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

The interstrand contact resistance (R-c) between crossing strands in Cable- In-Conduit Conductors (CICC's) determines the coupling loss and the stabili ty 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 i n the strand and cable production, the plating procedure determining the cr ystalline structure and by the heat treatment. A new process of making a mo re stable oxide has been developed and characterised. The Cr coating is act ually build up out of two different layers. The first layer is a hard Cr co ating, identical to the standard Cr layer previously used. On top of this a layer of black Cr oxide is deposited electrolitically. The coupling loss t ime constant and R-c are measured on a 48-strands CIC Conductor with this d ouble-coated strand material. The void fraction amounts to 36 % and the str and, cabling and jacketing are identical to those used in the previous chro me vendor comparison action. The results, presented in terms of R-c, time constant nz, and the atomic co ncentration of oxygen (acO) in the peripheral region of the strand, are com pared to previous results from single coated strands.