Flux penetration and surface losses in niobium

Measurements made on cylindrical niobium samples with test coils in ac magnetic fields of 290 Hz frequency have shown that the superconducting penetration depth λ can be expressed as
\lambda = \lambda_{0} + aH^{2} + bH^{4} + ...
, where λ
0
is the penetration depth at zero field and
H
is the surface field. In the region of
H = 800
to 1300 gauss, λ can be approximated as
\lambda = \lambda_{0}bH^{4}
, where
\lambda_{0} = 580
Å,
b = 8.3 \times 10^{-12}
Å/G
⁴
. The voltage waveforms from the test coils displayed on an oscilloscope showed good agreement with predicted curves based on this penetration depth. From the voltage signals surface losses could be determined. Loss measurements made on cylinders of niobium with cold worked surfaces gave surprising results. The measured losses were low if the sample was cooled to the measuring temperature quickly, but the losses could be several hundred times higher if the cooling rate was slow. After the sample was annealed, the losses were low and independent of the cooling rate. Data is also given for loss measurements at various temperatures.