Endoscope diagnostic for tomography, spectroscopy and thermography on Wendelstein 7-X

Plasma-surface interaction (PSI) in the divertor region of Wendelstein 7-X (W7-X) will be of great importance for operational phase OP1.2. While the erosion of the divertor will have an impact on its lifetime and is therefore a critical subject of investigation, fundamental PSI studies in the divertor region are in many ways equally significant. These plasma-wall interactions will be influenced by impurity transport, where the complex 3D magnetic geometry will play a crucial role, but this magnetic geometry could itself be influenced by plasma effects such as Pfirsch-Schlueter and bootstrap currents. Therefore, along with measurements of obvious quantities such as heat flux, PSI research in the divertor region will also require measurements of the temperature in the plasma edge and of the concentration and distribution of different impurities, in combination with modelling of impurity transport. In order to provide the measurements necessary to address these physics questions, a set of endoscopes has been designed for visible and ultraviolet spectroscopy and tomography of the plasma edge, along with infrared thermography of the divertor tiles. An overview of this endoscope diagnostic system is presented. Details of the measurements to be taken and their relationship to physics issues such as impurity transport and erosion of the divertor are discussed.