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A detailed study of the near-surface structure and composition of Nb, the material of choice for superconducting radio-frequency accelerator (SRF) cavities, is of great importance in order to understand the effects of different treatments applied during cavity production. By means of surface-sensitive techniques such as grazing incidence diffuse x-ray scattering, x-ray reflectivity, and x-ray photoelectron spectroscopy, single-crystalline Nb(100) samples were investigated in and ex situ during annealing in an ultrahigh vacuum as well as in nitrogen atmospheres with temperatures and pressures similar to the ones employed in real Nb cavity treatments. Annealing of Nb specimens up to $800\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ in a vacuum promotes a partial reduction of the natural surface oxides (${\mathrm{Nb}}_{2}{\mathrm{O}}_{5}$, ${\mathrm{NbO}}_{2}$, and NbO) into NbO. Upon cooling to $120\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, no evidence of nitrogen-rich layers was detected after nitrogen exposure times of up to 48 h. An oxygen enrichment below the Nb-oxide interface and posterior diffusion of oxygen species towards the Nb matrix, along with a partial reduction of the natural surface oxides, was observed upon a stepwise annealing up to $250\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. Nitrogen introduction to the system at $250\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ promotes neither N diffusion into the Nb matrix nor the formation of new surface layers. Upon further heating to $500\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ in a nitrogen atmosphere, the growth of a new subsurface ${\mathrm{Nb}}_{x}{\mathrm{N}}_{y}$ layer was detected. These results shed light on the composition of the near-surface region of Nb after low-temperature nitrogen treatments, which are reported to lead to a performance enhancement of SRF cavities.

The installation of Superconducting Radio Frequency (SRF) Crab Cavities is one of the key upgrades in the framework of the High Luminosity Large Hadron Collider (HL-LHC) at CERN. These devices – built out of 4mm thick niobium sheets – are shaped into complex geometries entailing very tight tolerances, in order to comply with strict SRF requirements. Numerical simulations of the sheet metal forming processes are used to optimize the fabrication of these SRF cavities by assessing different shaping approaches and tooling. Results of thickness distribution and geometrical shapes obtained from simulations are explored for an advanced design of the tooling. The tooling yields optimized shapes respecting the rigorous demands from the RF design. The following contribution details the numerical model used, presents the procedure and outcome of an advanced modeling of tooling and evaluates fabricated pieces.

International audience; Bulk niobium is the most common material used in the fabrication of rf superconducting cavities for accelerators. Predicting and reducing the rf surface dissipation in these cavity structures is mandatory, since it has a tremendous cost impact on the large accelerator projects. In this paper the author hopes to demonstrate that sources of dissipation usually attributed to external causes (mainly flux trapping during cooldown and hydrides precipitates) are related to the same type of crystalline defects that affect the local superconducting properties and can be at the source of early vortex penetration at the surface. We also want to show how these types of defects can explain some of the discrepancies observed from other laboratories in niobium cavity doping experiments. Understanding the origin and the role of these defects could provide direction for improving material specifications as well as improving fabrication control from sheet material to completed cavity. In particular, we will demonstrate that dislocation entanglements, due to the fabrication damage layer, have the strongest impact for the pinning behavior of trapped flux, as well as hydrogen segregation in cavity niobium. The author wishes to present to the superconducting radio frequency (SRF) accelerator community the synthesis of experimental results scattered in the literature, completed with some personal results. The results of this effort provide a new perspective on recently published work in the domain of SRF cavity doping and sensitivity to trapped flux during cooldown. I will also try to draw whenever possible, some conclusions about other types of superconductors used for SRF applications including Nb/Cu thin films and to discuss their possible change of behavior with field or frequency. I will concentrate on surface and material science aspects since the experimental results on rf cavities have already been treated elsewhere.

