Aluminum strand coating for increasing the interstrand contact resistance in Rutherford type superconducting cables

The interstrand contact resistance (Rc) in Rutherford type cables for fast cycling superconducting magnets must be sufficiently high in order to limit eddy current losses. The required value for Rc depends on the cable and magnet geometries and on the foreseen cycling rate, but is typically of the order of one mW. 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 Rc values higher than 500 Omega are achieved. The native Al2O3 oxide layer formed at ambient temperature in air is sufficient to reach this high contact resistance. A 6 h-200 °C oxidation heat treatment in air with 100% relative humidity further increases Rc to values above 600 μOmega . Due to the high thermal and mechanical stability of Al2O3 only a relatively moderate Rc drop of about 40 % is obtained during a 190 °C heat treatment under 50 MPa pressure (the so-called curing cycle of the coil insulation) subsequent to the 6 h-200 °C oxidation heat treatment.