Steady state magnetic sensors for ITER and beyond: Development and final design (invited)

The measurements of the magnetic field in tokamaks such as ITER and DEMO will be challenging due to the long pulse duration, high neutron flux, and elevated temperatures. The long duration of the plasma pulse makes standard techniques, such as inductive coils, prone to errors. At the same time, the hostile environment, with repairs possible only on blanket exchange, if at all, requires a robust magnetic sensor. This contribution presents the final design of novel, steady-state, magnetic sensors for ITER. A poloidal array of 60 sensors mounted on the vacuum vessel outer shell contributes to the measurement of the plasma current, plasma-wall clearance, low-frequency MHD modes and will allow for crosscheck with the outer-vessel inductive coils. Each sensor hosts a pair of bismuth Hall probes, themselves an outcome of extensive R&D, including neutron irradiations (to 10²³ n/m²), temperature cycling tests (73-473 K) and tests at high magnetic field (to 12 T). A significant effort has been devoted to optimize the sensor housing by design and prototyping. The production version features an indium-filled cell for in situ recalibration of the onboard thermocouple, vital for the interpretation of the Hall sensor measurement.