Characteristics of Trapped Field Magnet Activation by Flux Pumping

Currents are set up in high temperature superconducting (HTS) material when it is cooled in a magnetic field (called field-cooling: FC), or exposed to a magnetic field after cooling (called zero-field-cooling: ZFC). The HTS material can be treated to make these currents persistent, thereby forming “trapped field magnets” (TFMs). TFMs have been improved over the past two decades by the efforts of a large number of worldwide research groups. However, applications using TFMs have lagged, in part due to the problem of high fields needed for activation. We describe herein experiments to characterize the behavior of TFM activation using repeated applications of permanent ferromagnets (called “pumping”). Significant partial activation is obtained using a non-uniform pumping field which is higher in the center of the HTS than at the periphery. ZFC, followed by pumping with such a field, results in trapping the full applied field, instead of half the applied field, as is trapped by zero-field-cooling followed by application of a uniform field. We also find that for FC partial activation, and subsequent additional activation by pumping, the resulting fields are additive. Additionally, we conclude that one contributing, positive component of activation by fluxoid pumping is the phenomenon of creep.