Fine filament NbTi and Nb3Sn conductors

Fine filament conductors are needed for applications requiring low hysteresis losses and/or low magnetization in small fields in order to reduce field distortions. Examples are accelerator magnets (dipoles and quadrupoles) and magnets for TOKAMAK nuclear fusion devices. The main task is to achieve low magnetization especially at low fields while maintaining high critical current density jC at high fields. The relevant parameters are the width 2ΔM of the magnetization curve and the area of the M-H-curve during a field cycle, respectively. In this work, NbTi and Nb3Sn fine filament conductors were produced on a large scale and characterized with respect to transport critical current density and magnetization behavior. In NbTi jc values of about 3000 A/mm2 at 5 T and 4.2 K were achieved in 5 μm filament diameter single stack type conductors. Deterioration of the order of 15 to 20 % was observed when using double stack techniques and/or when going to 2.5 μm type conductors. Possible reasons were identified and work is in progress to improve jc further. At external fields smaller than 100 mT proximity effect induced excess magnetization was observed in Cu matrix conductors and its dependence on filament spacing, twist pitch, field sweep rate, and temperature was measured. The effect is significant, but plays a role only in applications where this low field regime is relevant