JLab, Newport News, Virginia, USA
A quality factor as high as 2 × 1010 at 1.5 GHz was achieved at a gradient of 35 MV/m by 800 °C annealing and N-infusion at 140 °C. A comparison of the field dependence of the surface resistance after N-infusion with a recent theoretical model that extends the calculation of the BCS surface resistance to high rf fields suggests an increase in the quasiparticles¿ relaxation time with increasing infusion temperature, which could be due to a decreasing density of subgap states. Nb coupons treated similarly showed the formation of thicker oxynitride layer on the surface beneath thin dielectric Nb2O5 layer. A plausible explanation for the improved Q0 is that the oxynitride layer on the Nb surface adds additional electron scattering within RF penetration depth.
JLab, Newport News, Virginia, USA
We presented the RF measurement on a 1.3 GHz single cell cavity fabricated at KEK using large grain ingot niobium with RRR=107. The cavity reached to 35 MV/m with Q0 = 2.0×10zEhNZeHn at 2.0 K, record performance on the cavity made from medium purity ingot niobium. The cavity was cool down with different temperature gradient along the cavity axis in order to understand the flux expulsion mechanism when the cavity does through the superconducting transition and effect of trap residual magnetic field on the residual resistance. The measurement showed the excellent flux expulsion with the flux trapping sensitivity of 0.29 nΩ/mG for electro polished surface and 0.44 nΩ/mG for cavity followed by low temperature baking at 120°C for 12 hours.
We acknowledge KEK for sending this cavity for evaluations.