Superconducting RF Research Articles

RF conditioning towards continuous wave of the RFQ of the Linear IFMIF Prototype Accelerator

The Linear IFMIF Prototype Accelerator (LIPAc) is designed to accelerate 125mA of deuteron beam to 9MeV in continuous wave (CW). The superconductive RF Linac has not yet been installed and the final accelerating stage now under commissioning is the RFQ. This system has been designed and developed by INFN (Italy) before installation in QST (Japan). The RFQ is the longest in the World with its 9.8m and requires RF power injection from 8 independent and synchronized coupler ports. LIPAc demonstrated the acceleration of 125mA deuteron beam at 5MeV for 1ms with a 1s repetition period in 2019. A fundamental milestone to extend beam operations to CW is the completion of the RFQ cavity RF conditioning up to CW. This work presents the strategy followed to successfully reach CW RF injection at 80% of the nominal 132kV vane voltage. The field distribution correction scheme (acting on cooling system at various power levels) was successfully verified. We discuss as well the main challenges encountered on the way, which include updates of the RF system, failure of a circulator (by arcs) and the damages occurred on some of the RF couplers. Finally, the recent status and outlook will be provided.

Development of EPICS-Based Data Acquisition system for Beam Loss Monitor and sX-map

Beam commissioning is underway at KEK’s Superconducting RF Test Facility (STF). Since beam diagnostics are important to realize stable operation, we have developed a time-resolved beam loss distribution monitor as one of the beam diagnostics. This monitor uses up to 16 PIN photodiodes as X-ray sensors to observe X-rays generated due to beam loss of a pulsed beam with a width of approximately 800 μs, which is divided into 100 μs intervals. Thus, 128 data can be acquired per bunch. The data is read out by an EPICS-based data acquisition system. On the other hand, we are also developing an EPICS-based data acquisition system for an inspection method for the superconducting cavities(sX-map) for the vertical test of bare cavities. sX-map strips are inserted inside the stiffener ring at the iris of superconducting cavities. In this presentation, an overview of these EPICS-based data acquisition systems and their integration with the existing control system will be presented.

Evidence for current suppression in superconductor–superconductor bilayers

Superconducting radio frequency (SRF) cavities, which are critical components in many particle accelerators, need to be operated in the Meissner state to avoid strong dissipation from magnetic vortices. For a defect-free superconductor, the maximum attainable magnetic field for operation is set by the superheating field, Bsh , which directly depends on the surface current. In heterostructures composed of different superconductors, the current in each layer depends not only on the properties of the individual material, but also on the electromagnetic response of the adjacent layers through boundary conditions at the interfaces. Three prototypical bilayers [ Nb1−xTixN (50nm)/Nb , Nb1−xTixN (80nm)/Nb , and, Nb1−xTixN (160nm)/Nb ] are investigated here by depth-resolved measurements of their Meissner screening profiles using low energy muon spin rotation (LE-µSR). From fits to a model based on London theory (with appropriate boundary and continuity conditions), a magnetic penetration depth for the thin Nb1−xTixN layers of λNb1−xTixN=182.5(31)nm is found, in good agreement with literature values for the bulk alloy. Using the measured λNb1−xTixN , the maximum vortex-free field, Bmax , of the superconductor-superconductor (SS) bilayer structure was estimated to be 610(40)mT . The strong suppression of the surface current in the Nb1−xTixN layer suggests an optimal thickness of ∼1.4λNb1−xTixN=261(14)nm.

A DFT study on nitrogen enhancing mechanism of SRF niobium cavity performance

High-temperature nitriding has been shown to improve the quality factor (Q) of the superconducting radio frequency (SRF) cavity recently, while its inherent promoting mechanism is still unclear. Therefore, based on first-principles calculations, the adsorption and dissociation processes of nitrogen molecules on the Nb(110) surface, and the adsorption, diffusion processes of nitrogen atoms on the Nb(110) surface, were studied in this paper to explain the principle of its performance improvement. To establish the reaction mechanism, adsorption preference sites were determined. All possible reaction paths were investigated for each step including the energy barrier based on the climbing image nudged elastic band (CI-NEB) method. We establish multiple reaction paths and the only optimal one was found. It is found that the Nb(110) surface can facilitate the dissociation of nitrogen molecules (with only a 1.14 eV energy barrier), and nitrogen atomic energy can readilyreach the subsurface (only a 1.73 eV energy barrier). Still, the high energy barrier (up to 6.64 eV) prevents it from diffusing inward. This revelation provides the nitriding mechanism of pure niobium and the rationale for enhancing niobium cavity performance after nitrogen mixing.

