Niobium Superconducting Radio Frequency Cavities Research Articles

The Influence of Strain Path and Heat Treatment Variations on Recrystallization in Cold-Rolled High-Purity Niobium Polycrystals

With the standard material and cavity production guidelines, the properties and performance of industrially produced high-purity niobium superconducting radiofrequency (SRF) cavities are unpredictably variable. Currently, assembled niobium SRF cavities undergo post-purification vacuum heat treatment at 800°C for three hours to remove hydrogen; however, recent work has shown significant improvements in cavity performance when annealed between 900-1000°C, causing recrystallization (Rx). Rx is a thermally activated process, so increasing the annealing temperature and/or the heating rate should facilitate the onset of Rx, resulting in a greater reduction in the density of geometrically necessary dislocations (GNDs) that are strongly associated with the degradation of cavity performance via trapped magnetic flux. Improving the SRF community's understanding of the influence of both heat treatment and deformation history on the resulting microstructure and Rx behavior in high-purity niobium is needed. This fundamental metallurgy is critical for identifying improved specifications for the consistent production of high-performing niobium SRF cavities. This research investigates the effects of heat treatment variations with different deformation paths, heating rates, and annealing temperatures on deformed samples of polycrystalline high-purity niobium. To assess the effects of strain path, samples were cold-rolled to ∼30% reduction prior to annealing, with half of the sample set rolled parallel to the original rolling direction of the as-received niobium sheet and the other half in the transverse direction.

(Industrial Electrochemistry and Electrochemical Engineering Division New Electrochemical Technology (NET) Award) Pulse Reverse HF-Free Electropolishing of Niobium Superconducting Radio Frequency (SRF) Cavities for Nuclear Physics and High Energy Physics Applications

On behalf of Faraday Technology, Inc., we are honored to receive the 2021 New Electrochemical Technology (NET) award of the Industrial Electrochemistry & Electrochemical Engineering (IE&EE) division for pulse reverse FARADAYIC® HF-FREE Electropolishing of niobium SRF cavities. The NET award was endowed by the Dow Chemical Company Foundation to recognize significant advances in industrial electrochemistry. We addressed the subject innovation as a multidisciplinary team of electrochemists, engineers and technologists.Niobium superconducting radio frequency (SRF) cavities are used in particle accelerators such as the Large Hadron Collider (LHC) to support nuclear/high energy physics applications. To achieve their required performance, the niobium SRF cavities are electropolished using nine-parts sulfuric acid to one-part hydrofluoric acid.[1] The conventional process follows the well-established “viscous salt-film” model for electropolishing.[2] The conventional electrolyte imparts an extreme hazard to workers requiring costly safety protocols with extensive worker protection (see Fig. 1a).[3] ,[4] To commercialize niobium SRF cavity electropolishing, a simpler, cost-effective industrially compatible process was required.We developed a pulse reverse process to electropolish niobium coupons in low concentration (<5 wt%) aqueous sulfuric acid electrolytes devoid of hydrofluoric acid.[5] The surface roughness was the same as that observed using conventional DC electropolishing.[6] We transitioned the work from coupons to single-cell niobium cavities SRF and the performance was the highest observed at the DOE’s Fermi National Accelerator Facility.[7] ,[8] Due to the avoidance of the safety protocols associated with the conventional process (see Fig. 1b), the capital and operating costs of the pulse reverse electropolishing process were considerably less than the conventional process.[9] Finally, the pulse reverse process was patented and transitioned to DOE’s Thomas Jefferson National Accelerator Facility.[10] ,[11] Finally, the pulse reverse process was adapted to other strongly passive materials.While reviewing the technology developments leading to and associated with the niobium pulse reverse electropolishing process, we will illustrate lessons learned during the development of industrial innovations.[12] [1] H. Tian, S. Corcoran, C. Reece, and M. Kelly, J. Electrochem. Soc., 155, D563 (2008). [2] P. A. Jacquet, Trans. Electrochem. Soc., 69, 629 (1936). [3] T. Dote and K. Kono, Jap. J. Occupational Medicine and Traumatology, 52, 3, 189 (2004). [4] J. Mammoser, “Chemical Safety Awareness for SRF Cavity Work” U.S. Particle Accelerator School, January 19, 2015 (accessed June 9, 2020) https://www.jlab.org/indico/event/98/ other-view?fr=no&detailLevel=contribution&view=standard& showSession=all&showDate=all [5] M. Inman, E. J. Taylor, and T. D. Hall, J. Electrochem. Soc., 160, E94 (2013). [6] M. Inman, T. Hall, E. J. Taylor, C. Reece, and O. Trofimova, Paper TOPO012, Proc. SRF2011, Chicago, IL (2011). [7] E. J. Taylor, T. Hall, M. Inman, S. Snyder, and A. Rowe, Electropolishing of Niobium SRF Cavities in Low Viscosity Aqueous Electrolytes without Hydroflouric Acid, Paper TUP054, Proc. SRF2013, Paris, France, Sept 2013. (ISBN 978- 3-95450-143-4) [8] A. Rowe, A. Grassellino, T. Hall, M. Inman, S. Snyder, and E. Taylor, Paper No. TUIOC02, Proc. SRF2013, Paris, France, Sept 2013. (ISBN 978-3-95450-143-4) [9] E. J. Taylor, M. Inman, T. Hall, S. Snyder, A. Rowe, and D. Holmes, Paper MOPB092, Proc. SRF2015, Whistler, BC, Canada (2015). [10] E. J. Taylor, M. E. Inman, and T. D. Hall, U.S. Pat. No. 9,006,147 issued April 14, 2015. [11] E. J. Taylor, M. E. Inman, and T. D. Hall, U.S. Pat. No. 9,987,699 issued June 5, 2018. [12] E.J. Taylor, M.E. Inman, H.M. Garich, H.A. McCrabb, S.T. Snyder, T.D. Hall in Electrochemical Engineering: The Path from Discovery to Product Alkire, Bartlett, Koper (eds) Wiley-VCH (2019). Figure 1

