  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.
