REBa_(2)Cu_(3)O_(7−x)(REBCO)coated conductors,owing to its high tensile strength and current‐carrying ability in a background field,are widely regarded a promising candidate in high‐field applications.Despite the great potentials,recent studies have highlighted the challenges posed by screening currents,which are featured by a highly nonuniform current distribution in the superconducting layer.In this paper,we report a comprehensive study on the behaviors of screening currents in a compact REBCO coil,specifically the screeningcurrent‐induced magnetic fields and strains.Experiments were carried out in the self‐generated magnetic field and a background field,respectively.In the self‐field condition,the full hysteresis of the magnetic field was obtained by applying current sweeps with repeatedly reversed polarity,as the nominal center field reached 9.17 T with a maximum peak current of 350 A.In a background field of 23.15 T,the insert coil generated a center field of 4.17 T with an applied current of 170 A.Ultimately,a total center field of 32.58 T was achieved before quench.Both the sequential model and the coupled model considering the perpendicular field modification due to conductor deformation are applied.The comparative study shows that,for this coil,the electromagnetic–mechanical coupling plays a trivial role in self‐field conditions up to 9 T.In contrast,with a high axial field dominated by the background field,the coupling effect has a stronger influence on the predicted current and strain distributions.Further discussions regarding the role of background field on the strains in the insert suggest potential design strategies to maximize the total center field.
REBa_(2)Cu_(3)O_(7−δ)(REBCO,RE=rare earth)tapes doped with 5%and 15%Zr have been scaled up to lengths more than 40 m in a pilot‐scale advanced metal organic chemical vapor deposition(A‐MOCVD)tool.The precursor compositions used for the long tapes were guided by a study of the influence of(Ba+dopant)/Cu content on the critical current density(Jc)of 5 and 15 mol.%Hf‐and Zr‐added tapes at 4.2 K and 13 T.The 40‐mlong tapes exhibited a critical current(Ic)over 4,000 A/12 mm at 4.2 K and 13 T as well as over 1,400 A/12 mm at 20 K and 20 T.The critical current densities of a 40‐m‐long tape doped with 5%Zr at 4.2 K measured at the National High Magnetic Field Laboratory(NHMFL)were>10 MAcm^(−2)and>5 MAcm^(−2)at 14 T and 30 T,respectively,which are over three times those of commercial REBCO tapes.The infield Jc of 5%Zradded 40‐m‐long tapes was similar to those of previously‐reported high‐performance short samples made with 15%Zr or Hf.These results demonstrate the excellent potential of A‐MOCVD for manufacturing high Ic REBCO tapes for use in ultrahigh‐field magnet applications.
