Preliminary study of a visible, high spatial resolution spectrometer for DEMO divertor survey

Developments towards DEMO Diagnostic and Control (D&C) system conceptual design are based on a subset of ITER mature diagnostic systems, whose eligibility for DEMO has been endorsed by their robustness, long lifetime expectancy and feasible remote maintenance [1]. They are devoted to ensure the machine operation in compliance with safety requirements and high availability. In particular, the evolution of divertor spectroscopic measurements on fusion experiments has demonstrated their potential as a control method for divertor protection via detachment control [2, 3] (near ultraviolet, 300-400 nm) and monitoring of the plasma-wall interaction (visible range, 400-700 nm) [4]. These characteristics make this method one of the leading candidates for DEMO detachment and radiation control power. In line with the application of a system engineering approach [5], initial assessments of design and feasibility of a VIS high spatial resolution spectrometer for the DEMO divertor survey based on early DEMO control requirements are presented and discussed. The proposed system is located at the equatorial port and it is composed of 3 oblique lines of sights (LoS), 9 toroidal mirrors, 6 plane mirrors and 6 spectrometers examining the outer, inner and X-point divertor region, optimized for the monitoring of chord-integrated NUV/VIS signals under parallel divertor plasmas observation. The wavelengths of interest, spatial resolution and main integration issues are reported.