Aluminum Strand Coating for Increasing the Interstrand Contact Resistance in Rutherford Type Superconducting Cables

The interstrand contact resistance (R_c) in Rutherford type cables for fast cycling superconducting magnets must be sufficiently high in order to limit eddy current losses. The required value for R_c depends on the cable and magnet geometries and on the foreseen cycling rate, but is typically of the order of one mOmega. Such values can be reached with a dedicated strand coating or with a resistive internal cable barrier. As a possible candidate Al strand coatings have been tested. For a Rutherford type inner conductor cable of the Large Hadron Collider (LHC) made of Al coated strands i?c values higher than 500 muOmega are achieved. The native Al₂O₃ oxide layer formed at ambient temperature in air is sufficient to reach this high contact resistance. A 6 h-200degC oxidation heat treatment in air with 100% relative humidity further increases R_C to values above 600 muOmega. Due to the high thermal and mechanical stability of Al₂O₃ only a relatively moderate R_c drop of about 40% is obtained during a 190degC heat treatment under 50 MPa pressure (the so-called curing cycle of the coil insulation) subsequent to the 6 h-200degC oxidation heat treatment.