Fabrication and Radio Frequency Properties of 3-GHz SRF Cavities Coated with MgB 2

Magnesium diboride (MgB2) is considered a potential material for superconducting radio frequency cavities. MgB2-coated Cu cavity will allow for a higher operational temperature than a bulk Nb cavity because of the higher transition temperature of MgB2 and the high thermal conductivity of Cu. Using the hybrid physical chemical vapor deposition technique, MgB2 coatings were successfully achieved on the inner wall of 3-GHz Cu cavities. The surface and superconducting properties of the coatings were characterized using small samples on Cu plugs mounted at different locations of mock cavities. RF measurement of a MgB2-coated single-cell 3-GHz test cavity was carried out, and it showed superconducting transition at 36 K. The quality factor of this test cavity was lower than expected due to poor connectivity and inclusion of Mg–Cu alloy in the MgB2 coating.