Superconducting RF Research Articles

Fermilab Cryogenic Test Facilities

Fermi National Accelerator Laboratory (Fermilab) has multiple cryogenic test facilities, undertaking testing of superconducting magnets, Superconducting Radio Frequency (SRF) cavities, SRF cryomodules and other helium cryogenic components. The test areas within Fermilab include: Meson Cryogenic Test facility (Meson), Industrial Building 1 (IB-1), Heavy Assembly Building/Illinois Accelerator Research Center (HAB/IARC), New Muon Lab, (NML) and the Cryomodule Test Facility (CMTF). Meson and HAB/IARC utilize repurposed Tevatron era reciprocating engine based cryogenic refrigerator systems, whereas CMTF and IB-1 utilize gas bearing turbine based coldboxes. Each of these test areas support the various Fermilab projects and collaborations including the Linear Coherent Light Source II (LCLS-II), Proton Improvement Plan II (PIP-II), High Luminosity Large Hadron Collider (HL-LHC), and Mu2e. This paper outlines the diverse and extensive cryogenic test capabilities within Fermilab.

Application of sub-cooled superfluid helium for cavity cooling at linac-based free electron lasers, energy recovery and proton linacs

In order to build a compact linear accelerator, high acceleration gradients of superconducting radio frequency (SRF) cavities have to be achieved. In many large accelerators, e.g. XFEL, CEBAF or SNS, operational limitations are caused either by a limit on available overall cooling power of refrigerators or on cooling capabilities of sc cavities. So, for the further improving of sc cavity cooling, it is possible to increase either a quality factor (Q 0) or to improve a heat transfer at the cavity surfaces. Application of a sub-cooled superfluid helium gives several advantages, e.g. higher heat flux densities, longer time for onset of a film boiling regime and shorter recovery time, reduced Kapitza resistances, etc. In the present paper, application of sub-cooled superfluid (sf) helium for linac-based Free Electron Lasers, Energy Recovery and Proton Linacs is considered. In order to limit the present discussion, its application to CEBAF/SNS-style cryomodules is discussed in detail. For operation at higher RF power levels, further cooling improvements of a fundamental power coupler are needed and design modifications also presented.

Experimental and numerical investigation of 2 K heat exchanger for superfluid helium cryogenic systems at KEK

Superfluid helium cryogenic systems at KEK are constructed for research and development of cryomodules of the compact Energy Recovery Linac (cERL) and the International Linear Collider (ILC). The niobium superconducting radio frequency (SRF) cavities in cERL and ILC operate at temperatures of 2.0 K or below. The SRF cavities are cooled with saturated superfluid helium, which is another phase of the liquid helium (LHe), when it is cooled below 2.17 K under saturation condition. To produce superfluid helium continuously, a Joule-Thomson (JT) valve is employed in the cryogenic system. Also, a 2 K heat exchanger (2K HX) is introduced in series with the JT valve to recover the coldness from 2.0 K gaseous helium (GHe) evaporating from the helium tanks of the SRF cavities. This increases the production rate of superfluid helium by reducing the incoming LHe temperature from 4.4 K to 2.2 K or above before the JT valve. At KEK, we have a 2K HX consisting of a helical coil and laminated fins for thermal loads up to 100 W. Its performance needs to be determined and is characterized by a factor known as effectiveness, which is the ratio of actual heat transfer to the maximum possible heat transfer between the fluids. The performance of the 2K HX has been determined experimentally using the heat exchanger test stand and numerically by computational fluid dynamics (ANSYS CFX®), respectively. In the heat exchanger test stand, the mass flow rate of incoming LHe is kept identical to the outgoing GHe through the 2K HX, using the level and pressure control of superfluid helium. An electric cartridge heater is immersed in the superfluid helium to vary the mass flow rate of evaporating superfluid helium. In the future, the optimization of the 2K HX design will be performed to improve its performance.

