Electrodeposition and characterisation of lead tin superconducting films for application in heavy ion booster

• PbSn alloy is investigated as a material for superconducting low and medium velocity rf resonators.
• It is easily electrodeposited with MSA chemistry at very low cost, has high Tc, and good performance at high fields.
• The optimum substrate preparation and coating conditions are established based on examination of the properties of substrate and superconducting films.
• A long term stability of the electroplated resonators has been showing no evidence of degradation of the rf properties over the last decade.
• The resonators high field performance limiting factors and their possible elimination have been evaluated.

The ANU has developed experimental systems and procedures for lead–tin (PbSn) film deposition and characterisation. The 12 split loop resonators have been electroplated with 96%Pb4%Sn film to the final thickness of 1.5 micron using methanesulfonic acid (MSA) chemistry. As a result, an average acceleration field of 3.6 MV/m off-line at 6 W rf power was achieved at extremely low technological cost. Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Heavy Ion Elastic Detection Analyses (HIERDA), Rutherford Backscattering Spectroscopy (RBS), Secondary Ion Mass Spectroscopy (SIMS) and Electron Backscattering Diffraction (EBSD) revealed correlation between the substrate and film structure, morphology and the rf performance of the cavity. The PbSn plating, exercised on the existing split loop resonators (SLR), has been extended to the two stub quarter wave resonator (QWR) as a straightforward step to quickly explore the superconducting performance of the new geometry. The oxygen free copper (OHFC) substrate for two stub QWR was prepared by reverse pulse electropolishing. The ultimate superconducting properties and long-term stability of the coatings have been assessed by operation of the ANU superconducting linac over the last few years.