Basic R&D on materials for superconducting radio frequency (SRF) applications requires RF measurements on samples with high resolution. The Quadrupole Resonator (QPR) at Helmholtz-Zentrum Berlin is a dedicated test cavity, enabling RF characterization of samples in a wide parameter space of temperature and RF field strength at three frequencies. Thereby, the instrument covers typical real-life accelerator conditions without being limited to it. Within the scope of this thesis, the QPR measurement capabilities were continuously expanded and improved, allowing calorimetric measurements of RF surface resistance at all three operating frequencies. Furthermore, magnetic penetration depth, critical temperature (Tc) and the RF critical field can be studied. From those directly measured values, superconducting and normal conducting quantities such as DC critical fields, Ginzburg-Landau parameter, mean free path and normal state resistivity can be derived, yielding a multi-parameter characterization of the investigated sample. In this work, two superconducting coatings were characterized: One single layer of Nb3Sn and a NbTiN on Nb multilayer structure. Nb3Sn is one of the most promising alternative materials to niobium and already showed high quality factors in coated research cavities. The sample investigated here underscores the potential of this high-Tc material for low surface resistance. Ultimately, this could enable low-loss continuous-wave particle accelerators. However the SRF performance failed to reach theoretical values, presumably due to off-stoichiometric areas that were subsequently identified using surface analysis. Furthermore, the RF quench field measurement indicated practical difficulties in reaching the superheating critical field of Nb3Sn with the current coating process. Multilayer structures of superconducting thin films are expected to increase the superheating critical field of a bulk superconductor, corresponding to higher maximum achievable accelerating gradients. With the second sample characterized in this work, the theoretical descriptions of magnetic penetration depth and superheating field for multilayer structures can be confirmed. The observation of non-monotonic surface resistance as a function of temperature reveals additional contributions that are not included in current theories and opens a promising field for future studies., Grundlagenforschung an Materialien für supraleitende Hochfrequenzfeldanwendungen erfordert hochauflösende Messungen an Proben in Hochfrequenzfeldern. Der Quadrupolresonator (QPR) am Helmholtz-Zentrum Berlin ist ein spezialisierter Hohlraumresonator, der die Charakterisierung von Proben in einem großen Parameterraum von Feldstärke und Temperatur sowie bei drei Frequenzen ermöglicht. Damit deckt das Instrument typische Betriebsbedingungen von Teilchenbeschleunigern ab, ohne darauf beschränkt zu sein. Im Rahmen dieser Arbeit wurden die Messmöglichkeiten des QPR kontinuierlich weiterentwickelt und ermöglichen nun die Messung des Hochfrequenzoberflächenwiderstands bei allen drei Betriebsfrequenzen. Außerdem können die magnetische Eindringtiefe, die Sprungtemperatur und das kritische Magnetfeld in Hochfrequenzfeldern untersucht werden. Von diesen Messgrößen können charakteristische Materialparameter sowohl für den supraleitenden als auch für den normalleitenden Zustand abgeleitet werden, wie z. B. kritische (statische) Magnetfelder, Ginsburg-Landau-Parameter, mittlere freie Weglänge und elektrische Leitfähigkeit. In dieser Arbeit wurden zwei supraleitende Beschichtungen charakterisiert: Eine einzelne Nb3Sn-Schicht sowie eine mehrlagige Struktur aus NbTiN und Nb. Nb3Sn ist derzeit die vielversprechendste Alternative zu Niob und beschichtete Testkavitäten haben schon hohe Resonanzgüten demonstriert. Die in dieser Arbeit untersuchte Probe unterstreicht das Potential dieses Supraleiters mit vergleichsweise hoher Sprungtemperatur für kleine Oberflächenwiderstände. Perspektivisch können dadurch verlustarme Dauerstrichbeschleuniger ermöglicht werden. Allerdings wurden bei dieser konkreten Probe nicht-stöchiometrische Bereiche mittels Oberflächenanalyse festgestellt, die die supraleitende Leistungfähigkeit limitieren. Außerdem weist die Messung des Hochfrequenzquenchfelds auf praktische Schwierigkeiten in der Erreichbarkeit des überhitzten kritischen Magnetfelds von Nb3Sn mit dem aktuellen Beschichtungsprozess hin. Mehrlagige Strukturen aus dünnen supraleitenden Schichten sollen das überhitzte kritische Magnetfeld eines massiven Supraleiters aus Vollmaterial erhöhen und damit zu höheren maximal erreichbaren Beschleunigungsfeldstärken führen. Mit der zweiten im Rahmen dieser Arbeit untersuchten Probe konnten die theoretischen Beschreibungen der magnetischen Eindringtiefe und des überhitzten kritischen Magnetfelds für mehrlagige Systeme bestätigt werden. Die Beobachtung von Oberflächenwiderstand mit nicht-monotonem Temperaturverhalten deutet auf weitere Widerstandsbeiträge außerhalb der bisherigen Theorie hin und eröffnet ein vielversprechendes Feld für weitere Untersuchungen.

