Dissipation in cylindrical type-II superconductors rotating in a magnetic field

A type-II superconductor in the mixed state which rotates with angular velocity ω in a magnetic field Hodirected perpendicular to the axis of rotation experiences a retarding torque due to pinning and viscous friction of vortices. A method is presented using the critical state concept that allows one to calculate the torque and associated losses for a cylindrical specimen of radius a and infinite length, if its equilibrium magnetization curve and the dependence of its flux-flow conductivity σfand critical current density Jcon the macroscopic flux density B are given. Results obtained for a substance with simple model behavior of σf(B), Jc(B), and magnetization are discussed. For constant a and Ho, a linear variation of the torque per unit length with σfω and with Jcis observed for sufficiently low values of these parameters. At higher values, a deviation from this behavior results from vortex curvature combined with strong flux-density gradients and from increasing explusion of flux from the sample.