Low BCS surface resistance and large superheating field make Nb3Sn a very attractive material for low-field SRF applications. At the same time, the performance of Nb3Sn at high RF fields can be limited by current-blocking grain boundaries, small lower critical magnetic field, poor thermal conductivity and high resistivity, which make Nb3Sn prone to premature penetration of vortices and...
We present the first comprehensive study of grain boundaries in Nb$_3$Sn from first principles. While most conventional superconductors, such as Nb, are not significantly impacted by the presence of grain boundaries, Nb$_3$Sn is much more sensitive to defects and disorder owing to its relatively short coherence length of ~3 nm. Indeed, experiments suggest a link between grain-boundary...
Incorporating nanoparticles into superconducting materials has been established as an efficient route to enhance their current-carrying capability. We explored vortex pinning by randomly distributed spherical nanoparticles using large-scale numerical simulations of time-dependent Ginzburg-Landau equations. First, we investigated a vortex lattice interacting with an isolated defect [1]. We...
Tunneling spectroscopy measurements were carried out at ANL and CEA on Nb3Sn/Nb samples made at Cornell and FNAL with différent growth conditions. We find a linear correlation between the average gap extracted from maps of the Density of states few hundreds microns of lateral size and the corresponding RF cavity tests made under the same conditions as the samples.
Superconducting Radio Frequency (SRF) cavities are being widely used in new generation particle accelerators, but their performance can be limited by surface defects which lead to cavity breakdown at high accelerating gradients. The microscopic origins of SRF cavity breakdown are still a matter of some debate. To study the electrodynamics of superconductors locally, a novel near-field magnetic...
Nb3Sn is currently the most promising material other than niobium for future superconducting radiofrequency (SRF) cavities. To achieve high accelerating gradient, the behavior of Nb3Sn thin films in an external magnetic field should be studied. The magnetic field at first flux penetration is one of the key physical parameters to characterize them. Therefore, it is important to have a simple,...
The field of first vortex penetration was measured on Nb$_3$Sn samples under DC and RF fields using the muon spin rotation technique and a quadrupole resonator. Those methods also enabled direct and indirect measurements of the London penetration depth from which the lower critical field and the superheating field are derived. The combined results confirm that Nb$_3$Sn cavities are indeed...
A tin vapor diffusion system was constructed for Nb3Sn cavity R&D at KEK. The vapor diffusion system consists of a vertical vacuum furnace, a coating chamber made of niobium, and a heating device for tin evaporation. In the thermal design of the coating system, the temperature distribution of cooling and coating part was investigated using ANSYS. After construction of the vapor diffusion...
There has been significant progress in Nb3Sn SRF cavities during the last decade and the current maximum accelerating electric field of Nb3Sn cavities exceeds ~24 MV/m for a single-cell. Collaborative research on Nb3Sn film growth studies between Fermilab and Northwestern University have been performed during the last three years. We explored effects of growth parameters such as Sn supply...
The tin vapor diffusion coating of Nb cavity interiors via a two-step nucleation-then-growth sequence appears to be the most promising path so far to produce Nb3Sn cavities. To elucidate the role of nucleation, we manipulated the accessible range of process variables and studied the niobium surface nucleated under varying process conditions using an array of materials characterization tools....
Following the recent progress made in the Nb₃Sn coatings on single-cell SRF cavities, development is ongoing to reproduce single-cell cavity results on practical SRF structures. Those structures may include multi-cell and single-cell cavities having a larger surface area than regularly coated ~1.5 GHz single-cell cavities. Early CEBAF five-cell cavities coated with a typical coating procedure...
The recently demonstrated performance of Nb3Sn cavities makes this material attractive for SRF accelerator applications. While the majority of research efforts are focused on the development of elliptical single-cell and multi-cell cavities, the potential of this material is evident to other cavity types, which may have complex geometries. We are working towards the development of Nb3Sn-coated...
Helium atom scattering (HAS) and Auger electron spectroscopy (AES) measured surface structure and composition of an unexplored regime of the oxidized Nb surface at SRF Nb3Sn cavity preparation temperatures. These in situ measurements revealed the high temperature stability of a NbO surface reconstruction, specifically the (3x1)-O Nb(100). HAS diffraction peak intensity, line shape, and...
The potential for lowered operating costs and higher quality factors (Q) motivate efforts to implement Nb3Sn based superconducting radio frequency cavities. These benefits are contingent upon the continued optimization of coating procedures resulting in smooth, homogeneous A15 Nb3Sn films. Specifically, the efficiency of Nb3Sn cavity coatings is limited by...
