Instabilities in transport current measurements of Nb3Sn strands

The critical current of Nb₃Sn strands used in Fermilab´s high field magnets was measured at low and high fields under various experimental conditions using the voltage-current (V-I) and voltage-field (V-H) methods. The strands were produced using Modified Jelly Roll, Restacked Rod Process and Powder-in-Tube technologies. V-I characteristics at fields above 10-12 T with a smooth transition from the superconducting to normal phase allowed determining strand critical current. V-I and V-H measurements at low fields showed premature quenches due to magnetic instability in strands with high critical current density and large effective filament size. The results of the strand critical current measurements at low and high fields are herein presented.