Characterization of dissipative regions of a N-doped superconducting radio-frequency cavity

Abstract

We report radio-frequency measurements of quality factors and temperature mapping of a nitrogen doped Nb superconducting RF cavity. Cavity cutouts of hot and cold spots were studied with low temperature scanning tunneling microscopy and spectroscopy, X-ray photoelectron spectroscopy and secondary electron microscopy. Temperature mapping revealed a substantial reduction of the residual resistance upon cooling the cavity with a greater temperature gradient and hysteretic losses at the quench location, pointing to trapped vortices as the dominant source of residual surface resistance. Analysis of the tunneling spectra in the framework of a proximity effect theory shows that hot spots have a reduced pair potential and a wider distribution of the contact resistance between the Nb and the top Nb oxide. Alone, these degraded superconducting properties account for a much weaker excess dissipation as compared with the vortex contribution. Based on the correlation between the quasiparticle density of states and temperature mapping, we suggest that degraded superconducting properties may facilitate vortex nucleation or settling of trapped flux during cooling the cavity through the critical temperature.

Oxide dissolution and oxygen diffusion scenarios in niobium and implications on the Bean–Livingston barrier in superconducting cavities

“…It is known that severe plastic deformation occurs in Nb affected by Q-disease, which may facilitate Bean-Livingston breakdown well below its theoretical maximum 99,100 topographically [101][102][103][104][105][106] or facilitate vortex entry through weakened superconductivity or proximity effect. 107 We suggest that nano-hydride precipitation may be more involved in setting E q0 ðB p0 Þ, while the extension of E q ðB p Þ with lowtemperature vacuum annealing is related to prevention of vortex entry of the imperfect surface. Recent calculations show that vortex nucleation and dissipation through hydrides may be especially lossy and bring about high-field Q-slope.…”

Magneto-thermal limitations in superconducting cavities at high radio-frequency fields

“…The change in the quench locations observed during tests with T-mapping and the hysteretic behavior of ΔT(B p ) at the quench locations suggest that trapped vortices may contribute to the quench. On the other hand, cavities may quench with P c < 10 W and ΔT < 10 mK at the breakdown field (Lechner et al, 2023). Quenching of such cavities likely has a magnetic field origin related to the behavior of the nonlinear BCS surface resistance at high rf fields.…”