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Critical current of an MgB2 coil with a ferromagnetic matrix

It is of great interest for magnet designers to know the maximum current that a coil can be operated with, i.e. the critical current of the coil. Magnesium diboride is a newcomer in the field of practical superconductors. Iron is a possible matrix metal for use in MgB2 conductors. Its ferromagnetic nature brings several new problems into the computation of the critical current of the coil. In this paper we show, by means of finite element method computations, that the measurement situation for the short-sample critical current in the case of an MgB2/Fe wire differs considerably from that for a conventional superconductor. Also, we study a computation of the critical current of an MgB2/Fe coil based on the engineering permeability. The computational model presented is applied to a small solenoidal MgB2/Fe coil. On the basis of the results, we propose a non-linear, conductor geometry dependent shift for the measured short-sample critical current curve to give values for the critical current of the coil.