Interstrand contact resistance and AC loss of a 48-strands Nb₃Sn CIC conductor with a Cr/Cr-oxide coating

The interstrand contact resistance (Rc) between crossing strands in Cable-In-Conduit Conductors (CICC's) determines the coupling loss and the stability against local disturbances. The surface oxidation, surface roughness and micro-scale sliding of the contact surfaces are key parameters in the R_c. The level of surface oxidation is influenced by manufacturing parameters in the strand and cable production, the plating procedure determining the crystalline structure and by the heat treatment. A new process of making a more stable oxide has been developed and characterised. The Cr coating is actually build up out of two different layers. The first layer is a hard Cr coating, identical to the standard Cr layer previously used. On top of this a layer of blade Cr oxide is deposited electrolytically. The coupling loss time constant and R_c are measured on a 48-strands CIC Conductor with this double-coated strand material. The void fraction amounts to 36% and the strand, cabling and jacketing are identical to those used in the previous chrome vendor comparison action. The results, presented in terms of R_c, time constant nτ, and the atomic concentration of oxygen (acO) in the peripheral region of the strand, are compared to previous results from single coated strands