Effects of lateral critical current nonuniformity on stresses in dry-wound high-field REBCO coils

Abstract The distribution of critical current density ( j c ) in rare-earth barium copper oxide (REBCO) coated conductors (CC) affects the magnetization behaviors and subsequently screening-current-induced stresses, particularly for solenoid magnets in high fields. This paper studies numerically the correlation between lateral j c profile across conductor width and stress distribution in pancake coils. The modeling framework considers bending, winding, thermal contraction, and magnetic forces including coupled electromagnetic-mechanical behaviors, i.e. the deviation of the perpendicular field away from axial direction due to tilting deformation. The lateral nonuniformity is introduced using trapezoidal functions, emulating typical j c profiles originated in pristine tapes and those caused by slitting. First, parametric studies are carried out on the small test coils previously reported in the ‘Little Big Coils’ (LBC) paper. It is shown that while slitting edge defects have a moderate impact on peak strains, imperfections in the pristine tape with a larger shoulder width can accelerate the penetration process, shifting peak force to the structurally resilient middle section. Similar behaviors are found in the LBC3 case study, suggesting that lateral j c nonuniformity may have contributed to the observed degradation states in pancakes with different slit-edge orientations. Furthermore, the manipulation of lateral j c profile is proposed as a strategy to manage stresses in high field solenoids. This is demonstrated in the design study of a hypothetical REBCO insert. By adjusting the lateral j c distribution and the overall scaling factor, magnet designs with a reasonable current margin and moderate peak strain can be found. The multi-width concept is then applied to allow for a higher operating current and a larger margin for the end pancakes. Albeit being a generic case, this study highlights the sensitivity of peak stresses in high-field magnets to j c distribution. This feature may be taken into account to fine-tune magnet designs and adjust coil assemblies for better overall performance. It also emphasizes the need for careful characterization and effective control of lateral j c distributions in REBCO CC.