Recent Results of High Temperature Vacuum Heat Treatment Program of SRF Resonators at IJCLab

In the framework of ESS, a vacuum furnace dedicated to High Temperature Heat Treatment under Vacuum (VH2T2) of SRF bulk niobium (Nb) cavities was developed and commissioned in May 2016. This furnace is mainly used for the reduction of interstitial hydrogen in niobium. VH2T2 is an effective process for curing SRF cavities from the so-called Q
₀
-deases phenomenon, which is due to Nb hydrid formation on the RF surface during the cool down of the resonator to cryogenic temperatures. IJCLab VH2T2 process is done at 650 °C during 10h00. The main differences between IJCLab recipe and the standard VH2T2 (SVH2T2) are 2 folds: 1) SVH2T2 is performed at 800 °C during 3h00, 2) IJCLab recipe is applied to spoke resonators equipped with Ti LHe tank. This process, will be applied to the whole series (26 cavities) of ESS 352 MHz Double-Spoke resonators. More than 28 cavities of various type (ESS Double-Spoke resonators, MYRRHA single spoke, ESS elliptical medium and high β cavities) were successfully heat treated according to our recipe and the last results are reported. Moreover, since 2 years we have launched the R&D program HELOISE with the financial support of IN2P3/CNRS. HELOISE is dedicated to Heat Treatment of SRF niobium cavities: it includes Low Temperature Baking, VH2T2, Nitrogen Infusion and Nitrogen Doping processes. HELOISE main objectives are: 1) understand the complex mechanisms of the physical phenomena involved in heat treatment processes, 2) investigate the effect of the parameters impacting the process, 3) develop and master, at large scale and with good reliability, heat treatment processes in order to produce low RF losses and high gradient SRF bulk niobium cavities. Finally, we will focus on the first experimental data on Nitrogen Infusion we obtained: the corresponding results are reported and discussed.