The effect of magnetic field on the superconducting cavity

The experiments to study the effect of external magnetic field on surface-resistance of niobium superconducting cavity were continued since the first weak-magnetic field experiment explored in the 0 to 140 mOe range. In this weak-magnetic field experiment, the external H-field was applied at non-superconducting state, and the noticeable effect was observed on the surface-resistance; 0.43 nΩ increasing per 1 mOe increasing. It is shown that the amount of flux trapping at phase-transition will be almost 100% even for much higher external field. The experiment at strong H-field (comparable or less than Hc1) were also started to see the effect by the external H-field that was applied at superconducting state. At this moment, no effect on the surface-resistance were observed up to 450 Oe. It is pointed out that a possibility of starting of magnetic-flux penetration at a seeable below the Hc1 external field, owing to the magnetic property of niobium (type II/1 superconductor) and owing to the cavity shape (demagnetization coefficient). To analyse the results, several parameters of niobium are estimated; Hc1=1700, Hc2=2500 Oe and κ=0.85 at T=0 K. (author)