Topological Hysteresis in the Intermediate State of Type-I Superconductors

Magneto-optical imaging of thick stress-free lead samples reveals two distinct topologies of the intermediate state. Flux tubes are formed upon magnetic field penetration (closed topology) and laminar patterns appear upon flux exit (open topology). Two-dimensional distributions of shielding currents were obtained by applying an efficient inversion scheme. Quantitative analysis of the magnetic induction distribution and correlation with magnetization measurements indicate that observed topological differences between the two phases are responsible for experimentally observable magnetic hysteresis.Comment: 4 pages, RevTex

Flux pinning characteristics in cylindrical ingot niobium used in superconducting radio frequency cavity fabrication

We present the results of from DC magnetization and penetration depth measurements of cylindrical bulk large-grain (LG) and fine-grain (FG) niobium samples used for the fabrication of superconducting radio frequency (SRF) cavities. The surface treatment consisted of electropolishing and low temperature baking as they are typically applied to SRF cavities. The magnetization data were fitted using a modified critical state model. The critical current density Jc and pinning force Fp are calculated from the magnetization data and their temperature dependence and field dependence are presented. The LG samples have lower critical current density and pinning force density compared to FG samples which implies a lower flux trapping efficiency. This effect may explain the lower values of residual resistance often observed in LG cavities than FG cavities

An Experimental and Analytical Study of Periodic and Aperiodic Fluctuations in the Critical Current of Long Coated Conductors

It is well known that the critical current of all coated conductors fluctuates along their length and from run to run for reasons that are seldom reported in detail and may not always be understood. Here, we report results obtained with a reel-to-reel transport and magnetization measurement apparatus that allows 77-Kmeasurementswith a resolution of about 20 and 1mm, respectively. Analysis of these data assesses both periodic and random contributions to the critical current fluctuations. We find multiple sources of the fluctuations, and, thus, many different types of behavior. A very positive result is that fluctuations are definitely decreasing in more recent conductors. We show how to reveal fluctuations due to variations of the active cross section of the conductor (i.e., an I-C variation) and to variations of the superconducting properties themselves. We found that the latter dominates in the vast majority of conductors studied