A.c. loss measurements in high-tc superconductors

Low a.c. loss is a key issue for electrical engineering applications of high-T_c superconductors at industrial frequencies. Different material options are characterized by three different a.c. loss measurement techniques which approach as much as possible the application conditions of the conductors. Melt-textured-growth YBa₂Cu₃O_7?x bulk samples, suitable for magnetic bearings, are characterized in an external a.c. field by magnetization measurements. Melt-casted Bi₂Sr₂Ca₂Cu₃O_8?x samples and powder-in-tube Bi₂Sr₂Ca₂Cu₃O_10?x tapes are suitable for current leads or power cables. In these applications the conductors are exposed to a.c. transport currents. The associated losses in self field are measured by a very sensitive electrical measurement technique. Finally, a calorimetric method is necessary when larger conductors (for instance Bi₂Sr₂Ca₂Cu₃O_8?x tubes) are tested under transport currents I_t, generating a transverse magnetic field H α I_t, as encountered in magnet windings for SMES, transformers or generators. The results show that the a.c. losses are sufficiently low for self field applications at industrial frequencies and a comparison of the different high T_c superconductors is given. The results show further that the a.c. losses are essentially hysteretic and can be modeled using the Bean model.