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Forged ingot Nb is a novel processing method to obtain Nb discs used for the fabrication of superconducting radio-frequency cavities. This contribution reports on the fabrication, surface treatment and cryogenic RF testing of two 1.5 GHz single-cell cavities made of this material, with a residual resistivity ratio of ~100, at Jefferson Lab.
The HL-LHC at CERN will be using new superconducting quadrupole magnets (MQXFA) made of Nb₃Sn that are in the process of being tested before installation in the LHC tunnel. MQXFA07 and MQXFA08 magnets failed because of high stress developed at the coil wedge/end spacer interface, where the Nb₃Sn coil, the stainless-steel end parts, and copper wedges exhibit different strain responses. Interrogating a cut piece of coil QXFP06 with optical fibers, we expect to get a better understanding of the stress response of the coil wedge/end spacer interface.
The large-aperture, high-gradient Low Beta Quadrupoles for the High Luminosity Large Hadron Collider at CERN are the cornerstones of this collider. They are first-of-the-kind accelerator magnets bringing Nb_3Sn conductor to High Energy Physics colliders. In the US they are being fabricated and tested by the HL-LHC Accelerator Upgrade Project (AUP), and in Europe by CERN. The design was developed by AUP and CERN in close collaboration. The Q1/Q3 cryo-assemblies contain two 4.2 m long magnets (MQXFA) fabricated and by AUP. They are tested vertically before use in a cry-assembly. In this paper we are going to present the main features of the MQXFA design, results of vertical tests, and main lessons learned. We will also discuss lessons learned for future use of Nb_3Sn magnets in HEP colliders.
The few nanometers of the surface exposed to RF field plays a major role in defining the RF performance of superconducting cavities. Over the past two decades, several pioneering surface treatment and processing methods have emerged, enabling remarkable improvements in cavity performance by simultaneously achieving high Q with increasing Eacc. These processing methods include: thermal treatment under ultra-high vacuum (UHV) conditions across lo¬¬¬¬¬¬¬w-, mid-, and high-temperature ranges and high temperature treatments under controlled N2 atmosphere. These processes also produce distinct surface oxide configurations with different valence states, thicknesses, and uniformity, as well as different oxygen concentration profiles in bulk Nb. In this work, we are trying to understand how do surface-processing methods and the resulting oxide/oxygen profiles affect the electronic structure of surface and the mechanism of superconductivity? With the help of Fermilab’s in-house X-ray photoemission facility and, in collaboration with the synchrotron-based angle-resolved photoemission (ARPES) facility at Argonne National Laboratory, we are investigating how the valence band structure and density of states (DoS) near the Fermi level modify with different surface treatments. Our observations show that different surface-processing methods lead to distinct evolutions of the valence-band states near the Fermi level during the superconducting transition. This behavior suggests variations in Nb-O orbital hybridizations and points towards the possibility of different underlying mechanisms of superconductivity governed by the surface chemistry and oxide configuration. We also correlate these distinct superconducting mechanisms with RF cavity performance, specifically focusing on measured surface resistance, the nature of the Q-slope, and quench fields observed in SRF measurements. These results will enable us to identify the potential limiting factors and relevant controllable parameters that can be further optimized to improve the performance of SRF cavities.
Medium-temperature (mid-T) baking, typically conducted at 300 350 C, enhances the quality factor of niobium (Nb) superconducting radio frequency cavities. However, surface contamination introduced during vacuum furnace baking can limit the cavity performance. To investigate this effect, a 1.3 GHz single-cell Nb cavity underwent mid-T baking, followed by a chemical treatment to remove the surface contaminants. Post-treatment measurements revealed a significant improvement in both the quality factor and the quench field.