Effect of inhomogeneous surface disorder on the superheating field of superconducting RF cavities

Recent advances in surface treatments of Niobium superconducting radio frequency (SRF) cavities have led to substantially increased Q-factors and maximum surface field. This poses theoretical challenges to identify the mechanisms responsible for such performance enhancements. We report theoretical results for the effects of inhomogeneous surface disorder on the superheating field --- the surface magnetic field above which the Meissner state is globally unstable. We find that inhomogeneous disorder, such as that introduced by infusion of Nitrogen into the surface layers of Niobium SRF cavities, can increase the superheating field above the maximum for superconductors in the clean limit or with homogeneously distributed disorder. Homogeneous disorder increases the penetration of screening current, but also suppresses the maximum supercurrent. Inhomogeneous disorder in the form of an impurity diffusion layer biases this trade-off by increasing the penetration of the screening currents into cleaner regions with larger critical currents, thus limiting the suppression of the screening current to a thin dirty region close to the surface. Our results suggest that the impurity diffusion layers play a role in enhancing the maximum accelerating gradient of Nitrogen treated Niobium SRF cavities.

Revealing the role of nitrogen on hydride nucleation and stability in pure niobium using first-principles calculations

Niobium provides the basis for all superconducting radio frequency (SRF) cavities in use, however, hydrogen is readily absorbed by niobium during cavity fabrication and subsequent niobium hydride precipitation when cooled to cryogenic temperatures degrades its superconducting properties. In the last few years the addition of dopant elements such as nitrogen has been experimentally shown to significantly improve the quality factor of niobium SRF cavities. One of the contributors to Q degradation can be presence of hydrides; however, the underlying mechanisms associated with the kinetics of hydrogen and the thermodynamic stability of hydride precipitates in the presence of dopants are not well known. Using first-principles calculations, the effects of nitrogen on the energetic preference for hydrogen to occupy interstitial sites and hydride stability are examined. In particular, the presence of nitrogen significantly increased the energy barrier for hydrogen diffusion from one tetrahedral site to another interstitial site. Furthermore, the beta niobium hydride precipitate became energetically unstable upon addition of nitrogen in the niobium matrix. Through electronic density of states and valence charge transfer calculations, nitrogen showed a strong tendency to accumulate charge around itself, thereby decreasing the strength of covalent bonds between niobium and hydrogen atoms leading to a very unstable state for hydrogen and hydrides. These calculations show that the presence of nitrogen during processing plays a critical role in controlling hydride precipitation and subsequent SRF properties.

Magnesium diboride coated bulk niobium: a new approach to higher acceleration gradient.

Bulk niobium Superconducting Radio-Frequency cavities are a leading accelerator technology. Their performance is limited by the cavity loss and maximum acceleration gradient, which are negatively affected by vortex penetration into the superconductor when the peak magnetic field at the cavity wall surface exceeds the vortex penetration field (Hvp). It has been proposed that coating the inner wall of an SRF cavity with superconducting thin films increases Hvp. In this work, we utilized Nb ellipsoid to simulate an inverse SRF cavity and investigate the effect of coating it with magnesium diboride layer on the vortex penetration field. A significant enhancement of Hvp was observed. At 2.8 K, Hvp increased from 2100 Oe for an uncoated Nb ellipsoid to 2700 Oe for a Nb ellipsoid coated with ~200 nm thick MgB2 thin film. This finding creates a new route towards achieving higher acceleration gradient in SRF cavity accelerator beyond the theoretical limit of bulk Nb.

