Speaker
Description
The native oxides of niobium cause surface losses during cavity operation arising from two-level systems/defects (TLS). These losses dominate the quality factor at low accelerating gradients (Eacc < 0.1 MV/m). In particular, the amorphous Nb2O5 is identified as a prominent host for the TLS. Nb2O5 dissociates when the material is baked above 200 °C for several hours in vacuum (the so-called Mid-T Bake), allowing for the modification or reduction of these losses. However, due to the inevitable exposure to air after the annealing, the surface reoxidizes and Nb2O5 regrows. The main goal is to reduce these TLS losses by introducing a passivating layer over the niobium which reduces oxygen diffusion into the substrate. When the cavity is coated with a passivating layer like Al2O3 and then subjected to the Mid-T Bake, this subsequent reoxidation of the niobium is inhibited. Here, we report the tests of several cavities tested at low fields, after Mid-T baking, with and without passivating coatings, and discuss these results in the framework of the TLS model.