Fermilab, Batavia, Illinois, USA
JLab, Newport News, Virginia, USA
Two CEBAF 5-cell accelerator cavities have been coated with Nb₃Sn film using the vapor diffusion technique. One cavity was coated in the Jefferson Lab Nb₃Sn cavity coating system, and the other in the Fermilab Nb₃Sn coating system. Both cavities were measured at 4 K and 2 K in the vertical dewar test in each lab and then assembled into a cavity pair at Jefferson Lab. Previous attempts to assemble Nb₃Sn cavities into a cavity pair degraded the superconducting properties of Nb₃Sn-coated cavities. This contribution discusses the efforts to identify and mitigate the pair assembly challenges and will present the results of the vertical tests before and after pair assembly. Notably, one of the cavities reached the highest gradient above 80 mT in the vertical test after the pair assembly.
ODU, Norfolk, Virginia, USA
Fermilab, Batavia, Illinois, USA
JLab, Newport News, Virginia, USA
A DC cylindrical magnetron sputtering system has been commissioned and operated to deposit Nb₃Sn onto 2.6 GHz Nb SRF cavities. After optimizing the deposition conditions in a mock-up cavity, Nb-Sn films are deposited first on flat samples by multilayer sequential sputtering of Nb and Sn, and later annealed at 950 °C for 3 hours. X-ray diffraction of the films showed multiple peaks for the Nb₃Sn phase and Nb (substrate). No peaks from any Nb-Sn compound other than Nb₃Sn were detected. Later three 2.6 GHz Nb SRF cavities are coated with ~1 µm thick Nb₃Sn. The first Nb₃Sn coated cavity reached close to Eacc = 8 MV/m, demonstrating a quality factor Q₀ of 3.2 × 108 at Tbath = 4.4 K and Eacc = 5 MV/m, about a factor of three higher than that of Nb at this temperature. Q₀ was close to 1.1 × 109, dominated by the residual resistance, at 2 K and Eacc = 5 MV/m. The Nb₃Sn coated cavities demonstrated Tc in the range of 17.9 ¿ 18 K. Here we present the commissioning experience, system optimization, and the first results from the Nb₃Sn fabrication on flat samples and SRF cavities.
ODU, Norfolk, Virginia, USA
Fermilab, Batavia, Illinois, USA
JLab, Newport News, Virginia, USA
JLab, Newport News, Virginia, USA
JLAB, Newport News, USA
Phase Space Tech, Bjärred, Sweden
GA, San Diego, California, USA
TechSource, Los Alamos, New Mexico, USA
TJS Technologies, Commack, New York, USA
After the success of designing a compact 1 MeV, 1 MW accelerator based on conduction-cooled SRF, Jefferson Lab is now pursuing a concept to provide a tenfold increase of the beam energy. The higher energy significantly extends the range of applications for environmental remediation and industry in general. The obvious challenge for SRF is to move from a single-cell to a multicell cavity while maintaining high efficiency and the ability to operate the machine without a complex cryogenic plant. The contribution presents the latest results of this design study with respect to its centerpiece, a Nb₃Sn coated 915 MHz five-cell cavity and its corresponding RF components, i.e. FPC and HOM absorber, as well as the conduction-cooling concept based on commercially available cryocoolers.
JLab, Newport News, Virginia, USA
Nb₃Sn offers the prospect of better RF performance (Q and Eacc) than niobium at any given temperature because of its superior superconducting properties. Nb₃Sn-coated SRF cavities are routinely produced by growing a few microns thick Nb₃Sn film on Nb cavities via tin vapor diffusion. It has been observed that a clean and smooth surface can enhance the performance of the Nb₃Sn-coated cavity, typically, the attainable acceleration gradient. The reduction of surface roughness is often linked with a correlative reduction in average coating thickness and grain size. Besides Sn supply’s careful tuning, the temperature profiles were varied to reduce the surface roughness as low as ~40 nm in 20 µm × 20 µm AFM scans, one-third that of the typical coating. Samples were systematically coated inside a mock single-cell cavity and examined using different material characterization techniques. A few sets of coating parameters were used to coat 1.3 GHz single-cell cavities to understand the effects of roughness variation on the RF performance. This presentation will discuss ways to reduce surface roughness with results from a systematic analysis of the samples and Nb₃Sn-coated single-cell cavities.
JLab, Newport News, Virginia, USA
Fermilab, Batavia, Illinois, USA
The mechanical vulnerability of the Nb₃Sn-coated cavities is identified as one of the significant technical hurdles toward deploying them in practical accelerator applications in the not-so-distant future. It is crucial to characterize the material’s mechanical properties in ways to address such vulnerability. Nanoindentation is a widely used technique for measuring the mechanical properties of thin films that involves indenting the film with a small diamond tip and measuring the force-displacement response to calculate the film’s elastic modulus, hardness, and other mechanical properties. The nanoindentation analysis was performed on multiple vapor-diffused Nb₃Sn samples coated at Jefferson Lab and Fermilab coating facilities for the first time. This contribution will discuss the first results obtained from the nanoindentation of Nb₃Sn-coated Nb samples prepared via the Sn vapor diffusion technique.
THIXA04
JLab, Newport News, VA, USA
GA, San Diego, California, USA
ODU, Norfolk, Virginia, USA
TechSource, Los Alamos, New Mexico, USA
TJS Technologies, Commack, New York, USA
Recent progress in the development of high-quality Nb₃Sn film coatings along with the availability of cryocoolers with high cooling capacity at 4 K makes it feasible to operate SRF cavities cooled by thermal conduction at relevant accelerating gradients for use in accelerators. We have developed a prototype single-cell cavity to prove the feasibility of operation up to the accelerating gradient required for 1 MeV energy gain, cooled by conduction with cryocoolers. The cavity has a ~3 ¿m thick Nb₃Sn film on the inner surface, deposited on a ~4 mm thick bulk Nb substrate and a bulk ~7 mm thick Cu outer shell with three Cu attachment tabs. The cavity was tested up to a peak surface magnetic field of 53 mT in liquid He at 4.3 K. A horizontal test cryostat was designed and built to test the cavity cooled with three cryocoolers. The rf tests of the conduction-cooled cavity achieved a peak surface magnetic field of 50 mT and stable operation was possible with up to 18.5 W of rf heat load. The peak frequency shift due to microphonics was 23 Hz. These results represent the highest peak surface magnetic field achieved in a conduction-cooled SRF cavity to date