Melt textured YBCO: High trapped fields and superconducting magnetic bearings

Bulk melt textured YBCO samples in the shape of discs were prepared by a top seeding method using Sm-123 crystals. The field distribution of the trapped field on the surface of the samples war measured at 77 K by means of a Hall probe. In single grains with diameter of 20 mm the trapped field reached maximum values up to 0.56 Tat 77 K: Investigating the temperature dependence of the maximum trapped field, a value of 5.5 Tat 30 K was found for the best sample. At the same temperature, degradation of the trapped field by a factor of about 10 was observed for a second sample after field cooling in a field of 7 T. This degradation is explained by damage to the sample due to magnetic stresses. At the centre of the sample a magnetic stress of II MPa is estimated to be responsible for this damage. Additionally YBCO material has been tested in a high speed motor with superconducting magnetic bearings (SMB) in order to demonstrate the passive stabilization properties of SMB. In first tests the shaft rotated with a speed of 12 000 rpm limited by air drag.