Superconducting-radio frequency (RF) cavities provide an efficient way to accelerate particle beams with extremely high acceleration gradients while generating very small power dissipation. The few nanometers of the surface play a critical role in defining the RF performance of superconducting Nb based cavities. Over the past two decades, several pioneering surface treatment and processing methods have emerged, enabling remarkable improvements in superconducting cavity performance by simultaneously achieving high quality factors with increasing maximum acceleration gradients. These processing approaches include chemical polishing, distinct multi-step thermal treatments under ultra-high vacuum (UHV) conditions over low to high temperature regimes, as well as high-temperature treatments under controlled nitrogen atmospheres. Beyond their macroscopic impact on RF performance, these methods produce distinct surface oxide configurations characterized by different valence states, oxide thicknesses, chemical uniformity, and oxygen concentration profiles extending into the near-surface bulk of niobium. In this work, we are trying to understand how the surface-processing methods and the resulting oxide/oxygen profiles affect the electronic structure of surface and the mechanism of superconductivity. Using a combination of X-ray photoemission and X-ray absorption spectroscopies, we investigate how the valence-band structure and the electronic density of states (DoS) near the Fermi level evolve under different surface treatments. By employing tunable photon energies across multiple elemental absorption edges, we use resonant photoemission to disentangle and identify the elemental contributions to specific valence-band features. Our observations show that different surface-processing methods lead to distinct temperature evolutions of the DoS and valence-band states near the Fermi level. Our results suggest variations in different Nb-O orbital hybridizations in distinct processes and point towards the possibility of different underlying mechanisms of superconductivity governed by surface chemistry and oxide configuration. We also correlate these distinct superconducting mechanisms with RF cavity performance, specifically focusing on measured surface resistance, the nature of the Q-slope, and quench fields observed in superconducting RF measurements. These results will enable us to identify the potential limiting factors and relevant controllable parameters that can be further optimized to improve the performance of superconducting RF cavities.
Main focus of this study is the investigation of thermodynamics phenomena responsible for the High Field Q Slope (HFQS) in SRF cavities by Internal Friction (IF) measurement. Mechanical spectroscopy is, indeed, a well-established technique to study precipitate formations in BCC materials and several works on the effects of impurities as N and O on the Snoek peak have been published so far and will be taken as reference to explain the mechanisms behind the observed dissipation effects. Internal Friction measurements were performed in Belgium at IMCE on Nb rectangular shape samples with different RRR values prepared at Fermilab by using Electro Polishing (EP), N-doping and heat treatments in order to reproduce the same conditions during the standard treatments applied on bulk Nb SRF cavities. From IF spectra, the H trapping mechanism by interstitial atoms (N and O and/or vacancies, depending on the purity level, RRR) can be easily recognized leading to results that perfectly corroborate previous findings on Q-disease, HFQS and RRR phenomena.
Interstitial hydrogen and deuterium in body-centered-cubic metals constitute archetypal quantum tunneling systems. Their relevance has been renewed by the connection between hydrogenic tunneling in Nb and defect-induced decoherence in superconducting qubits, motivating a predictive microscopic theory. Existing theoretical treatments invoke an adiabatic separation between the light interstitial and the host lattice, an assumption whose validity has not been rigorously established for hydrogenic species. Here, we show that the experimentally measured tunnel splittings of O-trapped H and D in bcc Nb are quantitatively reproduced only within a five-dimensional (5D) Lattice-Renormalized Born-Oppenheimer (LRBO) framework. This approach treats three interstitial modes and two judiciously selected lattice modes, which includes a transition-state mode, on equal quantum footing. By recasting nested Born-Oppenheimer hierarchies within this same formalism and benchmarking against modern \textit{ab initio} potential energy surfaces, we show that adiabatic separation of the light particle from lattice dynamics is satisfied only in the positive-muon ($μ^{+}$) mass limit. In contrast, tunneling for H and D is fundamentally a collective, nonadiabatic process mediated by anharmonic lattice couplings. Finally, we show that the breakdown of adiabaticity can be anticipated from simple energy estimates involving the ground-state light-particle energy evaluated at a small number of fixed lattice configurations, providing a practical criterion for assessing the validity of adiabatic tunneling theories in other systems.
P2NbS8 crystallizes in the tetragonal P-4n2 space group. The structure is three-dimensional. Nb5+ is bonded to eight S+1.88- atoms to form distorted NbS8 hexagonal bipyramids that share edges with two equivalent PS4 tetrahedra and a faceface with one NbS8 hexagonal bipyramid. There are a spread of Nb–S bond distances ranging from 2.52–2.71 Å. P5+ is bonded to four S+1.88- atoms to form PS4 tetrahedra that share corners with two equivalent PS4 tetrahedra and an edgeedge with one NbS8 hexagonal bipyramid. There are a spread of P–S bond distances ranging from 1.99–2.14 Å. There are four inequivalent S+1.88- sites. In the first S+1.88- site, S+1.88- is bonded in an L-shaped geometry to one Nb5+ and one P5+ atom. In the second S+1.88- site, S+1.88- is bonded in a 2-coordinate geometry to two equivalent Nb5+ and one S+1.88- atom. The S–S bond length is 2.02 Å. In the third S+1.88- site, S+1.88- is bonded in an L-shaped geometry to one Nb5+ and one P5+ atom. In the fourth S+1.88- site, S+1.88- is bonded in a water-like geometry to two equivalent P5+ atoms.