Keynote: Electropolishing of Niobium Superconducting Radio Frequency Cavities in a Low-Viscosity, Water-Based, HF-Free Electrolyte: From Coupons to Cavities

Niobium superconducting radio frequency cavities (SRF) are required for the International Linear Collider as well as other high energy physics projects. In order for these cavities to achieve the required particle acceleration gradients, electropolishing is conducted as a final surface finishing operation. Conventional electropolishing of SRF cavities is based on the well-established viscous salt film theory [1] and utilizes a viscous electrolyte consisting of a mixture of sulfuric acid (95-98%) and hydrofluoric acid (49%), in a 9:1 volume ratio.[2] The HF is included to depassivate the niobium oxide film during electropolishing.[3]Based on prior surface finishing activities using pulse reverse waveforms in low viscosity aqueous electrolytes, we speculated that a new paradigm was evolving and could be applied to electropolishing of SRF cavities. Initial research funds were provided from the Small Business Innovation Research program to demonstrate the feasibility of electropolishing niobium coupons in aqueous electrolytes using “Bipolar EP.” Although coupon polishing was successful, further funding was not provided due at least in part to skepticism by the review community familiar with the conventional viscous salt film theory. Subsequently, the American Recovery and Reinvestment Act funded a high risk/high payoff project to demonstrate the bipolar EP process on single cell SRF cavities. The ARRA funds did not allow for mechanistic studies or preliminary experiments on curved sections representative of the SRF cavity geometry.Within these funding constraints, Faraday used guidelines and observations from 1”x1” and 3”x3” coupons to scale the bipolar EP process to single cell SRF cavities of ~1,800cm2 surface area. We recently reported these results at the 16th International Conference on RF Superconductivity in collaboration with Fermi National Accelerator Laboratory who declared that this cavity demonstrated the highest performance of any cavity previously processed at FNAL.The presentation will focus on our development activities from coupons to cavities and the evolution of key observations required to electropolish niobium.[4] These key observations on niobium coupons were used to guide the development of bipolar EP process parameters on single cell SRF cavities even while the detailed mechanistic aspects were not completely understood. We conclude that innovation requires a careful balance between current fundamental understandings combined with evolving observation which do not necessarily fit the current paradigm. The financial support of Faraday corporate, DOE P.O. No. 594128 and DOE Contract No. DE-SC0004588, and the technical support of Mr. Allan Rowe at Fermilab are acknowledged. [1] P.A. Jacquet Trans. Electrochem. Soc. 69, 629 (1936). [2] H. Tian, S. Corcoran, C. Reece, M. Kelly, J. Electrochem. Soc., 155, D563-568 (2008).[3] MacDougall, B. (1995), The Importance of Surface Oxide Films in Corrosion, Semiconductor and Environmental Research, Proceedings of the Symposium on High Rate Metal Dissolution Processes, Electrochemical Society Proceedings Vol. 95-19, Eds. M. Datta, B. MacDougall and J. Fenton, The Electrochemical Society, Pennington, NJ, pp 16-31.[4] Inman, M., E.J. Taylor and T.D. Hall, Electropolishing of Passive Materials in HF-Free Low Viscosity Aqueous Electrolytes, J. Electrochem. Soc., 160 (9), E94-E98, 2013.

