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DOE OSTI2022

Thermodynamic re-modelling of the Cu–Nb–Sn system: Integrating the nausite phase

Currently available Cu–Nb–Sn phase diagrams lack the recently discovered nausite phase (Cu,Nb)Sn 2 , which is an important intermediate in the course of thermal processing of superconducting Nb 3 Sn wires. Processing decisively determines the resulting microstructure of Nb 3 Sn and, thus, its superconducting properties. Lack of suitable and complete phase diagrams, however, obstructs rational design of such thermal processing procedures. To close this gap and to obtain valid knowledge of homogeneity and stability range of nausite, various Cu–Nb–Sn samples, which are heat-treated between 300 °C and 500 °C, are investigated. By means of energy-dispersive X-ray spectroscopy (EDX), a temperature-dependent homogeneity range of nausite is observed, which covers average mole fractions of Cu between 0.09 and 0.15. This is correlated with a change in the mean atomic volume and can be seen in the lattice parameters determined by X-ray diffraction (XRD). Additionally performed first-principles calculations on different CuSn 2 and NbSn 2 model structures confirm this trend. Furthermore, the peritectic decomposition of nausite to NbSn 2 and liquid at 586 °C is determined by means of in situ XRD and differential scanning calorimetry (DSC). By using the CALPHAD (CALculation of PHase Diagrams) approach, all these findings are used to extend a previous thermodynamic description of the Cu–Nb–Sn system by including the nausite as an additional phase. Finally, with this noteworthy integration, the updated modelling of the Cu–Nb–Sn system can be used for optimizing the multistage heat-treatment steps during processing superconducting Nb 3 Sn wires.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Commissioning of a two-target DC cylindrical magnetron sputter coater for depositing Nb 3 Sn film on Nb superconducting radiofrequency cavities

A DC cylindrical magnetron sputter coater was commissioned and used to coat Nb 2.6 GHz superconducting radiofrequency (SRF) cavity with Nb 3 Sn. The sputter coater has two identical cylindrical magnetrons that can move, with a controlled speed, along the axis of the SRF cavity to coat the inside surface of the cavity. The design of the sputter coater is discussed, along with its performance. Initially, a sample holder that allows coating on flat substrates at positions similar to the equator and beam tubes of a 2.6 GHz SRF cavity was used to test conditions for fabricating Nb 3 Sn layers. Multilayers of Nb and Sn were sequentially sputtered on flat Nb and sapphire substrates mounted on the equivalent positions of the cavity's beam tubes and the equator using the two identical cylindrical magnetrons. Then, the Nb/Sn multilayers were annealed at 950 °C for 3 h. Here, the ~1.2 μm thick Nb 3 Sn film did not show any other Nb–Sn compounds and had a superconducting transition temperature of 17.61–17.76 K. The 2.6 GHz SRF cavity was coated using similar conditions as flat samples. Cryogenic RF testing of the Nb 3 Sn-coated cavity demonstrated a quality factor of 3.2 × 10 8 at an accelerating gradient of 5 MV/m at 4.4 K.

43 PARTICLE ACCELERATORS↗

DOE OSTI2026

Microwave Vortex Motion Characterization of Nb$_{3}$ Sn Coatings for Applications in High Magnetic Fields

In this work, microwave measurements carried out in dielectric-loaded resonators exposed to high magnetic fields are exploited to yield the surface impedance of Nb$_3$ Sn superconducting coatings deposited via two different techniques: vapor tin diffusion, and DC magnetron sputtering. The obtained data lead to qualitative interpretations on both the Nb$_3$ Sn superconducting properties, and vortex-dynamics and pinning, of each coating separately, as well as simple distinctive features when comparing those. When examining the respective surface impedances at varying field, it is expected that the studied films perform at substantially diverse magnitudes of flux-flow resistivity, but also in well-differentiated pinning regimes, yet the obtained surface resistances of both samples are comparable, thus demonstrating that there is room for film optimization at the expense of certain compromise between the parameters involved.

