Modeling of screening currents in coated conductor magnets containing up   to 40000 turns
  Screening currents caused by varying magnetic fields degrade the homogeneity and stability of the magnetic fields created by $RE$BCO coated conductor coils. They are responsible for the AC loss, which is also important for other power applications containing windings. Since real magnets contain coils exceeding 10000 turns, accurate modeling tools for this number of turns are necessary for magnet design. This article presents a fast numerical method to model coils with no loss of accuracy. We model a 10400-turn coil for its real geometry and coils of up to 40000 turns with the continuous approximation, which we checked that introduces negligible errors. The screening currents, the Screening Current Induced Field (SCIF) and the AC loss is analyzed in detail. The SCIF is maximum at the remnant state with a considerably large value and decreases substantially after one day of relaxation. The instantaneous AC loss for an anisotropic magnetic-field dependent $J_c$ is qualitatively different than for a constant $J_c$, although the loss per cycle is similar. Saturation of the magnetization currents at the end pancakes causes that the maximum AC loss at the first ramp increases with $J_c$. The presented modeling tool can accurately calculate the SCIF and AC loss in practical computing times for coils with any number of turns used in real windings, enabling parameter optimization. 
Enric Pardo
cond-mat.supr-con