Thermal annealing of DC sputtered Nb3Sn and V3Si thin films for superconducting radio-frequency cavities

Nb 3 Sn and V3Si thin films are promising candidates for the next generation of superconducting radio-frequency (SRF) cavities. However, sputtered films often suffer from stoichiometry and strain issues. This exploratory study investigates the structural and chemical effects of thermal annealing, both in−situ and post-sputtering, on DC-sputtered Nb3Sn and V3Si films with varying thicknesses, deposited on Nb or Cu substrates. Building upon our initial studies [Howard et al., Proceedings of the SRF’21, East Lansing, MI (JACoW, 2021), p. 82.], we provide fundamental insights into recrystallization, phase changes, and the issues of stoichiometry and strain. Through annealing at 950 °C, we have successfully enabled the recrystallization of 100 nm thin Nb3Sn films on Nb substrates, yielding stoichiometric and strain-free grains. For 2 μm thick films, elevated annealing temperatures led to the removal of internal strain and a slight increase in grain size. Moreover, annealing enabled a phase transformation from an unstable to a stable structure in V3Si films, while we observed significant Sn loss in 2 μm thick Nb3Sn films after high-temperature annealing. Similarly, annealing films atop Cu substrates resulted in notable Sn and Si loss due to the generation of Cu–Sn and Cu–Si phases, followed by evaporation. These results encourage us to refine our process to obtain high-quality sputtered films for SRF use.

Effect of defects on flux expulsion of pure niobium superconducting radio frequency cavities via spatial temperature gradient

Since flux vortices are easily pinned by defects in superconducting radio frequency (SRF) cavities, trapped magnetic flux increases the surface resistance of the SRF cavity. It has been experimentally demonstrated that trapped magnetic flux can be expelled by cooling down a superconducting cavity under a large spatial temperature gradient. In this paper, we use the time-dependent Ginzburg–Landau (TDGL) theory to theoretically investigate the effect of defects on the flux expulsion ability of pure niobium superconducting cavities with an applied spatial temperature gradient. It is shown that the residual flux vortex density displays a notable stepwise decreasing trend with increasing temperature gradient at smaller residual fields, while it shows a stepwise rising with increasing defect size. Both larger defect number and size make vortices difficult to be expelled. Moreover, long cooling time is beneficial for vortices to be expelled, and the cooling time has nearly no significant effect on flux expulsion under conditions of large cooling times. The results in this paper provide some theoretical guidelines for improving the performance of superconducting cavities in practical situations.

Insight into reactive oxygen plasma characteristics and reaction mechanism on SRF accelerator plasma cleaning

Reactive oxygen plasma treatment is an effective technique to eliminate hydrocarbon and improve the performance of superconducting radio frequency (SRF) cavities. This work investigated the reaction mechanism between reactive oxygen plasma and hydrocarbon, surface reaction kinetics, and cleaning process optimization through experiments, analytical models, and numerical simulations. The experimental results declare that the reaction between oxygen plasma and hydrocarbon is dominated by ion-assisted chemical sputtering, and the hydrocarbon attenuates exponentially, increasing the work function exponentially. To study the surface reaction kinetic process in-depth, we proposed a plasma cleaning rate model based on the Langmuir–Hinshelwood theory. This study found that the plasma cleaning rate primarily depends on the sheath potential, electron temperature, O atoms density, O+ ions, and Ar+ ions densities. Furthermore, we did a control-parameter simulation and found that increasing gas pressures or O2 ratios are conducive to enhancing the chemical reaction rate between O atoms and hydrocarbon. Also, the power increase can enhance the physical effect of ions. It shows that increasing the gas pressure and power and reducing the oxygen content can achieve a better cleaning effect while reducing the radio frequency power loss caused by the oxide. Those results provide valuable guidance for optimizing the cleaning process, deepening the understanding of the cleaning mechanism, and improving the performance of SRF cavities.

