Performance Of Superconductors Research Articles

Magnetic field reversion by SC/FM bilayers in the critical temperature vicinity

The electrical properties of bilayer structures consisting of a superconducting (SC) and a ferromagnetic (FM) layer deposited on a LAO substrate were studied by a contactless technique in the temperature range from 77 K in the vicinity of the critical temperature. Two series of bilayers were investigated: – SC (YBCO) layers with thickness varying from 35 to 80 nm on a FM (LSMO) layer with a fixed thickness of 40 nm, and – a fixed-thickness YBCO layer (70 nm) on LSMO layers with thickness varying from 10 to 40 nm; YBCO layers deposited under the same conditions as reference. An interesting effect was observed, namely, when the temperature is reduced below the critical value, the bilayer inverts the magnetic field passing through it – the signal picked up by the receiving coil inverts its phase. The measurements of the critical current and temperature showed that when the YBCO layer is thicker than the LSMO layer, the latter acts as a buffer layer improving the SC performance. These is in contrast with the results published earlier by other authors on proximity effect and spin injection for bilayers and super lattices of SC/FM layers that are an order of magnitude thinner. The SC performance worsens only in the case when the YBCO and LSMO layers have a thickness of approximately 40 nm.

A cooling concept for improved field winding performance in large superconducting ac generators

An analytical study of a flow circuit for large superconducting generator rotors is presented. The flow circuit provides regulation of the level of liquid in the rotor externally by adjusting the helium supply pressure. It also protects the vapour cooled structural members of the rotor from overcooling during transient periods of operation. Furthermore, it is capable of reducing the winding temperature below 4.2 K thereby enhancing the superconductor's performance. For example, a large generator rotor with NbTi superconducting field winding experiences approximately a 50% increase in its critical current density compared to that at 4.2 K.