Nb$_{3}$Sn has been identified as a promising next-generation material for superconducting radio frequency (SRF) cavities and there is significant interest in developing protocols resulting in pristine Nb$_{3}$Sn coatings. Recent work has shown that Nb$_{3}$Sn surface structure and composition significantly affects the cavity quality factor (Q) of Nb$_{3}$Sn SRF cavities; Sn homogeneity,...
We have investigated Nb3Sn film growth via bronze route by magnetron sputtering of Nb films on to bronze substrates for potential application in SRF cavities. Two main routes were followed: 1) deposition of 500 nm equivalent Nb onto hot (650 °C to 775 °C) bronze substrates, where Nb3Sn formed during the Nb deposition; 2) deposition of Nb onto bronze substrates at lower temperature (200 °C)...
Electrochemical methods are presented to prepare substrates, deposit Nb and bronze coatings, and set up reactions for Nb3Sn for a potential application in SRF cavities. Specifically, this paper presents firstly our understanding on the electrochemical mechanisms that hinder the proper preparation of substrate and coating layers and secondly methods of overcoming such limitations developed in...
Due to higher superconducting critical temperature and superheating field, and lower BCS surface resistance, Nb3Sn is considered a promising alternative to standard niobium for SRF application. Multilayer sequential sputtering method is a promising alternative to the conventional vapor diffusion method to grow Nb3Sn films inside a niobium cavity. In this method, multiple thin layers of Nb and...
In this contribution, we explore the growth of thin superconducting (SC) films, such as Nb3Sn and V3Si, on copper as possible candidates for the reduction of the operational surface resistance of superconducting radio-frequency (SRF) coated cavities for particle accelerators.
For an optimal SRF performance, the SC layers, grown using magnetron sputtering, have to be free of any...
Ultramet, an industry leader in the manufacture of refractory metal and ceramic components by chemical vapor deposition (CVD) and chemical vapor infiltration (CVI), continues to investigate and adapt CVD-based methods for the production of advanced high-gradient capable superconducting radiofrequency (SRF) cavities and components to meet the needs of the accelerator community. Ultramet...
The progress in the development of Nb3Sn cavity coating by vapour diffusion method at IMP was reported. Several 1.3GHz single cell cavities were coated and vertically tested. Up to now, the unloaded Q value of IMP Nb3Sn cavity at 4.2K reached 7.6e9 at the low field region, which is about three times lower than the Cornell results. Meanwhile, the quench field was only Eacc=8MV/m. Although the...
Two 1.5 GHz 5-cell accelerator cavities have been coated with Nb3Sn in JLab Nb3Sn cavity coating system. The cavities were qualified at 4 K in the vertical dewar test and are progressed towards their installation into a cryomodule. One of the cavities was used to study fundamental limitation of the coating material using two-mode excitation. Results from the fundamental study as well as the...
In this contribution, we present measurements from vertical testing of Nb3Sn cavities at Fermilab. Results include measurements on single cell cavities as well as 9-cell cavities. Most results are at 1.3 GHz, with some at other frequencies. Correlations are made to coating appearance.
A high-current electron source capable of generating high charge electron bunches at MHz repetition rates is currently being prototyped. The source is based on a 650-MHz single-cell superconducting cavity modify to include a reentrant cathode holder optimized to significantly enhance the electric field on the cathode surface. The electrons are produced via field emission from a...
Fermilab recently demonstrated practical accelerating gradients (~6.5 MV/m cw) on a Nb$_3$Sn SRF cavity with cryocooler conduction-cooling, without using the conventional liquid helium bath. The successful integration of this cryocooling scheme with an SRF cavity is a stepping-stone for realizing compact SRF based e-beam sources for high-throughput industrial applications of electron...
Jefferson Lab is funded by a grant from the DOE Accelerator Stewardship to demonstrate operation of an SRF cavity with a cryocooler to an accelerating gradient compatible with an electron energy gain of 1 MeV for possible use in an accelerator for environmental remediation. This presentation describes the current plan and initial test results on a 952 MHz single-cell cavity coated with Nb3Sn
A new frontier in SRF research is the use of simplified cooling methods
that will allow easier access to SRF technology for industrial applications.
At Cornell, we have developed a new conduction cooling setup that utilizes
a manufactured cryocooler to provide the necessary heat dissipation for
operation of a 2.6 GHz Nb$_3$Sn-coated SRF cavity. We report on various methods used to increase...
