Hysteretic losses of a type-II superconductor in parallel ac and dc magnetic fields of comparable magnitude

Abstract

Measurements of 50‐Hz hysteretic losses in Nb3Gematerial have been performed in the presence of a dc‐bias magnetic field. The measurements were made by an electrical technique with both the ac and dcmagnetic fields parallel to the axis of long Nb3Ge‐coated cylinders. With a fixed ac amplitude h 0, the losses are observed to decrease as the dc field H 0 is raised from zero, to pass through a minimum, and then to increase monotonically. The position of the minimum is observed to occur when H 0=h 0−H s , where H s is the field at which initial entry of flux into the sample is observed. Minimum depths which reach 30% of the zero‐bias loss are found for samples in which H s is large (?900 Oe). This ’’minimum’’ effect and other aspects of ac losses in the presence of a dc‐bias field are explained qualitatively by a critical‐state model that takes account of the magnetic field dependences of the critical current density J c (B) and the surface barrier ΔH (B). The experimental results are also compared with the results of calculations based upon a recent theoretical treatment that allows the use of independently determined functional forms for J c (B), ΔH (B), and H (B). Qualitative agreement is obtained between theory and experiment.