Demonstration of CW accelerating gradients on a cryogen-free, cryocooler conduction-cooled SRF cavity

Conduction-cooling of superconducting radiofrequency (SRF) cavities using closed-cycle cryocoolers can lead to compact linear accelerators by eliminating liquid helium and the associated infrastructure. In this paper, we present the first-ever experimental demonstration of continuous wave (CW) accelerating gradients on a niobium SRF cavity that is cooled without using liquid helium. In a newly developed experimental apparatus, a single-cell, 650 MHz niobium cavity was conductively coupled to a 2 W @ 4.2 K pulse tube cryocooler using a high-purity aluminum thermal link. The CW accelerating gradient slightly exceeded 1.5 MV/m, a limit imposed by our RF power supply. Using simple scaling, we project that the niobium cavity when coated with Nb3Sn and operated on the same experimental setup can produce >10 MV/m CW accelerating gradients.

Study of valence band electronic states of near-surface atoms of niobium used for superconducting cavity

The results of investigations conducted on the valence band structure of the near-surface atoms of niobium (Nb) sample, taken from a sheet that is used for fabricating superconducting radio frequency (SRF) cavity, are presented. The valence band electronic states were excited using low energy (≤50 eV) ultraviolet radiation. It was observed that a) the valence band spectrum primarily consists of oxygen 2p-derived states, b) a typical feature expected near the Fermi edge in the valence band of Nb is conspicuously absent. Analysis of the wide band profile provides a number of electronic states peaked at ∼6.3, 7.6, 9.2, and 10.9 eV. After sputtering the surface, a small peak appeared at ∼1.4 eV near the Fermi energy edge. The Nb 4d states showed a resonance involving intra-atomic transition and also hybridization with the states of O 2p. The present study will be useful in identification of the electronic states of Nb that are accountable for superconductivity related issues in an SRF cavity.

Back recursive estimation of unknown frequency sinusoidal disturbance in superconducting RF cavities

A novel back recursive estimation (BRE) scheme is proposed for superconducting radio frequency (SRF) cavities. Microphonic,the main source of cavities detuning is modeled as unknown frequency sinusoidal disturbance. The disturbance property is excited by an auxiliary filter and the frequency information is estimated in observer framework. Furthermore, the sinusoidal disturbance is rearranged as a series of dynamics form using virtual disturbances. Back recursive signal is calculated according to the correlation between virtual disturbance and equivalent input disturbance. As a result, the asymptotic stability of estimation error can be obtained based on Lyapunov function, and robustness can be obtained if another external bounded disturbance exists. Simulations verify the effectiveness of the proposed method.

First operation of the superconducting Darmstadt linear electron accelerator as an energy recovery linac

The superconducting Darmstadt linear electron accelerator (S-DALINAC) has been operated as an energy recovery linac (ERL) for the first time. The S-DALINAC is a recirculating superconducting radio-frequency (SRF) accelerator and had been upgraded with an additional recirculation beamline. It features a path length adjustment system that provides a freedom of choice of 360\ifmmode^\circ\else\textdegree\fi{} for the rf phase difference between the electron bunches recirculated through the new beamline and the phase of the accelerating ${\mathrm{TM}}_{010}$ mode of the oscillating electromagnetic field in the SRF cavities of the accelerator. A choice of around 180\ifmmode^\circ\else\textdegree\fi{} for this phase difference results in a deceleration of the recirculated beam and a corresponding transfer of the particles' kinetic energy back to the energy of the electromagnetic field in the cavities. The main components relevant for ERL operation are described and data of the first operation as an SRF-ERL are presented.

Grain-boundary structure and segregation in Nb3Sn coatings on Nb for high-performance superconducting radiofrequency cavity applications

We report on atomic-scale analyses of grain boundary (GB) structures and segregation in Nb3Sn coatings on Nb, prepared by the vapor-diffusion process, for superconducting radiofrequency (SRF) cavity applications, utilizing atom-probe tomography, high-resolution scanning transmission electron-microscopy and first-principles calculations. We demonstrate that the chemical composition of Nb3Sn GBs is correlated strongly with the diffusion of Sn and Nb at GBs during the coating process. In a sample coated with a relatively large Sn flux, we observe an interfacial width of Sn segregation at a GB of ∼3 nm, with a maximum concentration of ∼35 at.%. After post-annealing at 1100 °C for 3 h, the Sn segregated at GBs disappears and Nb segregation is observed subsequently at GBs, indicating that Nb diffused into the Nb3Sn GBs from the Nb substrate. It is also demonstrated that the amount of Sn segregation in a Nb3Sn coating can be controlled by: (i) Sn flux; and (ii) the temperatures of the Nb substrates and Sn source, which may affect the overall kinetics including GB diffusion of Sn and Nb. An investigation of the correlation between the chemical compositions of GBs and Nb3Sn SRF cavity performance reveals that the Nb3Sn SRF cavities with the best performance (high-quality factors at high accelerating electric-field gradients) do not exhibit Sn segregation at GBs. Our results suggest that the chemical compositions of GBs in Nb3Sn coatings for SRF cavities can be controlled by GB engineering and can be utilized to optimize fabrication of high-quality Nb3Sn coatings for SRF cavities.

