Design and R&D of Thermal Sensors for ITER Neutral Beam Injectors

In-vessel components of Megavolt ITER Injector and Concept Advancement, the ITER neutral beam injector prototype, will be instrumented with different sensor technologies mainly for protection and thermal control: thermocouples (TCs) will be used as thermal sensors distributed on all the in-vessel components, accelerometers as boiling detectors mounted on tubes of cooling channels subjected to high heat fluxes, fiber Bragg gratings as thermal sensors on high-voltage panels, strain gauges (SGs) as strain sensors on panels subjected to out-of-plane deformations. ITER in-vacuum cabling requires to use mineral insulated (MI) cables that exhibit radiation hardness and Ultra High Vacuum (UHV) compatibility. A common issue related to sensors cabled with MI (TCs, accelerometers, and SGs) involves the realization of UHV compatible terminations needed with provide vacuum tightness and mechanical robustness. Prototypes of MI TCs with 0.5-, 0.8-, and 1.0-mm cable diameter and with metallic/ceramic brazed terminations have been procured and tested; all sensors passed electrical and leak tests, including electrical insulation, continuity, dielectric, helium bombing, and thermal cyclic tests, so demonstrating the measuring capability and UHV compatibility. Custom sensor fastenings, custom remote handling compatible connectors, and safety important signal feedthroughs have been designed considering all the boundary conditions and the required number of sensors as obtained from the analyses.