Hybrid Physical Chemical Vapor Deposition Of Magnesium Diboride Inside 3.9 Ghz Mock Cavities

Magnesium diboride (MgB2) 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-s(BCS)) 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 MgB2 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. MgB2 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 MgB2 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 MgB2 coatings for SRF cavities.