Microwave Losses in a DC Magnetic Field in Superconducting Cavities for Axion Studies

The hypothetical axion particle is an excellent cold dark matter candidate. Models predict an axion mass of tens of μeV (GHz frequency range). In principle, it is possible to convert galactic axions into a monochromatic microwave signal or to detect them with an electron-spin flip. In both cases, to measure the resulting vanishing signal (P ≈ 10 -23 W), a high-Q cavity is needed, with Q ~ 10 6 . Superconducting RF technology can satisfy the requirement of operation in moderate-to-high dc magnetic fields. In this paper, we present an experimental study of the behavior of superconducting cavities in the vortex state. This study has been realized using 14 GHz Cu-NbTi and Cu-NbTiNx film cavities. A new cavity geometry is presented. We measured the magnetic field dependence of the Q-factor and Δf res /f res , where f res is the resonant frequency in the range B = 0-6 T at 4.2 K. Also, Q versus temperature at fixed B in zero-field-cooling and field-cooling setups has been considered. A comparison was made with Cu and Nb cylindrical bulk cavities. The data for Q(B) were analyzed in terms of the elastic and dissipative motion of flux lines, thus including flux flow and flux pinning. We discuss whether the field-induced quasiparticle contribution is relevant in our field and temperature range.