Optical design study for divertor observation at the stellarator W7-X

The stellarator W7-X will be capable of running in a quasicontinuous operating mode with 10 MW of electron cyclotron heating (ECRH) heating for 30 min, the duration only being limited by the capacity of the available cooling reservoir. The integrated ten discrete water cooled divertor modules need to be closely monitored by ten high resolution infrared imaging (Δx∼10 mm) real time control systems to prevent local overheating which could lead to a destruction of the tiles. Filter based (Hα, C II, C III, etc.) visible imaging systems will initially be used to study divertor symmetry. A first design study of a 2 m long combined IR/visible mirror based endoscope compatible with the boundary conditions of ∼107 degree sign viewing angle, thermal vessel movements, front end heat loads of ∼50-100 kW/m2, and maximum ECRH stray radiation levels of 50 kW/m2 has been performed. The system will be bakeable and contain an integrated shutter/calibrator. A 45 degree sign mirror in front of the secondary mirror of the back-end Cassegrain optics gives extra flexibility to install further optical components, e.g., a vessel illumination system, a relative calibration light source, and spectroscopic systems such as a coherence imaging spectrometer or fiber optically coupled grating spectrometers, a scanning laser for an active IR diagnostic, or possibly even an erosion monitor