Development of a MODEL superconducting helical undulator for the ILC positron source

The helical undulator for the proposed International Linear Collider (ILC) positron source requires high-permeability steel poles and superconducting coils to meet the ILC parameters. A short-model undulator with a period of 14 mm was designed, fabricated including high-permeability steel poles, and tested in LHe. The ends of the model were designed to wind the Nb₃Sn double helix without any conductor joints. After a few quenches in the first excitation test, the current density in the coil reached 1.28 kA/mm², which was approximately 90% of the estimated short-sample critical current density. The periodic on-axis fields were mapped at two azimuth angles. Excluding the end fields, the standard deviation of the field amplitudes and higher harmonic coefficients for the periodic field were less than 7times10^-3 and 5times10^-3, respectively.