Accurate Ellipsometric Magnetic-Field Sensor Used to Align the LNE Watt Balance Magnetic Circuit

Magnetic-…eld sensors based on the Faraday e¤ect in a Tb 3 Ga 5 O 12 crystal are investigated in terms of their sensitivity, accuracy and directionality. The possibility of an increased sensitivity of such Faraday e¤ect sensors due to the application of multipass sensitive elements is considered. Signal-to-noise measurements on 5 mm diameter 20 mm long sample yield magnetic …elds equivalent noise of 300 nT=Hz 1=2 (= 1 s). The sensor has a sensitivity of 5.457(1) rad=T. Using spinning quarter waveplate ellipsometer, we can measure magnetic …elds to 1 T. The data acquisition rate could be up to 80 Hz. This sensor is used to check the homogeneity and orientation of the …eld provided by the LNE watt balance magnetic circuit which the aim is to rede…ne the SI unit of mass in the near future.