HTS Zimmerman SQUIDs, associated electronics and some anomalies

In the early '90 we developed a laboratory technology for HTS Zimmerman SQUIDs manufacturing. It includes tools, working procedures, instruments for testing and measuring of the resulting devices. Taking an YBCO pellet as a promising material, a systematic activity to improve it followed. The effects of pressure, annealing temperature and annealing time on SQUID characteristics like signal to noise ratio and manufacturing yield were studied. So, we determined the corresponding technological parameters and prepared a good superconducting material for SQUID manufacturing. In the process, tests revealed anomalous SQUIDs. These exhibit a triangle like flux-voltage characteristics but any two adjacent triangles have different heights. When making the weak link thinner, some anomalous SQUIDs become normal. The first ones present a practical interest because their response signal to flux deviations is higher than that of the usual SQUIDs. We developed flux meters having superconducting transducer. Their noise limited resolution was rather poor: 0.1 Φ0 when integrator time constant equals 10 ms (Φ0 = 2.05 x 10-15 Wb is the flux quantum). This value is due to both the noises of the electronics and to the magnetic environment. We succeeded to improve the electronics up to the level where its input noise equivalent voltage equals the noise voltage generated by the resonant circuit coupled to the SQUID. That's why we account the result as an end point on this way. Superconducting shields are very effective: the attenuation factor of the magnetic induction B external axial component is larger than 600, in the symmetry center, when the length to inner diameter ratio of a cylindrical shell equals 3. When the external magnetic induction supersedes the critical value (Bc = 12 mT) and the external field has a harmonic component too, the inner harmonic component is opposite in phase. This is an unexpected result. The work is in progress. (authors)