Superconducting Photocathode Development Using High QE Coatings on Nb Substrates

A hybrid approach is used to create superconducting photocathodes with quantum efficiencies (QE) enhanced over bulk Nb for potential use in SRF linear accelerators. The QE and other relevant properties of photocathodes consisting of bulk Nb substrates coated with thin films of semiconducting Cs₂Te and metallic Mg are reported. The Cs₂Te coatings reveal a surprisingly high QE ~ 6% for thicknesses as low as 1.5 nm. A phenomenological model based on the Spicer 3-step model is used to fit the measured thickness dependence. Thin films of Mg (10 nm) on Nb are shown to have QE at least ten times greater than bulk Nb and are robust in an RF gun up to 60 MV/m. Previous work on the superconducting proximity effect of Nb/Mg bilayers show that the induced gap in the Mg layer is close to that of bulk Nb.