Development of Ultra-Low-Resistance Splicing of Nb3Sn and NbTi Superconducting Wires

During the last decade, Budker Institute of Nuclear Physics (BINP) manufactured nine multipole superconducting wigglers on the basis of NbTi superconductor. These wigglers are operating at various synchrotron radiation centers worldwide. High magnet parameters were achieved due to using NbTi wire of high NbTi/Cu ratio, up to a factor of two, and by grading the current density in the coils. So, further increase in the magnet parameters is limited by the properties of NbTi conductor. Another important feature of the BINP wigglers is the magnet design, with superconducting coils manufactured separately and connected with a splicing resistance of well below 0.1 nΩ at an operating current of about 1 kA. That yields significant decrease in the heat load on cryocoolers because the total amount of splicings in one wiggler exceeds 200. The existing demand in superconducting wigglers and undulators for higher magnet parameters can be realized by using Nb
₃
Sn wire. The ultimate aim of the on-going work at BINP is to make an Nb
₃
Sn superconducting wiggler with separate coils, the terminals of which will be connected by NbTi wires with low splicing resistance, below 0.1 nΩ. During assembling of such a wiggler, these coils will be connected via NbTi terminals by the technology available at BINP. This article reports on first results of testing five samples of Nb
₃
Sn-NbTi wire splicing. The experimental setup is described. The samples are two-turn loops made of Nb
₃
Sn and NbTi wires. The samples were charged by currents of up to 500 A, and the resistance of these superconducting wire connections was evaluated from the current decay using a Hall sensor. All samples have demonstrated very low resistance, estimated as several Ohm.