Promising effects of a new hat structure and double metal ring for mechanical reinforcement of a REBaCuO ring-shaped bulk during field-cooled magnetisation at 10 T without fracture

We have investigated a new reinforcement hat structure for a RE-Ba-Cu-O (REBaCuO, RE: rare earth element or Y) ring-shaped bulk superconductor (ring bulk) shrink-fitted with a double Al alloy ring and set on the cold stage of a cryogenic refrigerator prior to field-cooled magnetisation (FCM). With the hat structure, a ring bulk having an outer diameter of 64 mm, inner diameter of 40 mm, and height of 20.5 mm achieved a trapped field as high as 6.8 T at 50 K during FCM with an applied magnetic field at B_app = 10 T without fracture; a similar ring bulk without the hat structure broke during FCM at B_app = 8.8 T. Using a numerical simulation for electromagnetic and mechanical properties, the potential benefit of the hat structure was confirmed. The magnetic field dependence of the average critical current density J_c(B) of the ring bulk used in the simulation was determined by experimental results of time step dependence of the trapped field B_z at the bulk centre. As a result, the total hoop stress σ_θ^total in the ring bulk including the resulting stress of cooling from 300 K to 50 K and subsequent FCM at B_app = 10 T is lower than the fracture strength of a typical Ag-doped REBaCuO bulk material. The effect of double Al alloy ring reinforcement was also analysed using a numerical simulation and compared with that of the conventional single Al alloy ring reinforcement. These results suggest that the double ring provides only a limited benefit, whereas the hat structure is fairly effective in reducing the electromagnetic hoop stress during FCM at B_app = 10 T. Using this reinforcement method, a 400 MHz (9.4 T) nuclear magnetic resonance bulk magnet system could be realised.