An approach to improve the tension constancy of a TF coil shape

•The analytical formula of trapezoidal prism current source is used to accurately calculate the toroidal field of a TF coil system.•Basing on accurate calculation of the toroidal fields, an Optimized-D coil shape with improved tension constancy is obtained through a semi-analytical iteration process.•The effect of varying the number of TF coils N and radial coordinates r2 of outer leg on the height and length of the TF coil centerline is discussed.