MgB$_{2}$ Coated Ellipsoids as an Approach to Investigate the Possible Enhancement of the Vortex Penetrating Field of SRF Cavities

Superconducting rf (SRF) cavities fabricated from bulk niobium (Nb) are a key component for modern particle accelerators. The magnetic field distribution on the inner wall of an SRF cavity is inversely similar to the field distribution on top of a superconducting ellipsoid when we put it in a magnetic field parallel to its axis. By measuring the vortex penetration into the magnetized superconducting ellipsoids, we can deduct the behavior of SRF cavities. Magnesium diboride (MgB$_{2})$ has potential to replace Nb as it has a higher $T_{c}$ of 39 K, a lower residual resistivity of $\sim$ 0.1 $\mu\Omega$ cm (at 42 K), and a higher thermodynamic critical field $H_{c}$ value compared to Nb. In this work, we successfully coated uniform MgB$_{2}$ layers on top of molybdenum and niobium ellipsoids. SQUID magnetometer measurements showed that the coated MgB$_{2}$ layer has a $T_{\mathrm{c}}$ above 38.5 K, and can provide a perfect magnetic shielding up to $\sim$ 500 Oe at 1.8K. By coating MgB$_{2}$ on Nb ellipsoids, we increased the vortex penetration field (the maximum field at which a cavity can be operated) by $\sim$ 500 Oe at 2 K.