Optimization of gaseous helium heater for 2 K cryogenic system for VECC’s superconducting electron linac

Niobium superconducting radiofrequency cavities are generally operated at around 2K temperature to achieve a high quality factor by reducing residual surface losses. 2K temperature is produce by lowering down the pressure of the helium by employing a sub-atmospheric vacuum pumping system. The cavities are immersed in liquid helium bath, maintained in the helium chamber. A special heater is optimized for warming up the helium gas coming out from the helium chamber to 300K before it enters the pumping system. Keeping in view the uninterrupted and reliable operation of the superconducting electron linac and safe running of the liquid helium plant, a tubular heat exchanger type of heater is designed. Current is passed through the tubes of the heater so as to let the tube banks themselves act as heating element. He gas, passing through the tubes, absorbs the heat and warms up to the desired temperature. Unlike common notion, it has been observed that heater with longer length could reduce the requirement of the heater power but at the cost of extra pumping power, required to counter balance the excess pressure drop caused by the additional length of the heater. Pressure drop is kept within 50Pa for 2g/s helium flow rate. The whole lot of tubes, divided into 4 bundles, are electrically connected in series so that current rating of the feed-through could be kept within 750A. This paper discusses the methodology used for optimizing the design of the heater.

Suppression of hydride precipitates in niobium superconducting radio-frequency cavities

Niobium hydride is a suspected contributor to degraded niobium superconducting radio-frequency (SRF) cavity performance by Q slope and Q disease. The concentration and distribution of hydrogen atoms in niobium can be strongly affected by the cavity processing treatments. This study provides guidance for cavity processing based on density functional theory calculations of the properties of common processing impurity species—hydrogen, oxygen, nitrogen, and carbon—in the body-centered cubic (bcc) niobium lattice. We demonstrate that some fundamental properties are shared between the impurity atoms, such as anionic character in niobium. The strain field produced, however, by hydrogen atoms is both geometrically different and substantially weaker than the strain field produced by the other impurities. We focus on the interaction between oxygen and hydrogen atoms in the lattice, and demonstrate that the elastic interactions between these species and the bcc niobium lattice cause trapping of hydrogen and oxygen atoms by bcc niobium lattice vacancies. We also show that the attraction of oxygen to a lattice vacancy is substantially stronger than the attraction of hydrogen to the vacancy. Additionally, hydrogen dissolved in niobium tetrahedral interstitial sites can be trapped by oxygen, nitrogen and possibly carbon atoms dissolved in octahedral interstitial sites. These results indicate that the concentration of oxygen in the bcc lattice can have a strong impact on the ability of hydrogen to form detrimental phases. Based on our results and a literature survey, we propose a mechanism for the success of the low-temperature annealing step applied to niobium SRF cavities. We also recommend further examination of nitrogen and carbon in bcc niobium, and particularly the role that nitrogen can play in preventing detrimental hydride phase formation.

