Armoring of the Wendelstein 7-X divertor-observation immersion-tubes based on NBI fast-ion simulations

[Display omitted] •Simulations predicted NBI fast ion losses to sensitive immersion tubes.•Protective stainless steel collars were installed on short notice.•First NBI experiments confirmed the need for armoring. The first neutral beam injector (NBI) experiments of the Wendelstein 7-X stellarator took place in summer 2018. The modelling of the fast ion production and slowing down processes predicts losses of the NBI fast ions to the first wall on the order of 15%. One location receiving a high load (possibly peaking at several M W/m2) is the immersion tube for optical and infrared monitoring of the divertor targets. The stainless steel face of the tube has three vacuum windows, which are sensitive to temperature gradients and overheating. To protect the windows from damage caused by the fast ions, different heat load mitigation techniques were investigated. Given the available time and resources until the first NBI experiments, a protective stainless steel collar mounted at the front of the immersion tubes was regarded the most realistic solution. This contribution describes the fast ion modelling of the loads, the new design, thermal modelling of the design, and finally experimental experience with the protective collar showing heat loads in excess of 1.5 M W/m2. The fast ion heat loads have been assessed computationally with the ASCOT code and experimentally with thermal imaging.