No abstract

We propose that in an HTS application, stability is lost more likely because of a global increase in temperature caused by heat generation distributed over the whole coil than because of a local normal zone which starts to propagate. For consideration of stability in HTS magnets, we present a computational model based on the heat conduction equation coupled with Maxwell's equations, whereby analysis can be performed by using commercial software packages for computational electromagnetics and thermodynamics. For temperature distribution inside the magnet, we derive the magnetic field dependent effective values of thermal conductivity, specific heat, and heat generated by electromagnetic phenomena for the composite structure of the magnet, while cooling conditions and external heat sources are described as boundary conditions. Our model enables the magnet designer to estimate a safe level of the operation current before a thermal runaway. Finally, as examples, we present some calculations of the HTS magnet with ac to review the effects of slanted electric field-current density E (J ) characteristics and high critical temperature of HTS materials.

In this paper, an optimization method to determine the magnetic field dependence of the intrinsic critical current density, Jc(B), is introduced. First of all the critical current is measured in various external magnetic fields. Then the Jc(B)-dependence that fits optimally to the measurements is searched for. In these computations the self-field of the sample is taken into account and thereby measurements performed in low external magnetic fields, below 0.1 T, can be exploited. Here the Jc(B)-dependence of YBCO material is described by the Kim model, which is modified to include the anisotropy also. Thus, there are four parameters to search for: zero field critical current density Jc0, reference field B0, Kim model exponent α and anisotropy scaling factor γ. The search for these parameters was computationally challenging but computation times were still satisfactory. As examples, the Jc(B)-dependences of two YBCO samples from different manufacturers were found. For both samples, all parameters except B0 were near each other. For example, Jc0 values for both samples were about 0.9 × 1010 A m−2.

No abstract

We constructed a model to estimate analytically the current transfer from a matrix to a superconducting region through a highly resistive barrier layer. Also, a numerical model was formulated in order to study the current transfer when the superconducting region has nonlinear resistivity. Because so far the definition for current transfer length (CTL) has not been well established we propose a definition which introduces a method to determine the CTL corresponding to a chosen criterion. Two possible criteria are suggested. One relates to electric field and the other to current in the superconducting region. As a consequence this paper gives an analytical formula which can be used for example in a short-specimen measurement to determine the disposable measurement area. As examples, the different CTL criteria were computed with different resistivities in the superconductor. Also, analytical and numerical models were compared. The analysis was carried out for the possible general structure of future MgB 2 conductors: superconductor, barrier layer and matrix metal. According to the results, the presented analytical model is a useful tool in determining the current transfer length when the barrier layer contact resistance is at least 25 μ mm 2 . Otherwise the presented numerical model must be used.