Enhancement of lower critical field in thin MgB$_{2}$ films and MgB$_{2}$/MgO multilayers

Magnesium diboride is a conventional superconductor with a high $T_{c}$ of 39 K, a low residual resistivity of \textless\ 0.1 $\mu \Omega $cm (at 42 K), and higher thermodynamic critical field $H_{c}$ values than Nb. These properties make MgB$_{2}$ a promising superconductor as an alternative to Nb for future SRF cavities. However, the lower critical field $H_{c1}$ of MgB$_{2}$ is low, and vortex dissipation above $H_{c1}$ can lead to degradation of the quality factor and low RF breakdown field. Here, we report an enhancement of $H_{c1}$ in thin MgB$_{2}$ films and MgB$_{2}$/MgO multilayers. The value of $H_{c1}$(5K) is increased from 40 mT in a 300 nm-thick MgB$_{2}$ film to 180 mT when the MgB$_{2}$ layer thickness is 100 nm either in a single-layer film or in a MgB$_{2}$/MgO multilayer with a total MgB$_{2}$ layer thickness of 300 nm. Superconducting MgB$_{2}$ thin films have been coated \textit{in-situ }on the inner wall of a SRF cavity using the hybrid physical chemical vapor deposition (HPCVD) technique. The characterization of the coating will be presented.