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 Bapp = 10 T without fracture; a similar ring bulk without the hat structure broke during FCM at Bapp = 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 Jc(B) of the ring bulk used in the simulation was determined by experimental results of time step dependence of the trapped field Bz at the bulk centre. As a result, the total hoop stress θtotal in the ring bulk including the resulting stress of cooling from 300 K to 50 K and subsequent FCM at Bapp = 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 Bapp = 10 T. Using this reinforcement method, a 400 MHz (9.4 T) nuclear magnetic resonance bulk magnet system could be realised.