Validation of the Linear IFMIF Prototype Accelerator (LIPAc) in Rokkasho

For the development of the International Fusion Materials Irradiation Facility (IFMIF) aiming at material tests for fusion power plants, the construction of the Linear IFMIF Prototype Accelerator (LIPAc) has been conducted at Rokkasho, Japan under the Broader Approach Agreement. The commissioning of LIPAc has been progressing significantly. The world most powerful RFQ successfully accelerated proton beam of 58 mA, and an important project milestone of the acceleration of deuteron beam to 5 MeV with the beam current of 125 mA in pulse mode was successfully achieved. The result proves the validity of the present RFQ design. The adjustment and tuning of the RF power system and the injector for enabling an operation at maximum performances played an important role in achieving this result. For the next step, the preparation of the high duty cycle operation of RFQ is underway. The installation of the HEBT (High Energy Beam Transport line) and the beam dump accepting 1 MW, CW beam, has been completed and a new beam transport line is under manufacturing. Also the assembly of the Superconducting RF (SRF) linac to accelerate the beam up to 9 MeV started in a clean room in Rokkasho.

The Interface Microstructure and Mechanical Properties of Niobium-316L Stainless Steel Explosively Welded Composite Plate

In order to manufacture stainless steel helium vessels for the superconducting radio frequency (SRF) cavities, niobium-316L stainless steel composite plates were fabricated by explosive welding technique. The microstructure and mechanical properties of the composite plates were investigated both right after explosive welding and after annealing. The microstructure measurement results demonstrated that there was not any brittle intermetallic layer formed nor any diffusion phenomenon observed after heat treatment processes. Due to the plastic deformation and work hardening near the interface, the hardness of the composite plates was higher near the bonding interface than inside the bulk metal regions. Meanwhile, ultimate tensile strength and shear strength of the composite plate reached their maximum values when the sample was annealed at 873 K for 10 h at room temperature. Charpy impact test results at liquid helium results showed that the toughness of composite plate meets the requirements from the SRF cavities’ helium vessels fabrication.

OFHC copper substrates for niobium sputtering: comparison of chemical etching recipes

Niobium sputtered on copper has been a popular alternative approach for superconducting radio frequency (SRF) community in the last few decades. Comparing to bulk materials of a few millimeters, high-purity niobium of merely a few microns is sufficient to realize superconductivity on the coated surface. Being niobium thin film, it has been widely acknowledged that surface quality of the substrate plays a vital role in obtaining a superior niobium coating with excellent SRF performance. Therefore, proper chemical treatment of the substrate before coating is crucial and the ultimate goal is to create a smooth and defect-free surface. Prior to the design of a cavity etching system, the mechanism of SUBU as well as two industry-used solutions is studied in detail on samples. Copper samples were first pre-treated by mechanical grinding to remove fabrication damages, obvious defects and visible impurities. Two chemical solutions widely used in industries were subsequently chosen to etch the samples. Finally, the established SUBU solution was used independently on these pre-treated samples for comparison. Surface morphology and etching rate were measured accordingly. Mirror-like copper surface was created by using the SUBU solution thus qualified for subsequent niobium sputtering, while the other two solutions used in industries were less effective with nonideal surface morphology. The chemical reactions, the experimental requisites and the involved processes are extensively elucidated for all three solutions. Limitations for SUBU were examined, and the optimum ratio of the chemical bath volume to sample surface area was also determined. These investigations will serve as an important guidance for the development of a chemical etching system for elliptical copper cavities.

