Heat Treatment Studies of Nb$_3$Sn RRP wires for Superconducting Planar Undulators

A project aiming to fabricate a full-length 2.8-m long Nb3Sn superconducting undulator for the storage ring was started last year at the ANL APS. These Nb3Sn undulators operate at a maximum magnetic field on the conductor of about 5 T. To address instabilities at this field, two Nb3Sn wires with small subelement size were used. Specifically, Restacked Rod Processed wires of 0.6 mm in diameter and with 144 and 150 superconducting subelements respectively, over 169 total. The equivalent subelement diameter, DS , of these wires is ∼35 μm. At these small DS values, the critical current density is known to deteriorate, and the Residual Resistivity Ratio is very sensitive to heat treatment. A delicate balance has therefore to be found to obtain parameters within operation specifications. In this paper we show performance results from different heat treatments.