Time dependence of magnetic flux penetration in Nb and Nb3Sn

The authors have investigated a single crystal sample of Nb3Sn at low frequencies, 100 Hz to 13 kHz, and found a value of Bc1 = (45.6 + or - 2.7) mT for the lower critical field extrapolated to T = 0 K at 497 Hz. The critical field, as we have defined here, was observed to increase by 32 + or - 5% in going from 0.5 to 13 kHz. Hysteretic energy loss was used to determine the critical fields and was observed to decrease rapidly with frequency, dropping a factor of ten between 0.5 and 13 kHz. Samples of Nb wire were pulsed with field rise times of 2 μsec and found to have a T = 0 K value of Bc1 = 158 + or - 8 mT, unchanged from lower frequency values. The Nb3Sn sample had undetectable signal levels under similar pulse conditions (up to 150 mT maximum field). Temperature dependence of the critical fields is consistent with 1-(T/T /SUB c/ )2 behavior from 2 K to T /SUB c/ (18.0 + or - 0.1 K for the Nb3Sn sample)