Interstrand coupling and magnetization in Nb$_{3}$Sn Rutherford cables

Abstract

It is well known by now that interstrand coupling in NbTi Rutherford cables can be suppressed by separating the strand layers with an “insulating” foil or core, typically of stainless steel (SS). Based on the results of an extensive series of studies of NbTi cables with coated strands and cores, the LASM group and its collaborators went on to measure the coupling losses and associated magnetizations of Nb3Sn-wound cables incorporating various kinds of cores and several core widths. In Nb3Sn cables the relatively large product of critical current density and effective strand diameter (Jc.deff) dictates a relatively large low-field persistent current magnetization. The relationship of this to the coupling magnetization at lowand high fields is discussed. BACKGROUND STUDIES: NB-TI CABLES Since the pioneering work of Wilson and his colleagues at the then Rutherford Laboratory [1][2] the results of AC conductor research and development have provided a vast literature on eddy-current and coupling loss and its suppression in terms of strandand conductor design. Cable-level coupling currents have been a recurrent problem in the design of NbTi accelerator magnet cables both as-wound and fully impregnated with resistive solder [3]. Reduced eddy current losses were obtained when a Cu-10Ni barrier was installed around the filamentary bundle within the strand. Lowest coupling losses occur when the strands are individually insulated, the advantage of which is offset by conductor instability. At both Fermilab (FNAL) and at the former SSC Laboratory strand coatings were favored as way of reducing coupling loss. The influence of coatings on the ramp-rate dependence of loss in Tevatron magnets was discussed by Wake et al. [4]. From just the standpoint of loss full ebonol coating was superior to so-called “zebra coating” (alternate windings of ebonol and stabrite) which in turn was superior to full stabrite coating. Subsequent work addressed the questions of pressure and curing temperature on cable loss. Coupling loss was found to be enhanced if the cables were exposed simultaneously to pressure and heat treatment before measurement [5][6][7]. Taking a cue from these early studies the present authors, now at the Laboratory for Applied Superconductivity and Magnetism (LASM) began a series of investigations into the effect of strand coatings and subsequently cable cores on coupling loss in subsize and full-size Rutherford cables. Initially such variants in cable design were compared and quantified just in terms of “loss per cycle” of an applied AC field. Later the losses were converted to crossover and side-by-side “interstrand contact resistances” (ICR), R⊥ and R|| , respectively, using the following standard network-derived expressions [8] in which Q⊥ represents the “face-on, FO” loss when the applied field is perpendicular to the broad face of the cable and Q⎟⎟ represents the “edge-on, EO” loss