Nb3Sn↗

DOE OSTI2020

Effect of layer thickness on structural, morphological and superconducting properties of Nb 3 Sn films fabricated by multilayer sequential sputtering

Superconducting Nb3Sn films can be synthesized by controlling atomic concentration of Sn. Multilayer sequential sputtering of Nb and Sn thin films followed by high temperature annealing is considered as a method to fabricate Nb3Sn films where Sn composition of deposited films can be controlled by controlling the thickness of alternating Nb and Sn layers. We report on the structural, morphological and superconducting properties of Nb3Sn films fabricated by multilayer sequential sputtering of Nb and Sn films on sapphire substrates with ex-situ annealing at 950 °C for 3 h. We have investigated the effect of Nb and Sn layer thickness on the properties of Nb3Sn films. The thicknesses of Nb and Sn layers were varied in two ways: (1) varied Nb:Sn thickness ratio (1:1, 2:1, 3:1, 4:1), and (2) varied layer thickness of both Nb and Sn layers, while keeping constant Nb:Sn thickness ratio of 2:1. The crystal structure, surface morphology, topography, and film composition were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and energy dispersive X-ray spectroscopy (EDS) respectively. The results showed Sn loss from the surface due to evaporation during annealing. Superconducting Nb3Sn films of critical temperature up to 17.93 K have been achieved.

Sayeed, M. N.↗

DOE OSTI2020

The strong influence of Ti, Zr, Hf solutes and their oxidation on microstructure and performance of Nb 3 Sn superconductors

The strong influence of Ti, Zr, Hf solutes and their oxidation on microstructure and performance of Nb 3 Sn superconductors Over the last few years a new type of Nb 3 Sn superconducting composite based on the internal oxidation approach has emerged and has demonstrated performance significantly superior to conventional Nb 3 Sn. It requires a supply of O and the use of a Nb alloy – Nb-X, where X is a solute element that can be selectively oxidized to form oxide particles within the Nb 3 Sn. Such oxide particles not only refine Nb 3 Sn grain size, but also have the proper size to act as artificial pinning centers (APC) directly restraining fluxon motion, and thus dramatically improve superconducting properties. In this article we show that the size and volume fraction of the oxide particles determine both the levels of grain refinement and the shift in the peak field of the flux pinning force (F p -B) curve. Additionally, we explore the factors influencing these microstructure and properties, which we find include: selection of the solute element X, solute content, O content, and heat treatment. For the selection of X, we searched the periodic table for all promising candidates but focused down on the group-IVB elements (Ti, Zr, Hf) here as the drawability of Nb 3 Sn wires made from Nb-Ti, Nb-Zr, and Nb-Hf alloys has been demonstrated in the past few decades. We found that while internally oxidizing Nb-1.5at.%Ti led to negligible grain refinement and F p -B peak shift, Nb-Zr alloys led to much more dramatic results, and internally oxidizing Nb-Hf alloys led to the strongest grain refinement and F p -B peak shift. For Hf alloying in particular, we compared our internal oxidation method with another method for grain refinement, which uses Hf alloying itself without oxidation, and found that internal oxidation led to much stronger grain refinement and F p -B peak shift. We also found that higher solute content and lower reaction temperature led to stronger grain refinement and F p -B peak shift. We conclude with a discussion of the possible mechanisms for the influence of these factors.

43 PARTICLE ACCELERATORS↗

DOE OSTI2025

Status and Challenges in the MQXFB Nb 3 Sn Quadrupoles for the HL-LHC

The inner triplet (or low-β) quadrupole magnets are among the components to be upgraded in LHC interaction regions for the HL-LHC project. The new quadrupole magnets, called MQXF, are based on Nb 3 Sn superconducting magnet technology, with a conductor peak field of 11.3 T. CERN is in charge of the fabrication of the MQXFB variant, the longest Nb 3 Sn accelerator magnets designed and manufactured up to now, with a magnetic length of 7.2 m. Two magnets, MQXFBP3 and MQXFB02, reached the HL-LHC project requirements. However, they still exhibited a limitation at 4.5 K with a phenomenology similar to the one observed on the first two prototypes. After improvements on the cold mass (longitudinal welding) and magnet assembly (elimination of overstress on the conductor during loading) procedures, a series of modifications were implemented in MQXFB03 at the level of the coil fabrication to address and/or reduce weaknesses in the coils. The magnet was tested and was the first to achieve performance requirements at both 1.9 K and 4.5 K, with no signs of conductor limitation at 4.5 K. MQXFB is now in the series production phase, with around 2/3 of the coils completed and half of the magnets assembled. We provide in this paper an overview of the MQXFB program, with a summary of the main recent achievements and an overall status of the fabrication.

