Recent Improvement of the Design of the ITER Steady-State Magnetic Sensors

A recent improvement of the design of the ITER outer vessel steady-state magnetic field sensors (OVSS) is presented. OVSS features a pair of Hall sensors with a sensing layer made of bismuth. Sixty OVSS will be installed on the ITER vacuum vessel outer skin to perform an absolute measurement of magnetic field. The bismuth Hall sensor measurement is temperature dependent, and the maximum allowed OVSS error of 4-mT limits the error of the measurement of the Hall sensor temperature by an on-board thermocouple to be less than 0.28 °C. This error consists of the temperature difference between the thermocouple and Hall sensors, thermocouple calibration error, and thermocouple measurement error. The improved design of the sensor housing reduces the temperature difference between the thermocouple and the Hall sensors bellow 0.04 °C. Procedures of the calibration prior installation and in situ calibration will allow calibrating the on-board thermocouple with the error less than 0.06 °C. The thermocouple measurement error limit was determined as 0.27 °C in which case OVSS meets the maximum allowed error.