MgB$_{2}$ thin films for SRF application

Superconducting RF (SRF) cavities are usually fabricated from bulk niobium (Nb), a material which is thoroughly studied and approaching its limits. Magnesium diboride (MgB$_{2})$ has a higher $T_{c}$ of 39 K, a lower residual resistivity of \textless 0.1 $\mu\Omega$ cm (at 42 K), and a higher thermodynamic critical field $H_{c}$ value comparing with Nb. These properties imply that a MgB$_{2}$-coated SRF cavity would work at a higher temperature with a lower energy dissipation. 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 a low RF breakdown field. In this work, we report an enhancement of $H_{c1}$ in both c-axis oriented epitaxial and polycrystalline MgB$_{2}$ thin films. The value of $H_{c1}$ (5 K) was increased from 60 mT in a 300 nm-thick MgB$_{2}$ film to 180 mT when the MgB$_{2}$ layer thickness was 100 nm in both cases. The microwave properties of the MgB$_{2}$ films were characterized as well by using the dielectric resonator technique.