Calibration of a Hall Probe Array for Equilibrium Reconstructions in the Compact Toroidal Hybrid

An array of 16 gallium arsenide Hall sensors spanning 121.5 mm has been implemented as a magnetic field diagnostic in the CTH device. The main advantage of using these sensors is their 2.25  surface area which allows for local field measurements. Local magnetic measurements are necessary for V3FIT reconstructions of the poloidal magnetic field produced by plasma currents. Benchtop calibrations of the Hall probe array determined the sensors have an average sensitivity of 1.25 volts per tesla for fields perpendicular to the sensor surface and are generally insensitive to fields parallel to the surface. In situ calibration in the CTH device revealed a significant effect due to a slight difference in the angular orientation of the individual sensors in the array ranging from -.8 to 1.9 degrees. The CTH vacuum vessel and coil frames are also known to produce a measurable flux due to eddy currents that arise from their mutual inductance with the CTH coils. When accounting for the sensitivity to orientation and the effects from eddy currents, the experimental magnetic field profiles are in good agreement with the modeled profiles from a Biot-Savart code.