Optical design study of an infrared visible viewing system for Wendelstein 7-X divertor observation and control.

For the Wendelstein 7-X stellarator, which will allow quasicontinuous operation (tau < or = 30 min) with 10 MW of electron cyclotron radiation heating power, a conceptual design study for an IR/visible viewing system (IVVS) has been elaborated. Ten such systems, as part of the machine protection system, will be required for real time monitoring of all ten discrete, water cooled divertor modules with high spatial (< 10 mm) resolution, in order to prevent local overheating of the target tiles, which could easily lead to their destruction. On the physics side, the systems will be used for divertor symmetry investigations by studying the power load distribution on all targets modules and by observing the island divertor plasmas in the light of H alpha, C II, and C III using the visible imaging section of the systems. The optics of the system can be divided into three parts: a mirror based optical head, creating an intermediate image, a Cassegrain telescope system, and individual lens based imaging optics adapted to the various detectors for IR (3-5 microm and 8-14 microm) and visible observations, with their optical light paths being separated by in-vacuum dichroic beam splitters.