Superconducting magnesium diboride coatings for radio frequency cavities fabricated by hybrid physical-chemical vapor deposition

Abstract

We have investigated the coating of an inner surface of superconducting radio frequency cavities with a magnesium diboride thin film by hybrid physical-chemical vapor deposition (HPCVD). To simulate a 6 GHz rf cavity, a straight stainless steel tube of 1.5-inch inner diameter and a dummy stainless steel cavity were employed, on which small sapphire and metal substrates were mounted at different locations. The ${\mathrm{MgB}}_{2}$ films on these substrates showed uniformly good superconducting properties including ${T}_{c}$ of 37--40 K, residual resistivity ratio of up to 14, and root-mean-square roughness ${R}_{q}$ of 20--30 nm. This work demonstrates the feasibility of coating the interior of cylindrical and curved objects with ${\mathrm{MgB}}_{2}$ by the HPCVD technique, an important step towards superconducting rf cavities with ${\mathrm{MgB}}_{2}$ coating.