Heat treatment optimization of differently alloyed Nb3Sn superconductors

For an upgrade of our HOMER II magnet facility from 20 T to 24 T one possibility is to build insert coils made of alloyed Nb₃Sn wires. These wires require a heat treatment (HT) to form the superconducting phase by solid state diffusion. The temperature and duration of the HT determines the size of the superconducting layers and the grain structure within these layers and hence physical properties like critical current and n-value. To optimize the HT with regards to the planned operation at very high magnetic fields, two differently alloyed conductors were heat treated between 600°C and 750°C for 50 h up to 350 h. Critical currents and n-values are determined by E(I)-measurements in magnetic fields up to 15 T. Special attention is drawn to the upper critical field and the maximum pinning force, which allow, in principle, an extrapolation of the measured data to higher magnetic fields. The results show differences of the optimum HT for different magnetic background fields, which will be discussed.