ABSTRACT
In the presence of a magnetic field it turns out that both type I and type II superconductors may enter inhomogeneous states which are finely divided into normal and superconducting regions, with magnetic flux passing through the normal regions. Flux trapping occurs very frequently when a field is removed from a type I superconductor which is strained or otherwise non-uniform. What happens is that the outermost normal domains migrate outwards, leaving a wholly superconducting ring which surrounds a few well-separated normal domains near the centre of the sample. Superconductors usually have high normal resistivities, and Joule heating may pose major problems in superconducting magnets: if a small spot in the wire is driven normal for some reason, thermal runaway may occur, with the whole magnet driven normal and a very large amount of magnetic energy suddenly dissipated in the coolant.
