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The risk of overheating arises if the stability of an MgB2 coil is lost. This is mainly due to the low thermal conductivity of the matrix metal, which is typically iron or nickel. Due to a strong chemical reaction, copper can be used in contact with MgB2 only in specific cases: in situ preparation route and low formation temperature. However, recent developments in the manufacture of MgB2 conductors have resulted in stabilized MgB2 tapes. In any case, the stability considerations of MgB2 conductors are of great importance. We studied computationally the stability of an MgB2/Ni/Fe/Cu tape manufactured by Columbus Superconductors. First, the effective material properties of the tape were computed. Based on these material properties, we computed the basic quench characteristics (minimum propagation zone, minimum quench energy and normal zone propagation velocities) for the tape at 15, 20 and 25 K. The tape unit cell model was used in computations. The computed stability results were compared with the measured ones of a monofilament MgB2/Cu/Ni tape and commercial LTS and HTS. According to the results, the basic quench characteristics computed fit between the ones of LTS and HTS materials.

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.

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