Studying on failure compensation in low-energy region of superconducting cavity in CAFe2

The high availability of Accelerator-Driven Sub-critical system (ADS) facilities poses a major challenge for the accelerators, which sets stringent requirements on the beam trip rate. However, beam trips caused by component failures in the low-energy region of the superconducting (SC) radio-frequency (RF) cavity are always a bottleneck for the long-term stable operation of high-power beams. In this paper, we present a compensation method. This method can be used to compensate for cavity faults that occur at low-energy ([Formula: see text][Formula: see text]MeV). The compensation is achieved by rematching the beam with nearby components and compensating for energy using all cavities. The new Epeak and RF phases of the remaining components are calculated in the simulation environment using a multi-objective genetic algorithm. The compensation scheme is then tested on the super-heavy element research accelerator II (CAFe2). The experimental results show that the scheme can compensate for the energy, maintaining the same level as the normal operation status, while keeping the transmission efficiency [Formula: see text]. The entire compensation process takes only 36[Formula: see text]s. This is the first compensation experiment in the low-energy region, and it demonstrates the feasibility of long-term stable operation.

Understanding and optimization of the coating process of the radio-frequency Nb3Sn thin film superconducting cavities using tin vapor diffusion method

A study was conducted to understand and optimize the coating process of Nb3Sn thin film superconducting radio-frequency (SRF) cavities using the tin (Sn) vapor diffusion method. The characterization of nanometer-scale Sn droplets, millimeter-scale Sn spots, and locally extremely thin patchy areas was carried out. The causes of their occurrence were analyzed, and their influence on RF performance was distinguished and clarified. Furthermore, a method of achieving high-quality Nb3Sn films was explored by increasing the adsorption of residual Sn vapor to suppress the generation of nanometer-scale Sn droplets while under the premise of ensuring uniform film growth by increasing the Sn vapor flux. It was seen that the performance of the 1.3 GHz single cell Nb3Sn SRF cavity coated at IMP was significantly enhanced after the optimization of the coating process with a maximum accelerating gradient (Eacc,max) of over 18 MV/m and an unloaded quality factor (Q0) of more than 1 × 1010 at Eacc = 12 MV/m and 4.2 K. This study provides quantitative insights for understanding the coating process and provides an essential reference for coating high-performance Nb3Sn SRF cavities using the Sn vapor diffusion method.

Mid-T heat treatments on BCP’ed coaxial cavities at TRIUMF

Mid-T heat treatments in the range from 250°C to 400°C on superconducting radio-frequency (SRF) cavities have been shown to provide high quality factors that rise with applied rf field strength in high frequency, electro-polished (EP), elliptical cavities operating at 2 K, similar to nitrogen doped cavities. The rise in quality factor is attributed to a decrease in the temperature dependent part of the surface resistance RBCS. Until now, no results have been reported for these new treatments on quarter-wave resonators (QWR) and half-wave resonators (HWR). The TRIUMF multi-mode coaxial cavities are dedicated test cavities that allow frequency and temperature resolved performance characterization of treatments without changing environments, therefore providing an excellent test vehicle to test these new treatments with rf frequencies ranging from 200 to 1,200 MHz. In this paper, performance measurements from both QWR and HWR cavities are reported and their performance compared with four different treatments: baseline, a conventional 120°C low temperature bake for 48 h, and two mid-T bakes at 300°C and 400°C for 3 h. In addition, sample analysis using SEM, EDX and SIMS of witness samples is also shown. It is found that the mid-T bakes are not directly transferable to low frequency cavities. In the fundamental modes of the two test cavities, no performance gain over the baseline treatment nor a decreasing temperature dependent component with rising rf amplitude was observed. At frequencies above 1 GHz and low temperatures, the mid-T bakes show a reduced field dependence of RBCS compared to both the baseline and 120°C treatments.