P2NbS8 crystallizes in the orthorhombic Cmce space group. The structure is two-dimensional and consists of two P2NbS8 sheets oriented in the (0, 0, 1) direction. Nb5+ is bonded in a 8-coordinate geometry to eight S+1.88- atoms. There are a spread of Nb–S bond distances ranging from 2.52–2.68 Å. P5+ is bonded to four S+1.88- atoms to form edge-sharing PS4 tetrahedra. There are a spread of P–S bond distances ranging from 2.00–2.15 Å. There are five inequivalent S+1.88- sites. In the first S+1.88- site, S+1.88- is bonded in a 2-coordinate geometry to two equivalent Nb5+ and one S+1.88- atom. The S–S bond length is 2.02 Å. In the second S+1.88- site, S+1.88- is bonded in an L-shaped geometry to two equivalent P5+ atoms. In the third S+1.88- site, S+1.88- is bonded in an L-shaped geometry to one Nb5+ and one P5+ atom. In the fourth S+1.88- site, S+1.88- is bonded in an L-shaped geometry to one Nb5+ and one P5+ atom. In the fifth S+1.88- site, S+1.88- is bonded in a 2-coordinate geometry to two equivalent Nb5+ and one S+1.88- atom.
NbCl6N2 is Fluorite structured and crystallizes in the cubic Fm-3m space group. The structure is zero-dimensional and consists of eight ammonia molecules and four NbCl6 clusters. In each NbCl6 cluster, Nb4+ is bonded in an octahedral geometry to six equivalent Cl1- atoms. All Nb–Cl bond lengths are 2.37 Å. Cl1- is bonded in a single-bond geometry to one Nb4+ atom.
NbBr6N2 is Fluorite structured and crystallizes in the cubic Fm-3m space group. The structure is zero-dimensional and consists of eight ammonia molecules and four NbBr6 clusters. In each NbBr6 cluster, Nb4+ is bonded in an octahedral geometry to six equivalent Br1- atoms. All Nb–Br bond lengths are 2.54 Å. Br1- is bonded in a single-bond geometry to one Nb4+ atom.
The microstructure of a γ-TiAl alloy containing niobium undergoes continuous transformations during annealing at 550 °C. These take place within the α 2 -phase of lamellar (α 2 +γ) colonies. We have characterized these changes by transmission electron microscopy and made a correlation to the elastic constants determined by resonant ultrasonic spectroscopy during in-situ annealing at the same temperature. Further, the results show a continuous increase of the E-modulus. This can be attributed to elastic strains acting within α 2 lamellae as a result of lattice transformations. After 5000h, a thermodynamic equilibrium was established with equal fractions of α 2 -phase and O-phase that differ only in lattice symmetry and Nb content. Generally, the presence of O-phase should contribute to lower elastic moduli, however in the present case the evolving microstructural characteristics were responsible for the observed increase in the E-modulus.
Recent developments in additive manufacturing (AM) technology have reignited interest in the fabrication of the Nb-based refractory C103 alloy offering solutions to the challenges posed by traditional manufacturing methods. However, the limited numerical and experimental studies on laser direct energy deposition (DED) of C103 have hindered the understanding of the relationships between process parameters and build quality. This has made it challenging to consistently produce parts with the desired quality and microstructure suitable for critical applications. In this study, we focus on optimizing the laser DED process for C103 by employing a hybrid approach that combines experimental techniques and computational fluid dynamics (CFD). This approach facilitates the development of process maps for defect detection and geometric precision. To achieve this, multi-layer C103 samples were fabricated using laser DED under various process parameters, enabling the creation of a process map for defect detection. Additionally, a multi-physics, multiphase simulation framework was developed within a high-performance computing (HPC) environment to establish process maps for geometric precision. Using these process maps, printability windows were identified for achieving both the desired geometric accuracy and defect-free prints. It was observed that prints with a power-to-velocity (P/V) ratio close to unity resulted in defect-free outcomes. This study provides a foundation for reducing design lead time and rejected parts, ultimately optimizing the laser DED process for C103.
