Development of a superconducting transverse holding magnet for the HIγS frozen spin target

We describe the design, construction, and performance of a set of saddle coils used to maintain the spin of a polarized proton target in the transverse direction with respect to the incident gamma-ray beam direction. The transverse coil assembly consists of two racetrack shaped coils formed from a single strand of thin NbTi superconducting wire. Four layers of NbTi per saddle coil were wet wound in a racetrack shape and then installed on a cylindrical support tube and epoxyed to prevent them from moving when the coils were energized. As expected from our model calculation, a set of two saddle coils produced 0.36 T in the middle of the coils with a 25 A current at 4 K, which is 63% of the critical current of the wire. The measured homogeneity of the field over the target volume has a maximum variation from the average value of 0.6%.