Surface characterization of nitrogen-doped high purity niobium coupons compared with superconducting rf cavity performance

We report the results from the measurements of high purity Nb samples and superconducting radio-frequency (SRF) cavities doped with nitrogen and followed by either electropolishing (EP) or buffered chemical polishing (BCP), in order to understand the role of the postdoping treatment on the performance of SRF cavities. The samples characterization via scanning electron microscope, x-ray photoelectron spectroscopy and secondary ion mass spectroscopy showed topographical differences on the surface of the samples after EP versus BCP treatment, but similar surface composition. Radio-frequency measurements were done on single cell cavities made from fine-grain and large-grain Nb treated by nitrogen doping followed by BCP and showed that improved ${Q}_{0}$ in the medium field in both fine-grain and large-grain cavities is possible with BCP postprocessing. However, there are differences between performances of large-grain versus fine-grain cavities after BCP. A cavity made from large-grain Nb showed a larger increase in ${Q}_{0}$ and a lower quench field compared to cavities made from fine-grain Nb.

Vortex penetration field measurement system based on third-harmonic method for superconducting RF materials

We develop a vortex penetration field (Hv) measurement system using the third-harmonic response of an applied AC magnetic field from a solenoid coil positioned above a superconducting sample. The parameter Hv is measured via detection of the third-harmonic component, which drastically changes when a vortex begins to penetrate the superconductor with temperature increase. The magnetic field locally applied to one side of the sample mimics the magnetic field within superconducting radio-frequency (SRF) cavities and prevents edge effects of the superconducting sample. With this approach, our measurement system can potentially characterize surface-engineered SRF materials such as Superconductor–Insulator–Superconductor multilayer structure (S–I–S structure). As a validation test, we measure the temperature dependence of Hv of two high-RRR bulk Nb samples and obtain results consistent with the literature. We also confirm that our system can apply magnetic fields of at least 120 mT at 4–5 K without any problem of heat generation of the coil. The field applied is higher than those reported in previous works, allowing for a more accurate estimation of Hv at lower temperatures.

Multienergy operation analysis in a superconducting linac based on off-frequency detune method

The free-electron laser facilities driven by a superconducting radio-frequency (SRF) linac provide high-repetition-rate electron beam, which makes it feasible to feed multiple undulator lines at the same time. In this paper, we study a method of controlling the energy of multiple electron bunches by off-frequency detuning in the SRF linac. Based on the theoretical analysis, we present the available discrete frequency detunes and the optimal linac energy allocation solutions with given energy overhead and periodic energy pattern. The required off-frequency detune is not larger than half of the beam repetition rate, which can be realized with the frequency tuner in SRF cavities. We adopt the configuration of the high energy upgrade of the Linac Coherent Light Source II (LCLS-II-HE) as an example to discuss the possible schemes in practice to support two undulator lines simultaneously. We also discuss the energy jitter increase due to the off-crest acceleration, the tunable energy range with fixed total SRF linac energy capacity, the beam energy chirp and lattice design for multienergy beam transport.

Beam breakup current limit in multiturn energy recovery linear accelerators

CBETA, the Cornell BNL Energy-Recovery-Linac (ERL) Test Accelerator, is the first multiturn ERL with superconducting radio frequency (SRF) cavities cavities and will therefore be the first ERL whose current could be limited by the beam break-up (BBU) instability. To investigate the threshold current, we calculate HOMs in CBETA's cavities, including their construction errors, and simulate the BBU threshold current with the established simulation code BMAD. For some construction errors, this current limit is not sufficiently large and means of increasing it are investigated. We find that some means that have been shown effective in single turn ERLs are not practical for multiturn ERLs. To make sure that simulations are trustworthy for CBETA, we revisit the BBU theory for multiturn configurations, and compare to simulation results. We also further explore the scaling law of the BBU threshold current for the case with symmetric ERLs, and a new scaling factor has been found. BBU suppression with lattice chromaticity has been investigated assuming bunches with a Gaussian energy spread. The ability to simulate BBU effect with multiple particles per bunch has been added into BMAD, and agreement with the derived theoretical formula is found. CBETA is currently under beam commissioning at Cornell University. Our simulations show that an optimized optics will push the BBU threshold current beyond the design current, for all realistic cavity production errors.

