Hybrid Physical Chemical Vapor Deposition of Magnesium Diboride Inside 3.9 GHz Mock Cavities

Magnesium diboride (MgB
₂
) is considered a candidate for the next generation superconducting radio frequency (SRF) cavities due to its higher critical temperature T
_c
(40 K) and increased superheating field (H
_sh
) compared to other conventional superconductors. These properties can lead to reduced BCS surface resistance (R
^BCS
_S
) and residual resistance (R
_res
), according to theoretical studies, and enhanced accelerating field (E
_acc
) values. We have investigated the possibility of coating the inner surface of a 3.9 GHz SRF cavity with MgB
₂
by using a hybrid physical-vapor deposition (HPCVD) system designed for this purpose. To simulate the actual 3.9 GHz SRF cavity, we employed a stainless steel mock cavity for the study. The film qualities were characterized on small substrates that were placed at the selected positions within the cavity. MgB
₂
films on stainless steel foils, niobium pieces, and SiC substrates showed transition temperatures in the range of 30-38 K with a c-axis-oriented crystallinity observed for films grown on SiC substrates. Dielectric resonator measurements at 18 GHz resulted in a quality factor of over 30 000 for the MgB
₂
film grown on a SiC substrate. By employing the HPCVD technique, a uniform film was achieved across the cavity interior, demonstrating the feasibility of HPCVD for MgB
₂
coatings for SRF cavities.