43 PARTICLE ACCELERATORS↗

DOE OSTI2023

A Metallurgical Inspection Method to Assess the Damage in Performance-Limiting Nb 3 Sn Accelerator Magnet Coils

The design and production of Nb 3 Sn-based dipole and quadrupole magnets is critical for the realization of the High-Luminosity Large Hadron Collider (HL-LHC) at the European Organization for Nuclear Research (CERN). Nb 3 Sn superconducting coils are aimed at enhancing the bending and focusing strengths of accelerator magnets for HL-LHC and beyond. Due to the brittle nature of Nb 3 Sn, the coil fabrication steps are very challenging and require very careful QA/QC. Flaws in the Nb 3 Sn filaments may lead to quenches, and eventually, performance limitation below nominal during magnet testing. A novel inspection method, including advanced non-destructive and destructive techniques, was developed to explore the root-causes of quenches occurring in performance-limiting coils. The most relevant results obtained for MQXF coils through this innovative inspection method are presented. This approach allows for precise assessment of the physical events associated to the quenches experienced b y magnet coils, mainly occurring under the form of damaged strands with transversely broken sub-elements. Coil-slice preparation, micro-optical observations of transverse and longitudinal cross-sections, and a deep etching technique of copper will be illustrated in the present work, with a focus on the results achieved for a CERN coil from a non-conforming quadrupole magnet prototype, and two coils fabricated in the US, in the framework of the Accelerator Upgrade Project (AUP) collaboration, from two different non-conforming quadrupole magnets, respectively. The results obtained through the proposed inspection method will be illustrated.

43 PARTICLE ACCELERATORS↗

DOE OSTI2024

Assessment of Training Performance, Degradation and Robustness of Paraffin-Wax Impregnated Nb 3 Sn Demonstrator Under High Magnetic Field

In the context of high-energy physics, the use of Nb 3 Sn superconducting magnets as a cost-effective and reliable technology depends on improvements in the following areas: long development and manufacturing cycles, conductor degradation after thermal cycling, long training, as well as a demonstration in accelerator magnets with a beam aperture of the full potential of modern Nb 3 Sn conductors. In short, performance, robustness, and cost are the three issues to be addressed. The Magnet Development project (MagDev) of the Swiss Accelerator Research and Technology initiative (CHART) at the Paul Scherrer Institute (PSI) aims to contribute to the solutions to each of these issues, re-thinking the manufacturing and design process. Here in our program, every innovation is to be validated by means of a panoply of fast-turnaround tools: from non-powered and powered samples and coils, tested under background field, to low-field subscale magnets and high field short prototypes. This work presents one element in this panoply of R&D vehicles: a stress-managed Nb 3 Sn coil called BigBOX, impregnated with paraffin wax, and tested, through a collaboration with the Magnet Development Program of the United States (US-MDP), in the background field of Brookhaven National Laboratory (BNL)’s common coils dipole DCC17.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2023

APC Nb 3 Sn superconductors based on internal oxidation of Nb–Ta–Hf alloys

In the last few years, a new type of Nb 3 Sn superconducting composite, containing a high density of artificial pinning centers (APC) generated via an internal oxidation approach, has demonstrated a significantly superior performance relative to present, state-of-the-art commercial Nb3Sn conductors. This was achieved via the internal oxidation of Nb-4at.%Ta-1at.%Zr alloy. On the other hand, our recent studies have shown that internal oxidation of Nb–Ta–Hf alloys can also lead to dramatic improvements in Nb 3 Sn performance. Here in this work we follow up on this latter approach, fabricating a 61-stack APC wire based on the internal oxidation of Nb-4at.%Ta-1at.%Hf alloy, and compare its critical current density (J c ) and irreversibility field with APC wires made using Nb-4at.%Ta-1at.%Zr. A second goal of this work was to improve the filamentary design of APC wires in order to improve their wire quality and electromagnetic stability. Our new modifications have led to significantly improved residual resistivity ratio and stability in the conductors, while still keeping non-Cu J c at or above the conductor J c specification required by the proposed Future Circular Collider. Further improvement via optimization of the wire recipe and design is ongoing. Finally, additional work needed to make APC conductors ready for applications in magnets is discussed.

43 PARTICLE ACCELERATORS↗

DOE OSTI2021

Assessment of MQXF Quench Heater Insulation Strength and Test of Modified Design

The HL-LHC interaction region magnet triplets (Q1,Q2, and Q3) will be composed of superconducting Nb 3 Sn quadrupoles. The MQXF quadrupole protection system is based on CLIQ (Coupling-Loss Induced Quench system) and outer layer quench heaters. Furthermore, this paper reports a summary of quench heaters to coil high voltage tests performed on MQXF short and long coils in air after fabrication, and in air and He gas after magnet training. Breakdown voltage values demonstrate good margin with respect to the Electrical design criteria for the HL-LHC inner triplet mag-nets. A modification in the quench heater installation-with an extra layer of fiber glass between the coil and the quench heater trace-has been proposed and tested in a mirror magnet to further increase electrical margins. Results demonstrated improvements of high voltage margin at the expense of a clear increase of hot spot temperature.The baseline heater to coil insulation was assessed to be able to guarantee safe operation for the Nb 3 Sn quadrupole magnets for the interaction regions of HL-LHC.