Accelerating gradient improvement in nitrogen-doped superconducting radio-frequency cavities for SHINE

Several state-of-the-art recipes have been studied to pursue high quality factor (Q) and higher accelerating gradient (Eacc) on superconducting radio-frequency (SRF) cavities for the Shanghai HIgh repetition rate XFEL aNd Extreme light facility (SHINE) project. Based on the newly constructed SHINE facilities for cavity surface treatment, a higher gradient has been achieved while maintaining high Q on single-cell and 9-cell cavities through cold electropolishing (EP) and an optimized nitrogen doping (N-doping) recipe. During the N-doping process, the furnace was first heated to 800 ℃ and maintained for 30 min to stabilize the temperature. Then, nitrogen was injected into the furnace. On single-cell cavities with Q0>4.0×1010 at a medium gradient, an average maximum accelerating gradient higher than 35 MV/m is achieved; among them, one cavity reaches its maximum Eacc=38.3 MV/m. On 9-cell cavities with Q0>2.8×1010 at a medium field, an average maximum accelerating gradient higher than 26 MV/m is achieved, which meets the SHINE requirements. The vertical test results of the nitrogen-doped cavities and the experience to improve the accelerating gradient will be presented in this paper.

Temperature measurement on copper surfaces for superconducting thin film cavity applications

Superconducting radio-frequency (SRF) thin film cavities on copper substrates are employed in several particle accelerators. However, these SRF cavities historically featured a progressive performance degradation with the accelerating field that is still not completely understood. The degradation of cavity performance, which limits the use of this technology in accelerators where the real-estate gradient has to be maximized, is manifested by the presence of heat losses in the superconducting film. However, measuring the temperature on the outer surface of copper substrates is challenging due to the higher thermal conductivity of copper at low temperatures compared to niobium. This study describes how temperature variations on copper surfaces can be satisfactorily measured in view of superconducting thin film cavity applications at liquid helium temperatures. Furthermore, we explore how the thermal exchange between thermometers and copper surfaces, and thermometers and helium bath must be tuned with respect to each other in order to measure accurately temperature rises in the thin film. Our findings suggest that engineering the copper surfaces can improve heat transfer into the helium bath and potentially enhance the performance of thin film SRF cavities.

Design and simulation of 500 MHz single cell superconducting RF cavity for SILF

Shenzhen Innovation Light source Facility (SILF) is a 3.0 GeV fourth generation diffraction limited synchrotron light source currently under construction in Shenzhen. The SILF storage ring is proposed to use two 500 MHz single cell superconducting radio frequency (SRF) cavities to provide 2.4 MV RF voltage. In this study, we examined the geometric structure of mature CESR superconducting cavities and adopted a beam-pipe-type extraction scheme for high-order modes (HOM). One of the objectives of SRF cavity design and optimization in this study is to reduce Ep/Eacc and Bp/Eacc as much as possible to reduce power loss and ensure stable operation of the cavity. To reduce the risk of beam instability and thermal breakdown, the HOM and Multipacting (MP) are simulated. Moreover, the mechanical properties of the cavity are analyzed, including frequency sensitivity from pressure of liquid helium (LHe), stress, tuning, Lorentz force detuning (LFD), the microphone effect, and buckling. By comprehensive design and optimization of 500 MHz single-cell SRF cavities, a superconducting cavity for SILF storage ring was developed. This paper will detailed present the design and simulation.

Smooth, homogeneous, high-purity Nb3Sn superconducting RF resonant cavity by seed-free electrochemical synthesis

Workbench-size particle accelerators, enabled by Nb3Sn-based superconducting radio-frequency (SRF) cavities, hold the potential of driving scientific discovery by offering a widely accessible and affordable source of high-energy electrons and x-rays. Thin-film Nb3Sn RF superconductors with high quality factors, high operation temperatures, and high-field potentials are critical for these devices. However, surface roughness, non-stoichiometry, and impurities in Nb3Sn deposited by conventional Sn-vapor diffusion prevent them from reaching their theoretical capabilities. Here we demonstrate a seed-free electrochemical synthesis that pushes the limit of chemical and physical properties in Nb3Sn. Utilization of electrochemical Sn pre-deposits reduces the roughness of converted Nb3Sn by five times compared to typical vapor-diffused Nb3Sn. Quantitative mappings using chemical and atomic probes confirm improved stoichiometry and minimized impurity concentrations in electrochemically synthesized Nb3Sn. We have successfully applied this Nb3Sn to the large-scale 1.3 GHz SRF cavity and demonstrated ultra-low BCS surface resistances at multiple operation temperatures, notably lower than vapor-diffused cavities. Our smooth, homogeneous, high-purity Nb3Sn provides the route toward high efficiency and high fields for SRF applications under helium-free cryogenic operations.