Nb3Sn is currently the most promising material other than niobium for future superconducting radiofrequency cavities. Critical fields above 120 mT in pulsed operation and about 80 mT in CW have been achieved in cavity tests. This is large compared to the lower critical field as derived from the London penetration depth, extracted from low field surface impedance measurements. In this paper direct measurements of the London penetration depth from which the lower critical field and the superheating field are derived are presented. The field of first vortex penetration is measured under DC and RF fields. The combined results confirm that Nb3Sn cavities are indeed operated in a metastable state above the lower critical field but are currently limited to a critical field well below the superheating field.

Transition edge sensors (TES) have recently been developed at CERN as second sound detectors for quench localisation on superconducting radio frequency (SRF) cavities. After validation of the concept of heat source localisation by TES in the laboratory, TES have been implemented as a diagnostic tool in SRF vertical testing cryostats at CERN SM18 facility. Two cavities were forced to quench by applying sufficiently high values of RF power. Simultaneously, RF and TES second sound signals were recorded. In this paper we present the particularities of each of the relevant tests, the nature of the read signals, as well as the analysis performed on them to locate the origin of the quenching spot.

The twofold goals of this work are (i) to reduce the lack of experimental data in literature concerning the effects of temperature and mechanical strain on magnetic properties of the Ni-Fe alloy Cryophy, and (ii) to validate its use as magnetic shield material for last-generation cryomodule prototypes of crab cavities used in particle accelerators for transverse deflection. The relative magnetic permeability was measured at room and cryogenic temperature, and its lowest value at 4 K fits the minimum design criteria of 100,000 for the crab cavities experiment at CERN. Permeability after uniaxial plastic deformation between 0% and 3% was also measured by means of an Epstein frame. Results show that deformation induces a significant decrease in the magnetic performances, underlining that particular care must be taken during all stages of handling and operation. Finally, the attenuation inside the magnetic shields was tested for the prototype Super Proton Synchrotron at CERN. Results highlight that at 150 mm from the opening, the magnetic field is shielded as required.

We propose an approach to search for axion dark matter with a specially designed superconducting radio frequency cavity, targeting axions with masses $m_a \lesssim 10^{-6} \text{ eV}$. Our approach exploits axion-induced transitions between nearly degenerate resonant modes of frequency $\sim$ GHz. A scan over axion mass is achieved by varying the frequency splitting between the two modes. Compared to traditional approaches, this allows for parametrically enhanced signal power for axions lighter than a GHz. The projected sensitivity covers unexplored parameter space for QCD axion dark matter for $10^{-8} \text{ eV} \lesssim m_a \lesssim10^{-6} \text{ eV}$ and axion-like particle dark matter as light as $m_a \sim 10^{-14} \text{ eV}$., Comment: 30 pages, 7 figures

A horizontal test facility is set up at the Raja Ramanna Centre for Advanced Technology to test the superconducting radio frequency dressed cavities. Along with the cryomodule, control instrumentation, and the power coupler, this facility incorporates a high-power solid-state amplifier for establishing the desired cavity voltage gradient during the testing. This article describes the design, construction, rigorous testing, and measured results of this high-power solid-state radio frequency amplifier and its constituent components. Its maximum output power is 36 kW (average) at the operating frequency of 650 MHz. Its main features are its modular and scalable design with in-house developed constituent components. These components include 500 W, 20 dB gain modules, novel two-tier radial dividers, combiners, power sensors, and aperture-coupled directional couplers. Their excellent reprise performance for the multiple quantities confirms the design methodology presented here. The measured wall plug efficiency of this 36 kW amplifier is 43.6%, and its power gain is 86 dB. The designed radial combiner is highly efficient (power-combining efficiency of 98.4%), and the directional coupler exhibits a very low loss (insertion loss of 0.05 dB).

