In pulsed field magnetization (PFM), the phenomenon of flux jump is capable of driving magnetic flux vortexes into GdBCO superconducting bulk center to aid full magnetization. Varieties of homogeneous critical current density (Jc) models have been implemented to reproduce flux jumps, but simulated multi-physical responses differ from experimental observations. This paper proposes a modified Jc model to consider r-z plane Jc inhomogeneity and simulates flux jumps under experimental conditions by solving a 2D axisymmetric electromagnetic-thermal coupled model. A numerical treatment is developed to reflect the break of shielding current during flux jumps. The accuracy of our model is verified by comparisons of the calculation results of trapped magnetic fields (BT) and the PFM and field cooling (FC) experimental results. On this basis, we investigate the improvement of inhomogeneous Jc model and obtain the multi-physical responses which have better agreement with the experimental results compared to homogeneous Jc model. Moreover, to further test the ability of the inhomogeneous Jc model to predict anisotropy of spatially magnetic field distribution, the simulated BT profiles in top and bottom surfaces of HTS bulk at 77 K are compared to the experiments. This study may provide a new approach for modelling the inhomogeneity of Jc characteristics and a useful analysis tool for industrial devices using high-temperature superconductor (HTS) bulk magnets.
The development of rare-earth barium copper oxide (REBCO) coated conductors with an extremely high critical current density under ultra-high fields opens up a high-field path towards large-scale fusion. The latest technology has inspired cable-in-conduit conductors (CICC) such as conductor on round core (CORC) wires, twisted stacked tape conductor (TSTC) cables and Rutherford cables with outstanding current-carrying capacities. In order to realise an inductance balance and decrease magnetic diffusion, these cables have been twisted or folded to a certain extent, thus breaking the mechanical behaviour of the ceramic superconductor and limiting their potential for ultra-high-field applications. One possible solution is to employ a non-twisted cable, which offers maximum protection of its mechanical properties and enables a parallel orientation of the toroidal field vector to the surface of REBCO coated conductors. However, the applied physics community’s attitude towards using non-twisted, parallel REBCO stacked-tape cables is one of scepticism, the main argument being that the nonlinear E-J behaviour associated with screening current in the parallel stack might lead to a field distortion and reduce the performance of superconductivity. Recent analyses have demonstrated that the effect of screening current decreases significantly owing to a wavelike magnetic field distribution along the cable. The authors obtained similar results using H-formulation and T-A formulation based finite element methods and demonstrated that the non-twisted cable may be feasible for DC current transmission toroidal field coils in magnetic-confinement devices. Furthermore, the electromechanical behaviour of toroidal field coils has been evaluated via the Maxwell stress, solved by using an A-V formulation. It was discovered that the stress generated by the toroidal field coils is within the stress tolerance of the REBCO coated conductor, something which is of great significance in promoting the application of REBCO coated conductors for ultra-high-field magnetic-confinement plasma devices.
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