Master thesis : Design and experimental realization of a 3-axis Hall probe for low temperatures : application to the measurement of the flux density in superconducting structures

Abstract

Applications involving superconductors generate an increasing interest nowadays in various fields (medical imaging, motors/generators, particle accelerators, shielding, etc.). Measuring the local magnetic flux density around the whole superconducting structure is a key element towards the improvement of these applications. The context of this master’s thesis is the need to measure and understand the details of the 3D spatial distribution of the flux density at cryogenic temperature (77 K). However, 3-axis Hall probes operating at cryogenic temperature for a flux density above the mT range are not commercialized anymore. The goal of this master’s thesis is to design, build and characterize experimentally a 3-axis cryogenic magnetic field sensor operating at several tens of mT.

In the first part of this work, guidelines are established in order to build an experimental chamber able to keep a region of space at room temperature while it is immersed into liquid nitrogen. This way, a commercial Hall probe suited for room temperature measurements can artificially be used in a cryogenic environment. The resulting device (experimental chamber with the probe) should be usable to measure the magnetic flux density over superconducting structures. As a result, the experimental chamber should be of minimal size and the Hall probe should be placed as close as possible to these structures. A first analytical model is developed, before validating the results experimentally.

In the second part, printed circuit boards are developed to interface and calibrate the 3-axis Hall probe to be placed in the experimental chamber. The probe shows a linear range of more than ~155 mT in each direction and a typical noise level of a few hundreds of µT. Additionally, the sensitivity and the offset are shown to be almost temperature independent between 5°C and 65°C.

In the last part, the final experimental chamber is assembled. The chamber outer diameter is 20 mm and the active region of the Hall probe is determined to be located at 2.2 ± 0.25 mm from the bottom end. A proportional integral (PI) temperature controller is implemented to ensure that the Hall probe stays around 25°C with few variations, irrespective of the possibly cryogenic environment. The resulting device is used to perform two experiments on superconducting structures; the measurement data are compared to both analytical and finite element models. As a result, the device can perform reliable and accurate measurements in cryogenic conditions (77 K) as soon as the flux density exceeds ~0.5 mT.