Behaviour of normal zones in a uniform ac superconductor

A description of normal zone behaviour in an ac superconductor over the whole range of current frequencies is given in terms of a model. A zone boundary motion equation is derived for the case of heat generation arbitrarily dependent on time. Using a numerical solution of this equation for a half-infinite normal zone, the dependences have been obtained which characterize zone propagation in terms of velocity and peculiarities of its behaviour. The evolution of finite-length zones is analyzed. The existence is shown of two characteristic lengths, Lsub(min) and Lsub(max) which are responsible for zone collapse and growth, respectively. A qualitative picture of the behaviour of zones of various length over the whole range of currents is given. (author)