Abstract-The Electron Ion Collider (EIC) will require large aperture quadrupole magnets for the Hadron beam in the insertion region. Key requirements include high field, compact size, and tight control of the fringe fields. A 120 mm aperture, 308.4 mm outer diameter actively shielded Nb 3 Sn quadrupole model is under development to support these goals. This work is being carried out by a collaboration of BNL, JLAB and LBNL. A compact shell-based structure preloaded with a bladder and key system was developed for this project. In this paper, the effect of the compact structure on the mechanical behavior was investigated. In particular, the impact of the assembly tolerances and coil size variations on the actual coil stresses and bladder pressures was computed and compared with results from larger bladder and key structures developed for the LHC IR. The longitudinal preload is provided by stainless steel rods. Differently from other bladder and key magnets, the rods are first preloaded axially before coil axial load is applied. This new design aims to increase the overall stiffness of the system and reduce the longitudinal coil displacement during powering. Based on the results of a 3D mechanical analysis, the preliminary preload targets for the EIC quadrupole assembly will be reviewed and discussed.
In a dipole or in a quadrupole accelerator magnet, the displacement of the coil turns induced by the electromagnetic forces can cause quenches limiting the magnet performance. For this reason, an azimuthal preload is applied to avoid azimuthal movements of the coil up to the required operational current. However, several tests showed that accelerator magnets can operate with a partial preload, i.e. that coil unloading during the ramp does not prevent reaching higher currents. This issue is particularly relevant for Nb 3 Sn magnets, where the loads applied to the Nb3Sn filaments can reach the degradation limits of critical current. In order to investigate the impact of coil preload on the quench performance, the MQXFS6 short model quadrupole for the High Luminosity Upgrade was tested under an azimuthal preload at 80% of the short sample current, reaching 93% of short sample current at 1.9 K. The preload was then released to 60%, still showing ability to operate in the range of 80-85% of short sample current as required by HL-LHC project. With this lower preload, the ability of going above 90% of short sample was lost, and a significant training appeared above 85%. When the preload was restored to the original 80% value, the magnet reached with few quenches 95% of short sample (13.4 T peak field). Magnetic measurements confirm the larger movement of the coil in the case with lower preload, and agree with finite element simulations.
Ameliorating electronic/ionic transport and structural stability of electrode materials is important to the development of power-intensive lithium ion batteries. Despite its great potential as a high-power anode, titanium niobium oxide (Ti 2 Nb 10 O 29 , TNO) still underperforms due to its unsatisfactory electronic/ionic conductivity. In this work, a powerful synergistic strategy by combining ion doping and spiral array architecture to boost high-rate performance of TNO is reported. Cr 3+ doped TNO nanoparticles (Cr-TNO) of 5-10 nm intimately grow on a conductive vertical graphene@TiC-C (VGTC) skeleton, forming novel Cr-TNO@VGTC spiral arrays. The unique spiral growth of TNO is achieved due to the confinement effect of VGTC skeleton. Meanwhile, a more open TNO crystal structure with faster ion transfer paths and enhanced structural stability is realized by Cr 3+ doping, demonstrated via density functional theory calculation and in situ synchrotron X-ray diffraction technique. Benefiting from the superior conductive network, enhanced intrinsic electronic/ionic conductivity of Cr-TNO and reinforced structural stability, the Cr-TNO@VTC arrays show prominent high-power performance with a large capacity of 220 mAh g -1 at 40 C (power density of ≈11 kW kg -1 ) and superior durability (91% retention after 500 cycles). This work provides a new path for the construction of widespread high-power electrodes for fast energy storage.
One-dimensional (1D) van der Waals materials, such as nanofibers or nanoribbons are considered as the future ultimate limit of downscaling for modern electrical and electrochemical devices. Here, for the first time, we successfully synthesize nanofibers of a solid solution transition metal trichalcogenide (TMTC), Nb 1-x Ta x S 3 , with outstanding electrical, thermal, and electrochemical characteristics rivaling the performance of the-state-of-the art materials for each application. This material shows nearly unchanged sheet resistance (≈740 Ω/sq -1 ) versus bending cycles tested up to 90 cycles, stable sheet resistance in ambient conditions tested up to 60 days, remarkably high electrical breakdown current density of ≈ 30 MA cm -2 , strong evidence of successive charge density wave (CDW) transitions, and outstanding thermal stability up to ≈900 K. Additionally, this material demonstrates excellent activity and selectivity for CO2 conversion to CO reaching ≈ 350 mA cm -2 at – 0.8 V vs RHE with an outstanding turnover frequency number of 25 for CO formation. It also exhibits an excellent performance in a high-rate Li-air battery with the specific capacity of 3000 mAhg -1 at a high current density of 0.3 mAcm -2 . This study uncovers the multifunctionality in 1D TMTC alloys for a wide range of high-impact applications and opens a new direction for the design of the next generation of low-dimensional materials with high performance.