Superheating field in superconductors with nanostructured surfaces

We report calculations of a DC superheating field Hsh in superconductors with nanostructured surfaces. Numerical simulations of the Ginzburg–Landau (GL) equations were performed for a superconductor with an inhomogeneous impurity concentration, a thin superconducting layer on top of another superconductor, and superconductor–insulator–superconductor (S-I-S) multilayers. The superheating field was calculated taking into account the instability of the Meissner state with a non-zero wavelength along the surface, which is essential for the realistic values of the GL parameter κ. Simulations were performed for the material parameters of Nb and Nb3Sn at different values of κ and the mean free paths. We show that the impurity concentration profile at the surface and thicknesses of S-I-S multilayers can be optimized to enhance Hsh above the bulk superheating fields of both Nb and Nb3Sn. For example, an S-I-S structure with a 90-nm-thick Nb3Sn layer on Nb can boost the superheating field up to ≈500 mT, while protecting the superconducting radio-frequency (SRF) cavity from dendritic thermomagnetic avalanches caused by local penetration of vortices.

Commissioning of a two-target DC cylindrical magnetron sputter coater for depositing Nb₃Sn 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 Nb3Sn. 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 Nb3Sn 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. The ∼1.2 μm thick Nb3Sn 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 Nb3Sn-coated cavity demonstrated a quality factor of 3.2 × 108 at an accelerating gradient of 5 MV/m at 4.4 K.

Design Study of a Superconducting Quadrupole Magnet System Sustainable Under Dark Current Heating in ILC Main Linac

The International Linear Collider (ILC) main linac comprises a series of 12 m-long cryomodules. The cryomodule contains eight 9-cell superconducting (SC) RF (SRF) cavities and an SC quadrupole magnet combined with dipole correctors to focus and steer electron and positron beams. The magnets are installed between the SRF cavity string and at the longitudinal center of a common cryomodule/cryostat. These magnets are conductively cooled by pure aluminum channels thermally connected to a 2 K two-phase helium pipe for cooling the SRF cavities. A recent study shows that field-emitted electrons, so-called “dark current” initiated in the SRF cavities, are transmitted through the SRF cavity string and reach the SC magnet. The energy is inevitably absorbed in the SC coil due to the magnetic field, resulting in risks of a quench caused by the coil heating. We are investigating alternate magnet designs by using Nb <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{3}$</tex-math></inline-formula> Sn or <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\text{MgB}_{2}$</tex-math></inline-formula> SC and by adding the dark current absorber surrounding the beam-pipe to realize sustainable magnet operation under the dark current heating. We report the design study of the magnet system and interfaces to the cryomodule accommodating it with the SRF cavities.

Nb$_{3}$Sn SRF Photogun High Power Test at Cryogenic Temperatures

Superconducting RF (SRF) photoguns are emerging as promising candidates to produce highly stable electrons for Ultra fast Electron Diffraction/Microscopy (EUD/UEM) applications due to the ultrahigh shot-to-shot stability compared to room temperature RF photoguns. SRF technology was prohibitively expensive for industrial use until two recent advancements: Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn [1] and conduction cooling [2], [3]. SRF gun can provide a CW operation capability while consuming only several Watts of RF power which eliminates the need of an expensive high power RF system and saves a facility footprint. Euclid is developing a continuous wave (CW), 1.5-cell, MeV-scale SRF conduction cooled photogun operating at 1.3 GHz. In this paper, we present first high power results of the gun covered with Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn.

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.

Construction of a New MgB2 Coating System for 1.3-GHz Superconducting RF Cavities at LANL

After many years of evaluating MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films prepared with various techniques for the application to superconducting radio-frequency (SRF) cavities, we have decided to build a system to coat full-size 1.3-GHz elliptical cavities. This paper describes the design and construction of the system. Additionally, we briefly describe experimental results with a small system and first tests with the new large system. We were able to obtain superconducting samples with a T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of up to 38 K with the small system, but we have not been able to get any superconducting samples with the new system yet.