Speaker
Description
For the high luminosity upgrade of the large hadron collider the US accelerator research program is developing the quadrupole magnet MQXF wound with a 40 strand (N = 40) Nb3Sn Rutherford cable QXF. During the field ramping of accelerator magnets interstrand coupling currents flow through the crossover- and adjacent-strand contact resistances represented by Rc and Ra, respectively, and a combination of them by Reff. In order to control Rc during reaction heat treatment (RHT) thin stainless steel cores of specified widths (core cover, W%) are often included in Nb3Sn Rutherford cables. Cables with cores of various widths were prepared and reaction heat treated (RHT) under various conditions. In earlier studies by our group the cables were RHT under face-on uniaxial pressure of 20 MPa and vacuum impregnated with resin under 5 MPa. The close crossover contact produced an uncored average Rc of 0.26 μΩ. But with increasing core width Reff(Rc,Ra) increased rapidly reaching, for example, more than 240 μΩ at W ~ 90%. Reff versus W% results for QXF cable stacks agreed with prediction based on the fortran code CUDI(c) assuming Rc = 0.26 μΩ and 1000 μΩ for the uncored and cored contact, respectively. More recently cable RHT and vacuum impregnation in the absence of mechanical constraint has been recommended; the magnet winding or test cable stack is placed in a closed channel just large enough to contain it during RHT when small known expansions in both width and thickness occur. Reff versus W% results for QXF cable stacks treated in this way are compared with the earlier experimental results and the fortran prediction. It was deduced that crossover contact was absent (Rc essentially infinite) and that Reff which did not vary much with W% depended entirely on Ra (of order 20 nΩ) following the relationship Reff = (N3/20)Ra.