Winding strain analysis for YBCO coated conductors

An analysis is made of the winding strains for YBCO coated conductors in solenoid coils. Expressions are developed for the bend strains that occur in pancake and layer wound coils as a function of the relevant parameters. The bend strain for the winding of radial pancakes or helical layers follows directly from the bending analysis of thin beams. In the case that there is a radial step in the windings, as might occur at a joint, a simple model for the step is introduced and an expression for the associated bend strain is derived. For layer winding, two patterns of layer end turn are suggested to meet the design objectives and to avoid radial build interference. Expressions for the wide direction bend strain are derived. The reported strain tolerance of the critical current of YBCO is reviewed to quantify acceptable limits for the bend strain. Bend strains are computed for various configurations of pancake winding and layer winding using the derived expressions and compared to the measured irreversible strain. At large winding diameters, bend strains are generally low but are shown to increase and become a design constraint at small diameters. Locations for joints are identified in the end turn regions of layer winding patterns. Depending on the diameter of the windings and the space available for the end turn, it is shown that the presence of a joint can greatly increase the bend strain of the end turn.