Hybrid magnetic diagnostics on DEMO: general concept and integration studies

As tokamaks advance towards steady-state fusion operation, most plasma diagnostics will have elements exposed to increasing nuclear and thermomechanical loads over the lifetime of the facilities. Since diagnostic failure due to neutron exposure is expected to occur in such environments, the approach on DEMO has been to develop a set of diagnostics with robust front-end components compatible with remote handling. This paper outlines a general concept of a magnetic diagnostic system for long plasma-duration operation, developed for DEMO but applicable to future full-scale fusion power plants and intermediate machines like the VNS. This is achieved by employing resilient enabling technologies, hybrid magnetic diagnostic chains and data fusion estimators. Following a detailed overview of the diagnostic subsystems under development, and the characterization of the radiation environment at the most exposed front-end locations, the main interfaces with other reactor systems are described and a remote handling classification scheme is developed, based on the one adopted on ITER. An in-depth discussion of the integration possibilities for in-vessel probes and cables on DEMO is then provided. To address the constraints identified in these studies, the Magnetic Strip concept is proposed, a modular structure conceived to house all the in-vessel magnetic probes and part of the cable looms that carry the signals to the vacuum vessel feedthroughs. The Magnetic Strip is designed to be attached to the inner surface of the vacuum vessel, anchored on the fixation bosses developed for ITER diagnostics, behind the breeding blanket. Integration with the remote maintenance procedures of the blanket is demonstrated through the development of a remote handling tool concept to manipulate the structure. Preliminary analyses underline the advantages of the Magnetic Strip concept, which allows the replacement of the front-end components and cabling in single remote handling operations while keeping temperatures and thermal gradients within the operating limits.