A DSP-enhanced AC susceptometer for characterisation of high temperature superconductors.

Abstract

DSP-based susceptometer was designed and implemented to study granular HTS Y1Ba2Cu3O7-x (YBCO) superconducting specimens. The AC susceptometer is made up of a primary coil driven by an AC signal and two sensing coils: a balancing coil and a pickup coil which senses the magnetic response of a superconducting specimen. A particular feature of the AC susceptometer was a digital lock-in amplifier which was designed and implemented in DASYLab instead of using a conventional lock-in amplifier. The lock-in amplifier was necessary to separate the real and imaginary parts of AC susceptibility response of the superconducting sample placed into one of the sensing coils. Granular YBCO material is known for having a low critical current density, , due to a weak link effect. Grain boundaries, which are responsible for the weak link effect, comprise of regions of high stress fields. In one experiment to test the apparatus, YBCO specimens were doped with hydrogen with the aim of relieving stresses to improve the intergranular critical current. The AC susceptometer was successfully used to characterise the intergranular critical current of YBCO samples. The relative intergranular critical current was successfully estimated using a critical state model, based on Bean model. The migration of H2 molecules into the YBCO‟s lattice was not successful in relieving the stresses at the grain boundaries, and resulted in the deterioration of intergranular critical current.