43 PARTICLE ACCELERATORS↗

DOE OSTI2025

Pathway to tune sputter coated Nb$_3$Sn with Zr

Superconducting radio frequency (SRF) cavities are essential elements of many modern-day particle accelerators such as, European XFEL (DESY), LCLS II-HE (SLAC) and PIP II (Fermilab). Nb$_3$Sn superconducting material promises significant potential to exceed the performance of conventional niobium based SRF cavities. We aim to advance the ongoing R$&$D efforts to achieve Nb$_3$Sn coated cavities with high material quality. In this work, we have developed to achieve a route for effectively reducing the intrinsic defects in magnetron sputter coated Nb$_3$Sn SRF cavities and eventually improve the overall RF performance. We have observed that Zr is mainly being incorporated as ZrO$_2$ precipitates of average dimensions ranging from 20-100nm. By increasing the concentration of Zr, we have three major effects in the physical properties. Firstly, we noted that the density of the surface and bulk voids as well as their average dimensions are dramatically reduced on increasing the Zr content in Nb$_3$Sn. These surface voids are believed to be detrimental to cavity applications due to increase the local dissipation of electric and magnetic fields. These results indicate that inclusion of Zr in Nb$_3$Sn sputtered coating will improve the material quality and might help to reduce the overall RF power dissipation. Secondly, we observed that increasing Zr concentration from 0 to 2.4% substantially improves both superconducting transition temperature (Tc) and upper critical magnetic field (Hc2). However, on further increasing the Zr concentration to 24%, we noticed a small decrement in the Hc2 value. These results infer that the increase in Zr concentration up to an optimal concentration will be beneficial to enhance the superconducting properties. Increase in Tc promises lesser RF losses and improvement in accelerating gradient, whereas the increase in Hc2 suggests more efficient flux pinning which could give rise to high quality factors in presence of multi-Tesla magnetic field. Additionally, we also observed that increasing Zr concentration possibly prevents the oxygen diffusion and leads to thinner formation of primary surface oxide (Nb$_2$O$_5$ and SnO). This study gives a route to tune Zr fraction in Nb3Sn to modify the physical properties that control the high field performance of Nb3Sn RF cavities.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2024

Improving Nb 3 Sn cavity performance using centrifugal barrel polishing

Abstract In this study we will show a new method of polishing for Nb 3 Sn cavities known as centrifugal barrel polishing (CBP). Using this method, Nb 3 Sn coated samples are polished to a surface roughness comparable to a traditional Nb cavity after electropolishing. We also investigate different methods of cleaning the Nb 3 Sn surface after CBP to remove residual abrasive particles. The polished Nb 3 Sn surface is analyzed using confocal laser microscopy, and scanning electron microscopy is used to image the surface and measure the surface roughness after polishing. Transmission electron microscopy is also used for high resolution analysis of the surface after polishing. Finally, we show that CBP can improve the performance of a Nb 3 Sn superconducting radiofrequency cavity.

43 PARTICLE ACCELERATORS↗

DOE OSTI2021

Progress in the Development of the Nb 3 Sn MQXFB Quadrupole for the HiLumi Upgrade of the LHC

The high-luminosity upgrade of the Large Hadron Collider (HL-LHC) requires new high field and large-aperture quadrupole magnets for the low-beta inner triplets (MQXF). With a nominal operating gradient of 132.2 T/m in a 150 mm aperture and a conductor peak field of 11.3 T, the new quadrupole magnets are based on Nb 3 Sn superconducting technology. After a series of short models constructed in close collaboration by LARP (LHC Accelerator Research Program) and CERN, the development program is entering in the series production phase with CERN on one side and the US Accelerator Upgrade Project (US-AUP) on the other side assembling and testing full-length magnets. This paper describes the status of the development activities at CERN, in particular on the cold powering test of the first MQFXB prototype and on the construction of the second full scale prototype. Critical operations such as reaction heat treatment, coil impregnation and magnet assembly are discussed. Finally, the plan towards the series production is described.

