from building-up coatings with poor transmission properties during boronisation and glow discharge cleaning, but also the faceplate of the periscope and, in particular, the observation windows need to be actively cooled. An ANSYS® study of a prototype of such a complete actively cooled periscope head, consisting of a faceplate, three windows and a rotating shutter, is being presented. Furthermore, in a follow up of a detailed theoretical investigation of the suitability of the various window materials available for the different spectral regions, using the finite element code ANSYS®, an experimental study of the prototype design of a window which can be actively cooled by water or gas (e.g. compressed air) is being presented. For the qualification of the window design, a special high heat load vacuum chamber equipped with a Boralectric® infrared heater was used, which allows to study its properties up to heat loads of 50 kW/m2, using initially the window material with the worst overall design properties, namely fused silica. Our sealing design was successfully tested not only with water (over 70 heat cycles) and compressed air cooling but also with helium at a static pressure of 30 bar for central window temperatures of 400 °C.