Further improvement of medium temperature heat treated SRF cavities for high gradients

The application of heat treatments on 1. 3 GHz TESLA type cavities in ultra-high vacuum at 250C to 350C is called medium temperature or mid-T heat treatment. In various laboratories such treatments on superconducting radio frequency (SRF) cavities result reproducible in three main characteristic features for the quality factor Q₀ in dependency of the accelerating electric field strength E₀₂₂. First, comparing mid-T heat treatment with a baseline treatment, a significant increase of Q₀ up to 510^10 at 2K can be observed. Second, with increasing accelerating gradient E₀₂₂ the Q₀ increases up to a maximum around 16 to 20 MV/m. This effect is known as anti-Q-slope. The third observation for a mid-T heat treatment compared to a baseline treatment is an often reduced maximum gradient E₀₂₂. The appearance of a high-field-Q-slope (HFQS) was reported after mid-T heat treatments of 3 hours at 350C or of 20 hours at 300C at DESY. Using the heating temperature and the heating time taken from the temperature profile of the furnace effective oxygen diffusion lengths l were calculated. In the follow-up study presented here, a set of three single-cell cavities with diffusion lengths l above 1700 nm, showing HFQS, were treated with an additional so-called low-T bake of 24-48 hours at 120C to 130C. The subsequent reproducible Q (E) -performances results indicate that the low-T bake procedure cures the HFQS like for cavities treated with the EuXFEL recipe of EP and following low-T treatments. As presented in the following, Q values of more than 310^10 at 16 MV/m and accelerating gradients of 32 to 40 MV/m are achieved.