Behaviour Prediction of Closed-loop HTS coils in Non-Uniform AC fields

TL;DR

This paper develops a numerical model to predict the field decay of closed-loop HTS coils in non-uniform AC fields, aiding magnet design and shielding strategies in applications like Maglev trains.

Contribution

It introduces a systematic simulation approach for field decay in HTS coils under AC fields, including analysis of current relaxation and shielding design.

Findings

Simulation aligns well with experimental data

Most decay occurs at coil turns innermost and outermost

Magnetic shielding effectively reduces decay rate

Abstract

Field decay rate is the key characteristic of the superconducting magnets based on closed-loop coils. However, in Maglev trains or rotating machines, closed-loop magnets work in external AC fields and will exhibit an evidently accelerated field decay resulting from dynamic resistances, which are usually much larger than joint resistance. Nevertheless, there has not been a numerical model capable of systematically studying this behaviour, which is the main topic of this work. The field decay curves of a closed-loop high-temperature-superconducting (HTS) coil in various AC fields are simulated based on H-formulation. A non-uniform external field generated by armature coils is considered. Reasonable consistence is found between experimental and simulation results. In our numerical model, the impact of current relaxation, which is a historical challenge, is analysed and subsequently eliminated with acceptable precision. Our simulation results suggest that most proportion of the field decay rate is from the innermost and outermost turns. Based on this observation, a magnetic shielding pattern is designed to reduce the field decay rate efficiently. This work has provided magnet designers an effective method to predict the field decay rate of closed-loop HTS coils in external AC fields, and explore various shielding designs.