Winding strain analysis for YBCO coated conductors

Abstract

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 coilsas a function of the relevant parameters. The bend strain for the winding of radial pancakesor helical layers follows directly from the bending analysis of thin beams. In the case thatthere is a radial step in the windings, as might occur at a joint, a simple modelfor the step is introduced and an expression for the associated bend strain isderived. For layer winding, two patterns of layer end turn are suggested to meet thedesign objectives and to avoid radial build interference. Expressions for the widedirection bend strain are derived. The reported strain tolerance of the critical currentof YBCO is reviewed to quantify acceptable limits for the bend strain. Bendstrains are computed for various configurations of pancake winding and layerwinding using the derived expressions and compared to the measured irreversiblestrain. At large winding diameters, bend strains are generally low but are shown toincrease and become a design constraint at small diameters. Locations for joints areidentified in the end turn regions of layer winding patterns. Depending on thediameter of the windings and the space available for the end turn, it is shownthat the presence of a joint can greatly increase the bend strain of the end turn.