Development of a split-toroid high-temperature superconducting cryogenic current comparator

Cryogenic current comparators (CCCs) based on split toroidal shields show promise for the construction of high-accuracy ratio devices using high-temperature superconductors. To test this approach we have carried out measurements on a low-temperature superconducting prototype built of three layers of toroidal shields. We find that ratio accuracies of about 1 in 10/sup 7/ are achievable for this prototype. After several attempts, we were also able to construct a toroidal two-layer CCC fabricated of YBCO (YBa/sub 2/Cu/sub 3/O/sub 7/) components using split shields. The results from measurements on this device at low temperatures (4 K) indicate that the shielding provided by these layers does not provide sufficient attenuation of the leakage flux. The challenge remains to find a processing technique to produce YBCO in a machinable form with satisfactory magnetic shielding properties.