Effect of cathode shape on vertical buffered electropolishing for niobium SRF cavities

This paper reports the research results of the effect of cathode shape during vertical buffered electropolishing (BEP) by employing a demountable single cell niobium (Nb) superconducting radio frequency (SRF) cavity. Several different cathode shapes such as, for instance, bar, ball, ellipsoid, and wheels of different diameters have been tested. Detailed electropolishing parameters including I–V characteristic, removal rate, surface roughness, and polishing uniformity at different locations inside the demountable cavity are measured. Similar studies are also done on conventional electropolishing (EP) for comparison. It is revealed that cathode shape has dominant effects for BEP especially on the obtaining of a suitable polishing condition and a uniform polishing rate in an Nb SRF single cell cavity. EP appears to have the same tendency. This paper demonstrates that a more homogeneous polishing result can be obtained by optimizing the electric field distribution inside the cavity through the modification of the cathode shape given the conditions that temperature and electrolyte flow are kept constant. Electric field distribution and electrolyte flow patterns inside the cavity are simulated via Poisson–Superfish and Solidworks respectively. With the optimal cathode shape, BEP shows a much faster polishing rate of ∼2.5μm/min and is able to produce a smoother surface finish in the treatments of single cell cavities in comparison with EP.

Laser nitriding of niobium for application to superconducting radio-frequency accelerator cavities

Particle accelerators are a key tool for scientific research ranging from fundamental studies of matter to analytical studies at light sources. Cost-for-performance is critical, both in terms of initial capital outlay and ongoing operating expense, especially for electricity. The major factor is the niobium superconducting radio frequency (SRF) accelerator cavities at the heart of many of these machines. Presently, niobium SRF cavities operate near 1.9 K, well below the 4.2 K atmospheric boiling point of liquid helium to obtain sufficient performance. The consequent electric power costs are the most significant limit to operate the SRF cavities at 1.9 K. Transforming the cavity interior surface from niobium to δ niobium nitride (δNbN) with a critical temperature (Tc) ≅ 17 K instead of 9.2 K, appears to be a promising approach to raising the operating temperature. The traditional furnace method has nitrided niobium, but apparently have not been able to obtain δNbN.1 Moreover, furnace nitriding requires exposing the complete SRF cavity to an aggressive time-temperature history, risking mechanical distortion. As an alternative, laser gas nitriding has been applied successfully to a number of metals.2 A very recent review is available.3 The beam dimensions and thermal diffusion length permit modeling in one dimension to predict the time course of the surface temperature for a range of per-pulse energy densities. As with the earlier work,2 we chose conditions just sufficient for boiling of the niobium surface as a reference point. The treated materials were examined by scanning electron microscopy (SEM), electron probe microanalysis and x ray diffraction (XRD). The SEM images show a sharp transition with fluence from a smooth, undulating topography to significant roughening, interpreted here as the onset of ablation. Electron probe microanalysis measurements found a constant value of the nitrogen/niobium atom ratio to depths greater than the SRF active layer thickness. Certain irradiation conditions resulted in atomic ratio values consistent with formation of δNbN, and XRD data indicated only δNbN on top of the niobium metal.

Development of Vertical Buffered Electropolishing for Its Post-Treatment Technology on 1.5 GHz Niobium SRF Cavities

We report the latest research development of vertical buffered electropolishing on its post-treatment procedure as well as the effects of several major post-treatment techniques for buffered electropolishing (BEP) processed 1.5 GHz niobium (Nb) superconducting radio frequency (SRF) cavities. With the established post-treatment procedure, an accelerating gradient of 28.4 MV/m is obtained on a single cell cavity of the cebaf shape. This is the best result in the history of BEP development. The cavity is limited by quench with a high quality factor over 1.2 × 1010 at the quench point. Analyses from optical inspection and temperature-mapping show that the quench should be originated from the pits that were already present on the cavity before this BEP treatment. All of these factors indicate that this procedure will have a great potential to produce better results if cavities without intrinsic performance limiting imperfections are used.