Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
Nitrogen-doping has been shown to consistently produce better quality factors in SRF cavities than is achievable with standard preparation techniques. Unfortunately, nitrogen-doping typically brings with it lower quench fields and higher sensitivities of residual resistance to trapped magnetic flux. Here we present work to understand these effects in hopes of mitigating them while maintaining the high Q desired by future projects. Using a nitrogen diffusion simulation, material parameters of nitrogen-doped cavities can be predicted prior to doping. These simulations results are consistent with SIMS data taken from samples treated with cavities. The nature of doping's effect on quench field has also been studied using CW and pulsed measurements. These results have allowed us to better understand the nature of nitrogen-doping and its effect on cavity performance.
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
Previous measurements at Cornell have shown that the sensitivity of residual resistance to trapped magnetic field in SRF cavities is heavily dependent on the mean free path of the RF penetration layer of the niobium. Here we report on a systematic study of ten cavity preparations with different mean free paths and the effect of these preparations on sensitivity to trapped magnetic flux. In the clean limit, longer mean free path leads to a lower sensitivity to trapped magnetic flux while in the dirty limit the opposite is true, shorter mean free path leads to lower sensitivity. These results are also shown to be in good agreement with theoretical predictions of RF losses due to oscillations of vortex lines.
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
Nitrogen-doping has become a standard tool for reaching high quality factors in SRF cavities in the medium field region at 2 K. This high Q has been shown to be a result of lowering of the temperature dependent BCS resistance. Here we show that this lowering of the BCS resistance is due to interstitial nitrogen in the niobium lowering the mean free path. The BCS resistance extracted from experimental data is shown to be consistent with theoretical predictions from BCS theory; that there is an optimal doping of which the mean free path is lowered to about half the intrinsic coherence length. These results provide insight into understanding the mechanisms behind nitrogen-doping and allow us to more accurately predict doping parameters to reach optimal cavity performance.
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
Impurity doping of bulk-niobium superconducting radio frequency (SRF) cavities is a relatively new field of study and the underlying physics is not yet fully understood. Previous studies have shown an increase in the intrinsic quality factor and the corresponding decrease of the temperature-dependent component of the surface resistance of nitrogen-doped cavities with increasing accelerating field.* Here we investigate the effects of alternative inert dopants on the surface resistance and thus the intrinsic quality factor of SRF cavities in pursuit of the optimal dopant and doping level.
A. Grassellino et al., Nitrogen and Argon Doping of Niobium for Superconducting Radio Frequency Cavities. Supercond. Sci. Technol., 26(102001), 2013
