Complex flux configurations in a type II superconductor

A reversible type 11 superconductor in the mixed state can carry a large loss-free transport current when the direction of the current is parallel to the external applied field. The origin of this effect lies in the arrangement of the flux vortices into a complex helical or 'force-free' structure. A study is presented of the details of this structure, the criterion for its breakdown and the nature of the subsequent flow state in a set of reversible cylindrical samples. (U.K.)