Calibration of a 3D Hall Probe Array for Equilibrium Reconstructions and MHD stability Studies in the Compact Toroidal Hybrid

Innovations in Hall sensor technology have led to the development of compact 3D sensors capable of measuring fields ranging from milli-Tesla to Tesla. An array of 8 of these sensors has been implemented as a magnetic diagnostic on the Compact Toroidal Hybrid (CTH) experiment. The array spans 45 mm allowing for simultaneous measurement of all fields within CTH at 8 positions. These local magnetic field measurements allow for improved knowledge of the current profile near the plasma edge using V3FIT reconstructions and will constrain the edge current gradient for MHD stability studies. The array has been calibrated on the benchtop using a 1-D Helmholtz coil, and in situ inside of CTH. Measurements demonstrate that effects from misalignment of the sensors’ internal components, and the planar Hall effect can both be neglected. Misalignment between the sensors making up the array was found to be less than one degree and can be accounted for. 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 plasma current. After accounting for sensor misalignment and eddy currents, the experimentally reconstructed magnetic field profiles are in good agreement with profiles from a Biot-Savart model.