For space and cost reasons, the Nb3Sn accelerator magnets for FCC should be graded. The joint between the two conductor grades should be conveniently done before the heat treatment (wind-and-react joints) for a compact design, without fragile protrusions from the winding packs. In collaboration between CERN and the Swiss Plasma Center (SPC) an R&D program is carried out to develop reliable, low resistance W&R joints and test them in the SULTAN test facility.
The geometry of the joint is a 100 mm long overlap of cables with the same width. After an initial review of applicable techniques, two methods are retained in the first phase of the R&D: soldered and diffusion bonded joints. Various options and parameters are implemented for the two methods, including preliminary electroplating, inter-leaved copper foil and variable applied pressure.
Screening tests for joint specimens are carried out at 4.2 K in the background field of a 15 T solenoid with operating current up to 1 kA. Four series connected specimens are assembled on a dedicated sample holder.
A more relevant test is carried out in the SULTAN test facility for the most promising joint specimens in the background of 11 T, at the nominal operating current of 18 kA and at variable operating temperature to achieve the nominal distance from the critical surface.
The progress of the R&D and the test results are discussed with an outlook in next phase.