43 PARTICLE ACCELERATORS↗

DOE OSTI2022

Assembly and Pre-Loading Specifications for the Series Production of the Nb 3 Sn MQXFA Quadrupole Magnets for the HL-LHC

We report the High Luminosity LHC (HL-LHC) Project is planning to install 16 cold-masses made with Nb 3 Sn quadrupole magnets in the LHC Interaction Regions to significantly increase its luminosity. Half of these cold masses are fabricated at BNL, FNAL, and LBNL under the US Accelerator Research Program (AUP). Each cold mass includes two identical Nb 3 Sn quadrupole magnets, called MQXFA with a magnetic length of 4.2 m. Currently, the AUP project has completed the fabrication and test of the first 5 pre-series magnets, and is working on the following 16 magnets for the series production. The brittleness and strain sensitivity of the Nb 3 Sn superconducting material requires a careful definition of the allowable maximum stress in the windings during magnet assembly and pre-load, and a tight control of their variation within the whole coil length. Therefore, a series of assembly and pre-load specifications have been defined with the goals of minimizing the risk of conductor degradation and providing the mechanical support required to reach the nominal current during powering. In this paper we present the specifications defined for the MQXFA magnets and applied during the different assembly phases and during the pre-load process of the first 5 pre-series magnets.

43 PARTICLE ACCELERATORS↗

DOE OSTI2020

Heat Diffusion in High-Cp Nb3Sn Composite Superconducting Wires

A major focus of Nb3Sn accelerator magnets is on significantly reducing or eliminating their training. Demonstration of an approach to increase the Cp of Nb3Sn magnets using new materials and technologies is very important both for particle accelerators and light sources. It would improve thermal stability and lead to much shorter magnet training, with substantial savings in machines’ commissioning costs. Both Hypertech and Bruker-OST have attempted to introduce high-Cp elements in their wire design. This paper includes a description of these advanced wires, the finite element model of their heat diffusion properties as compared with the standard wires, and whenever available, a comparison between the minimum quench energy (MQE) calculated by the model and actual MQE measurements on wires.

43 PARTICLE ACCELERATORS↗

DOE OSTI2020

Heat Treatment Studies of Nb3Sn Wires for Superconducting Planar Undulators

A project aiming to fabricate a full-length 2.8-m long Nb 3Sn superconducting undulator for the storage ring was started last year at the ANL APS. These Nb 3 Sn undulators operate at a maximum magnetic field on the conductor of about 5 T. To address instabilities at this field, two Nb 3 Sn wires with small subelement size were used. Specifically, Restacked Rod Processed wires of 0.6mmin diameter and with 144 and 150 superconducting subelements respectively, over 169 total. The equivalent subelement diameter, DS, of thesewires is35 mu m. At these smallDS values, the critical current density is known to deteriorate, and the Residual Resistivity Ratio is very sensitive to heat treatment. A delicate balance has therefore to be found to obtain parameters within operation specifications. In this paper we showperformance results from different heat treatments.

critical current density↗

DOE OSTI2026

Superconducting low-beta Nb 3 Sn cavity for ATLAS and future ion accelerators

We report on a Nb 3 Sn-coated low-beta superconducting radio frequency (RF) cavity intended for accelerating ions. We aim to apply the cavity in ATLAS, our Argonne National Laboratory user facility for nuclear physics studies with ion beams in the energy range of 5–20 MeV u −1 . The Nb 3 Sn-coated cavity, a 145 MHz quarter-wave optimized for ions moving with velocity β = $v/c$ = 0.08 exhibits an order-of-magnitude reduction in RF losses into helium at 4.4 K compared to a superconducting niobium (Nb) cavity at the same frequency and temperature. Experimentally measured fields are among the highest to date for any Nb 3 Sn-coated cavity, reaching a peak surface magnetic field of 105 mT. We also present a practical solution to the problem of cavity frequency tuning. Tuning by mechanical deformation has been a challenge with Nb 3 Sn due to its brittle nature, however, using a set of techniques tailored to the properties of thin-film Nb 3 Sn on Nb, we can repeatably tune the cavity to the ATLAS master clock frequency after it is cooled, while maintaining the excellent performance characteristics. The same Nb 3 Sn cavity technology offers broad benefits for future ion accelerators.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2024

Nuclear magnetic resonance investigation of superconducting and normal state Nb 3 Sn

The superconductor Nb3Sn has important applications for construction of very high-field superconducting magnets. In this work we investigate its microscopic electronic structure with 93 Nb nuclear magnetic resonance (NMR). The high-quality Nb 3 Sn powder sample was studied in both 3.2 T and 7 T magnetic fields in the temperature range from 4 K to 300 K. From measurement of the spectrum and its theoretical analysis, we find evidence for anisotropy despite its cubic crystal structure. Magnetic alignment of the powder grains in the superconducting state was also observed. The Knight shift and spin-lattice relaxation rate, T 1 -1 , were measured and the latter compared with BCS theory for the energy gap Δ(0) = 2.7 ± 0.3k B T c at 3.2 T and Δ(0) = 2.33 ± 0.07 k B T c at 7 T, indicating suppression of the order parameter by magnetic field.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