Trapped field evolution of two approaching bulk superconductors with parallel c-axes

Although the properties of individual trapped field magnets are well documented, there is little scientific information concerning their trapped field behaviour when several of them are brought in close vicinity. In this context, the translational approach of two cylindrical bulk YBCO superconductors (diameter 15.7 mm, height 8.8 mm) with parallel c-axes is experimentally investigated. The evolution of the trapped field at the surface of a fixed superconducting sample is measured while a second sample is approached. The following configurations are considered: (i) face to face (with anti-parallel trapped field orientation) approach and (ii) sideways (with parallel trapped field orientation) approach. The studied distance between the samples ranges from 120 mm, where the samples are observed to have no influence on each other, to contact (0 mm). The experiments are carried out at 77K and the samples are magnetised independently in field cooled under 1.2 T. The motion actuation is performed using a computer-controlled miniature linear actuator which is capable of overcoming the repulsive force arising between the trapped field magnets. Experimental trapped field data are compared to analytical calculations assuming fully magnetized superconductors. The trapped field of the fixed superconducting sample is measured to be altered by the second one when the separating distance is smaller than their diameters. If the distance is smaller than this limit, an irreversible diminution of the trapped field is measured when the samples are separated again. Repeated approach cycles showed that the irreversible loss of trapped field is more important for the first approach.