Relaxation Properties of the Trapped Flux of Bulk High-Temperature Superconductors at Different Magnetization Levels

Abstract

The magnetic relaxation phenomenon is a crucial subject for the engineering applications of bulk hightemperature superconductors (HTS) in which the trapped field or levitation force behaves with a time-dependent decay due to the intrinsic flux creep inside the HTS materials. To fully exploit the high trapped field of bulk HTS, we have experimentally investigated the trapped flux relaxation properties, especially at different magnetization levels. With different excitation fields, the dependence between trapped field and relaxation rate was analyzed and compared in both field-cooling magnetization (FCM) and zero-fieldcooling magnetization (ZFCM) conditions. In parallel, the relaxation rates for different trapped flux can be measured all at once during a single magnetization process. The relaxation rate is closely correlated to the trapped field and useful to reflect the effect of the following remagnetization processes. To suppress the relaxation, we further checked possible methods from the three aspects of material improvement, working temperature and flux annealing effect.

A trapped field of 17.7 T in a stack of high temperature superconducting tape

“…The creep of the central trapped field was very low for 15 and 8 K (less than 0.1% decay after 30 min), however the creep of the field at the edges of the sample is higher as illustrated by figure 8(b) as flux first begins to leave the sample from the outer edges. Such flux creep is not a concern for applications because the decay in-field is logarithmic and can be effectively eliminated by lowering the sample temperature a few Kelvin below the magnetization temperature [20,21]. The data for the previous bulk experiment [1] is also plotted for comparison and shows a creep rate between the 15 and 30 K as expected given the bulk temperature was in between the two at 26 K. A more detailed comparison would require using the same temperatures, but creep rates for bulks and stacks can broadly be considered similar.…”

Flux creep after field trapping in YBa2Cu3O x foams

“…All experiments were repeated at least twice; for t=5 min and 15 min, three runs were performed in order to check the reproducibility and accuracy. These experimental problems are also a reason why such experiments have not often been performed in the literaturethe time-dependence of the local fields are very often investigated only via simulations [24][25][26][27][28].…”

Early stages of magnetization relaxation in superconductors

“…Vortex structures in conventional and high-temperature superconductors display remarkable complexity both in equilibrium 1,2 and dynamic regimes. [3][4][5][6][7][8][9][10][11][12] Relaxation of the magnetic moment of superconductors is achieved through initial viscous flux flow [13][14][15][16][17][18] and slow, logarithmic in time, thermally activated creep. [19][20][21][22][23] Thermally-assisted hopping of vortices and vortex bundles between local minima in the random pinning potential is characteristic of both the creep and the flux flow under a driving force.…”