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 7 x 10-3 and 5 x 10-3, respectively.