Cornell University, Ithaca, New York, USA
Brigham Young University, Provo, USA
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
Research linking surface hydrides to Q-disease, and the subsequent development of methods to eliminate surface hydrides, is one of the great successes of SRF cavity R\&D. We use time-dependent Ginzburg-Landau to extend the theory of hydride dissipation to sub-surface hydrides. Just as surface hydrides cause Q-disease behavior, we show that sub-surface hydrides cause high-field Q-slope (HFQS) behavior. We find that the abrupt onset of HFQS is due to a transition from a vortex-free state to a vortex-penetration state. We show that controlling hydride size and depth through impurity doping can eliminate HFQS.
Cornell University, Ithaca, New York, USA
Recent experiments suggest that noble-metal deposition on niobium metal surfaces can remove the surface oxide and ultimately improve superconducting radio-frequency (SRF) cavities performance. In this preliminary study, we use density-functional theory to investigate the potential for noble-metal passivation of realistic, polycrystalline niobium surfaces for SRF. Specifically, we investigate the stability of gold and palladium monolayers on niobium surfaces with different crystal orientations and evaluate the impact of these impurities on superconducting properties. In particular, our results suggest that gold can grow in thin layers on the niobium surface, whereas palladium rather tends to dissolve into the niobium cavity. These results will help inform ongoing experimental efforts to passivate niobium surfaces of SRF cavities.
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
Cornell University, Ithaca, New York, USA
Nb₃Sn is a promising alternative material for SRF cavities that is close to reaching practical applications. To date, one of the most effective growth methods for this material is vapor diffusion, yet further improvement is needed for Nb₃Sn to reach its full potential. The major issues faced by vapor diffusion are tin depleted regions and surface roughness, both of which lead to impaired performance. Literature has shown that the niobium surface oxide plays an important role in the binding of tin to niobium. In this study, we performed various chemical treatments on niobium samples pre-nucleation to enhance tin nucleation. We quantify the effect that these various treatments had through scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). These methods reveal information on tin nucleation density and uniformity, and a thin tin film present on most samples, even in the absence of nucleation sites. We present our findings from these surface characterization methods and introduce a framework for quantitatively comparing the samples. We plan to apply the most effective treatment to a cavity and conduct an RF test soon.
Cornell University, Ithaca, New York, USA
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
Experiments on superconducting cavities have found that under large RF fields the quality factor can improve with increasing field amplitude, a so-called anti-Q slope. We numerically solve the Bogoliubov-de Gennes equations at a superconducting surface in a parallel magnetic field, finding at large fields there are surface quasiparticle states with energies below the bulk superconducting gap that emerge and disappear as the field cycles. Modifying the standard two-fluid model, we introduce a ‘‘three’’-fluid model where we partition the normal fluid to consider continuum and surface quasiparticle states separately. We compute dissipation in a semi-classical theory of conductivity, where we provide physical estimates of elastic scattering times of Bogoliubov quasiparticles with point-like impurities having potential strengths informed from complementary ab initio calculations of impurities in bulk niobium. We show, in this simple yet effective framework, how the relative scattering rates of surface and continuum quasiparticle states can play a role in producing an anti-Q slope while demonstrating how this model naturally includes a mechanism for turning the anti-Q slope on and off.
S. Deyo, M. Kelley et al. Phys. Rev. B 106, 104502 (2022)
Cornell University, Ithaca, New York, USA
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
The first experimental studies of zirconium-doped surfaces verified that zirconium can enhance the critical temperature of the surface, resulting in a lower BCS resistance than standard-recipe niobium. However, they also produced a disordered oxide layer, resulting in a higher residual resistance than standard-recipe niobium. Here, we show that zirconium-doped surfaces can grow well-behaved thin oxide layers, with a very thin ternary suboxide capped by a passivating ZrO2 surface. The elimination of niobium pentoxide may allow zirconium-doped surfaces to achieve low residual resistance.