Normal-mode splitting in coupled high-Q microwave cavities

Three-dimensional radio frequency cavities demonstrate excellent frequency selectivity and, as such, are known for their use in RF filters. These cavities have potential applications in quantum information science, precision displacement metrology, and quantum electrodynamics. Additionally, coupled cavities that form a spectral doublet allow for parametric gain when incorporating mechanical elements. Here, we investigate normal-mode splitting in a pair of quarter-wave stub microwave cavities at room temperature and cryogenic environments in order to identify coupling mechanics for normal and superconducting systems. Superconducting quarter-wave stub cavities with a resonant frequency of 10 GHz are made from reactor-grade niobium and exhibit Q ranging from 105 to 109. We varied cavity-to-cavity coupling to observe several normal-mode splittings of increasing peak separation until we observed a mode crossing. The minimum observed peak separation was 7 MHz for room temperature tests and 200 kHz for cryogenic tests. We also report on values of an intrinsic quality factor for the tuning cavity as a dielectric rod is translated along its symmetry axis. The realization of coupled superconducting radio frequency (SRF) cavities of this type is a necessary step toward implementation of parametric SRF-mechanical gain.

Simulation of Quench Detection Algorithms for Helmholtz Zentrum Berlin SRF Cavities

The Helmholtz Zentrum Berlin is carrying out two accelerator projects which make use of high gradient Continuous Wave (CW) Superconducting Radiofrequency (SRF) cavities: bERLinPro and BESSY-VSR. In both projects, a prompt detection of a quench is crucial to avoid damages in the cryomodules and cavities themselves. In this paper, the response of real time estimation of the cavity parameters using the transmitted and forward RF signals is simulated, in order to perform the quench detection. The time response of the estimated half bandwidth is compared with the dissipated energy in the cavity walls for the different type of SRF cavities used in both projects, i.e., bERLinPro's photoinjector, booster and linac, and BESSY-VSR 1.5 GHz and 1.75 GHz cavities. As an intermediate step prior to the implementation in an mTCA.4 system together with the LLRF control and test with a real cavity, the algorithm has been implemented using a National Instruments FPGA board to check its proper behavior.

Simulation of quench detection algorithms for Helmholtz Zentrum Berlin SRF cavities

The Helmholtz Zentrum Berlin is carrying out two accelerator projects which make use of high gradient Continuous Wave (CW) Superconducting Radiofrequency (SRF) cavities: bERLinPro and BESSY-VSR. In both projects, a prompt detection of a quench is crucial to avoid damages in the cryomodules and cavities themselves. In this paper, the response of real time estimation of the cavity parameters using the transmitted and forward RF signals is simulated, in order to perform the quench detection. The time response of the estimated half bandwidth is compared with the dissipated energy in the cavity walls for the different type of SRF cavities used in both projects, i.e., bERLinPro’s photoinjector, booster and linac, and BESSY-VSR 1.5 GHz and 1.75 GHz cavities. As an intermediate step prior to the implementation in an mTCA.4 system together with the LLRF control and test with a real cavity, the algorithm has been implemented using a National Instruments FPGA board to check its proper behavior.

Upgrade of the Cryogenic Control System for SRF Modules at the Taiwan Light Source

An upgrade of the cryogenic control system for superconducting radio-frequency (SRF) modules of the Taiwan Light Source (TLS) has been completed. The biggest challenge was to recover all protection and operational functions, while minimizing the quantity of vented helium from SRF modules while replacing valve controllers. Gradually, this work was finished within several one- and ten-day scheduled machine shutdown periods for accelerator maintenance. No large helium vent nor pollution of the cryogenic system occurred during all component replacements and function verifications. Functions of the cryogenic electronics were improved, whereas the valve controllers are upgraded to new versions to increase reliability and availability. Communications with the data acquisition system was also secured by buffered signal processing module so that device shutdown of the data acquisition system will not interrupt the cryogenic valve operation.

Design of a compact integrated high-average power superconducting radio-frequency (SRF) electron beam source

There exists a need for reliable, high-power electron sources for applications including those in discovery science, security, medical and industrial engineering. Today, there are many examples of high-average-power electron sources, but they are not, for instance, simultaneously compact, highly efficient, and available at a reasonable cost. Exploiting recent advances in superconducting, radio-frequency (SRF) cavities and RF power sources as well as innovative solutions for the SRF gun and cathode system, we have developed a design concept for a truly compact, SRF, high-average power electron linac source integrating a thermionic cathode system. Potentially capable of 50 kW average power and continuous-wave operation, this accelerator design will produce electron beams with energies up to 10 MeV. In this paper, we present in detail the electromagnetic design results of our 9-cell accelerating structure with its integrated, thermionic cathode structure.