Maxim Osipov

Relaxation of Levitation Force of a Stack of HTS Tapes

Nowadays, magnetic bearings are used to design motors, generators, separating platforms, and flywheel energy storage systems. High-temperature superconducting (HTS) magnetic bearing construction is based on YBa2Cu3O7-x (YBCO) bulks through its ability to capture and hold significant magnetic field for a long time. However, that material requires external mechanical reinforcement and has low thermal conductivity, which provides additional restrictions on practical application. Modern HTS tapes do not have such disadvantages. Its structure provides better rigidity and thermal stability in comparison with HTS bulk. One more significant advantage of tapes is simplicity of cutting to required form. In order to construct magnetic bearings from stacks of HTS tapes, it is necessary to have data about stability of system stack magnet under different dynamic processes. In this work, relaxation of levitation force was researched in both static and vibration modes with vertical oscillation frequencies of 15, 20, and 30 Hz. The measurements were performed for stacks with different amounts of 12 mm × 12 mm REBCO tapes in a stack (n = 1, 5, 10, 15, 20) at temperature T = 77 K. Stacks were placed over permanent magnet with a minimal gap of 4 mm. FC cooling method was realized. All static and dynamic relaxation measurements were carried out at 11.6-mm gap.

Magnetization and Levitation Characteristics of HTS Tape Stacks in Crossed Magnetic Fields

In this paper, we present the results of the measurements of HTS tape stack magnetization and magnetic levitation forces between the permanent-magnet NdFeB and the stacks composed of n = 5-100 tapes of ReBCO (with the size of 12 mm × 12 mm) in the field cooling and zero-field cooling modes in a crossed magnetic field. The alternating magnetic field parallel to the surface of the tapes was produced by a copper coil with the amplitude up to 80-Oe frequency was varied in the range from 50 to 400 Hz. Measurements were carried out in liquid nitrogen. The dependencies of the magnetization of the stacks and the levitation force on the magnitude and frequency of the alternating magnetic field were obtained. The suppression of the levitation force and magnetization in a crossed field for the stacks of various thicknesses was shown. The results were compared with the characteristics of the bulk superconductors.

Investigation of HTS Tape Stacks for Levitation Applications

Currently, high-temperature superconductor (HTS) bearings are most commonly designed with bulk REBa2Cu3O7-x (REBCO), but these HTS bulks have several drawbacks, such as brittleness and the complexity of sample preparation. An alternative for levitation systems is to use stacks of REBCO coated conductors (HTS tapes). The main advantages of HTS tapes are the simplicity of manufacturing elements in the required shape and high strength stability. For the development of HTS bearings, it is important to characterize both the levitation force along the axis connecting the magnet and the superconductor, and the stability of the system in response to lateral displacement. In this paper, we present new results of measurements of the levitation force between a permanent magnet and a stack of HTS tapes containing from 10 to 100 tapes of 12 mm × 12 mm. Measurements were conducted in zero-field cooling mode. We observed hysteresis of the levitation force in the case of lateral displacement. In addition, we have investigated the decay in the levitation force due to lateral displacement with each cycle of movement. It is shown that the decay rate tends to be constant on increasing the number of tapes in the stack.

Analysis of the magnetic field distribution and estimation of the critical current density based on a magnetooptical study of the superconducting strip composites

The results of magnetic flux penetration into high-temperature superconducting composites of the second generation have been analyzed and described. The distribution of magnetic flux was observed experimentally using the magnetooptical visualization technique at various temperatures of 4.2–80 K. The experimental data have been found to coincide with the results of analytical and semi-numeric calculations of magnetic field profiles carried out for regimes of full and partial field penetration into superconducting composites. The method of estimating the critical current density based on measurements of the topography of the transverse magnetic flux has been considered, and an approach to estimating the critical current density for strips from measurements of the tangential field component has been proposed.

Investigation of the hysteresis losses in CC tapes after laser filamentation

Study of AC loss has been conducted on the 4 mm wide CC-tapes manufactured by SuperOx. Filamentation was carried out by laser cutting of the commercially available tapes with a copper coating. Also for comparison, original tapes and commercial multifilament tapes (manufactured by SuperOx by using chemical etching method) were studied. Losses were obtained from the magnetization curves measured on a vibration sample magnetometer in the temperature range from 4 K to 77 K. In addition, the current carrying characteristics of tapes were studied, and the effect of filamentation on the critical current value were examined. We present an analysis of the experimental results, as well as a comparison of data for different types of samples. The possibility of decreasing the energy losses due to filamentation tapes was demonstrated.

Magnetic and transport properties of HTS MgB2 wires

Critical current Ic and magnetization M of industrial MgB2 tape and wire were measured in order to estimate their suitability for production of liquid helium free cryomagnetic system. Samples were subjected to bending with different diameters Db and critical current was measured at temperature T = 4.2 K in magnetic fields up to 10 T. The dependences of the critical current on the diameter of the bend Ic (Db) were found and used for optimization of liquid helium free cryomagnetic system. The magnetization of the wire was measured by a vibrating sample magnetometer at temperatures T = 4.2,10, 15, 20, 25 K in magnetic fields up to 14 T. It was found that the magnetization curve M (H) is influenced by ferromagnetic response of the metal matrix. Ferromagnetic contribution had been taken into account and magnetization loops caused by the diamagnetic contribution of the superconducting phase were extracted. Dependencies Ic(T) and Ic(H) were obtained from the data.

Magneto-Optical Investigation of the Critical State in Coated Conductors: Peculiarities of the Magnetic Field Distribution and the Determination of the Critical Current Density

In this paper, we present a detailed analysis of the results of the magneto-optical measurements of the fields topography in the coated conductor (CC) tapes at various temperatures in order to study the distribution of the field and develop a simple method for determining the critical current density. It is shown that the fitting of calculated field profiles to the results of the magneto-optical measurements gives a satisfactory agreement between the model and the experiment. In addition, we proposed a method for the determination of the critical current density from measurements of the topography of the transverse field that is insensitive to the precise form of the current lines. Furthermore, it is shown that the experimentally observed asymmetry of the field profile is probably due to a slight misalignment between the MOI film and the CC tape, which is possibly due to the presence of a small transverse curvature of actual CC tapes.