Measurements of flux pumping activation of trapped field magnets

Abstract

Large grains of high temperature superconducting (HTS) material can be utilized as trapped field magnets (TFMs). Persistent currents are set up in the HTS when it is cooled in a magnetic field, or exposed to a magnetic field after cooling. 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 designed to observe the behaviour of TFM activation using repeated applications of low fields (called 'pumping'). Significant partial activation is obtained using a non-uniform pumping field (e.g., a small permanent magnet) which is higher in the centre of the HTS than at the periphery. Cooling in zero field followed by pumping with such a field results in trapping the full applied field, in comparison to half of the applied field being trapped by cooling in zero field followed by application of a uniform field. We find that for partial activation by cooling in a field and subsequent activation by pumping, the resulting fields are additive. We also conclude that for activation by fluxoid pumping, creep assists the process.

A study of pulsed activation of trapped field magnets: effects of multiple pulsing

“…This remarkable behaviour has been observed previously [6]. The data in the present experiment confirms the log N behaviour over a wide range of I EM and B T (r) not accessible in [6].…”

“…This approximation will be amended in the region of core saturation. This distribution of applied field meets the aim of the earlier flux pumping experiment [6], i.e., that most of the applied field distribution is inside a ring of HTS. Such localized field distributions have been found, experimentally and theoretically, to be advantageous in pulsed activation [6,9,12].…”

“…Activation studies have been in progress for several years. In 2010 an experiment [6] and analysis [7] on activation of TFMs by means of flux pumping was performed. Activation was accomplished by repeated passage of a small permanent dipole magnet (PM) through a hole in the centre of a 2 cm diameter YBCO superconductor.…”

“…The present series of reports is an extension of that experiment, in which the PM has been replaced by a wire-wound split-field electromagnet (EM) with iron cores. The EM, including iron cores and wires, has a smaller diameter than that of the TFM, in order to maintain a spatially limited applied field, as in [6]. The split-coil EM permits experiments with a much larger variety of controlled variables than did the reciprocating PM.…”

A novel approach for x-ray scattering experiments in magnetic fields utilizing trapped flux in type-II superconductors

“…While in situ use of pulsed magnetic fields and flux pumping are desirable, at present these techniques remain an active field of research on their own right. [21][22][23][24] Commonly, an external resistive or superconducting magnet is used with a TFM cooled either in an applied field (field cooled, FC), or in zero field with subsequent application of fields at low temperatures (zero-field cooled, ZFC). While we are able to employ a ZFC method for activating a TFM using our setup described below, there is a significant drawback: applied field needs to be at least twice the maximum field that a TFM can trap in order to guarantee full flux penetration into the bulk.…”