The basic topics for AC losses in superconductors are reviewed briefly. The AC losses are affected by various factors such as the variation in critical current density, strong nonlinearity of transport property and the complicated electromagnetic environment in superconducting apparatus. Several general expressions to evaluate AC losses are presented from the physical viewpoint of local power dissipation, Poynting's vector and the area surrounded by a magnetization curve. The fundamental properties of AC losses are discussed for each step, from the initial stage of superconducting materials to the final stage of a superconducting apparatus system. The measurement techniques for AC losses based on both electromagnetic and thermal principles are explained for some practical cases. Finally, the guidelines to suppress AC losses in typical superconducting power devices are proposed.
With the continuing improvement of properties of High Tc Superconductors (HTS), their application to accelerator technology is becoming increasingly attractive. While the medium to long-term goal for their use in this field will be for high field magnets, the materials now available enable a number of specific applications which benefit from their characteristics and make them a valid alternative to conventional Low Tc Superconductors (LTS). If devices for electrical utilities generally seek to use nitrogen as cryogen, large superconducting accelerators rely on helium cryogenics, and allow operating at lower temperatures with consequent gains in electrical performance and temperature margin. After an overview of commercial HTS materials, applications of HTS to accelerators are presented, with examples of present and future use of HTS in the Large Hadron Collider.
A high-precision nitrogen boil-off method has been developed for measuring AC losses calorimetrically in HTS coils. A sensitivity of about 0.05 W was achieved through control of convection in a sample vessel and the use of a cryostat. In the case of a simple LN2 reservoir instead of the cryostat, convection control in the sample vessel made it possible to measure losses with a sensitivity of about 0.5 W. AC loss measurements of Bi-2223 and Y-123 coils were examined using the cryostat and the simple reservoir, respectively. The results show that the reliability of the measurement is the same as for electrical measurement, and that the measuring procedure is simpler than any other method.
Search-coils, which can detect magnetic field variations, have been applied to the measurement of superconducting coil displacements. The advantage of the search-coil is that it can measure displacements which occur in an arbitrary section of a superconducting coil without making contact with a winding. In our experiment, the radial displacements in the straight and curved sections of a racetrack coil were measured and compared through the results of FEM analysis. The results showed that micrometer-order sudden displacements were detected during the ramping of the racetrack coil current, and the sum of sudden displacements increased with the racetrack coil current. When the racetrack coil current reached the maximum value of 1000 A, the total displacement of the straight section was three times greater than that of the curved section. This measured result agrees relatively well with the numerically calculated result.