Solutions for thermally mismatched brazing operations for ceramic tokamak magnetic sensor

Abstract To monitor high-frequency fluctuations of the equilibrium magnetic field in tokamaks, a 3D magnetic sensor has been developed. The sensor, which is positioned inside the vacuum vessel behind the protective tiles of the tokamak and is exposed to potential temperatures up to 400°C, is based on thick-film and LTCC (low-temperature co-fired ceramic) technology. To connect the sensor to the cabling that runs inside the vacuum vessel, mineral-insulated cables have to be brazed to the sensor to ensure electrical connection together with mechanical robustness and sufficient thermal stability. As the brazing temperature is about 600°C, direct brazing to the alumina sensor substrate can cause failure by cracking induced by thermal stresses. It arises both by temperature gradients stemming from the localised heating and by the high thermal mismatch of alumina with the braze and wire materials. In previous work, high stresses from temperature gradients were efficiently decoupled by brazing indirectly to alum...