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 250{}C to 350{}C 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 Eacc. First, comparing mid-T heat treatment with a baseline treatment, a significant increase of Q₀ up to 5·10¹⁰ at 2K can be observed. Second, with increasing accelerating gradient Eacc 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 Eacc. The appearance of a high-field-Q-slope (HFQS) was reported after mid-T heat treatments of 3 hours at 350{}C or of 20 hours at 300{}C 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 120{}C to 130{}C. 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 3·10¹⁰ at 16 MV/m and accelerating gradients of 32 to 40 MV/m are achieved.