Investigation of vortex structures in a current-carrying Nb wire

We describe small angle neutron scattering measurements of the vortex structure within a current-carrying wire in the presence of a longitudinal magnetic field. This situation involves the combined effects of the applied field and the self-field of the current and has attracted a great deal of theoretical and experimental work as exotic helical vortex structures have been predicted. We find that the voltage characteristics, a complete absence of spreading of the Bragg peaks and loss of scattered intensity with increasing current are inconsistent with a helical vortex state. Our data are qualitatively consistent with a model where the crossing of self-field loops with vortex lines results in a ring of kinks leaving the remaining longitudinal lattice unperturbed.