Operational availability is a critical performance measure for an accelerator facility in modern time. A high availability enables the facility to serve a wide range of users simultaneously. Consequently, besides pure accelerator physics considerations, newly proposed accelerator facilities account for the availability and reliability aspects in the design phases. It allows incorporation of appropriate mitigation strategies for the most vulnerable systems in the machine and therefore, minimizes unscheduled interruptions during the operation. This paper lays out a methodology for the availability assessment of the complete particle accelerator facility and presents an initial assessment of the availability of the newly proposed Proton Improvement Plan-II (PIP-II) accelerator facility at Fermilab. The paper describes a comprehensive reliability model of the PIP-II facility that comprises not only 800 MeV linear accelerator (linac) system but also essential utility systems in the form of cryogenic, water, power and air systems. The paper details estimations of the availability of the PIP-II facility for two operational modes i.e. the nominal operational mode featuring 800 MeV beam and critical operational mode involving operation with the lowest objective beam energy of 600 MeV.

Here, we report a demonstration of electron beam generation in a superconducting radio frequency (SRF) Teraelectronvolt Energy Superconducting Linear Accelerator 1.3 GHz gun equipped with a field emission cathode and operated at 2 K. The cathode is a submicrometer film of nitrogen-incorporated ultrananocrystalline diamond [(N)UNCD] deposited atop a Nb RRR300 cathode plug that is 4 mm in diameter. Measurements of the output current showed that it increased exponentially as a function of the gun gradient. Our results demonstrate a feasible path toward simplified fully cryogenic SRF injector technology. At a maximal gradient of 0.9 MV/m, a current density of 22 μA/cm2 was obtained [calculated as a 2.75 μA current over the total cathode surface area of π(2 mm)2]. One important finding is that the electron emitter made of (N)UNCD, a material long been known as a highly efficient field emission material, demonstrated a record low turn-on gradient of 0.6 MV/m. A hypothesis explaining this behavior is proposed.

China Spallation Neutron Source (CSNS) has been in operation for about 2 years, and an upgrade is proposed to compensate the space charge effect. A superconducting radio frequency (SRF) linac with double spoke cavities and medium-beta elliptical cavities has been designed to increase the injection energy from 80MeV to 300MeV, and institute of high energy physics (IHEP) has developed a prototype double spoke cavity at β 0 0.5. The cavity shape was optimized to minimize Ep/Eacc while keeping Bp/Eacc reasonably low. Meanwhile, mechanical design was applied to check stress, Lorentz force detuning and microphonic effects, and to minimize pressure sensitivity. A new input power coupling scheme was proposed to avoid electrons hitting directly on ceramic window. After fabrication and post processing of cavity, the cavity was vertical tested for several times, and mechanical polishing was applied to improve the performance. The cavity finally reached Bp of 140mT at Eacc = 16.1MV/m and Q 0 = 1 . 42 × 1 0 10 under vertical test at 2K.

650 MHz multicell superconducting radio frequency (SRF) elliptical cavities are proposed for effective acceleration of proton beam in the Chinese initiative Accelerator Driven Subcritical System (CiADS). Two families of such cavities will be used in the driver superconducting linac to accelerate the H + beam from 175 MeV to 500 MeV, with the possibility to upgrade the energy to 1 GeV and higher. Detail designs of a six-cell β o p t = 0.62 elliptical cavity and a five-cell β o p t = 0.82 elliptical cavity will be presented in this paper, including the multi-parameter electromagnetic design and optimization, high order modes analyses, multipacting simulations, mechanical and engineering analyses. Sub-systems of the cavity such as the fundamental power coupler antenna tip design and mechanical design of cavity-helium vessel-tuner system will also be discussed in detail. Three single cell prototype cavities were fabricated and vertical tested to verify the electromagnetic properties, the fabrication and post-processing technologies. Four multi-cell prototype cavities are under manufacturing and expected to be ready in 2021.

In this study, the deposition of high Tc NbN thin films onto copper substrates, in the form of both single layers and as part of an ML SIS structure, using DC magnetron sputtering (DC MS) was explored. The effects of the deposition parameters on the film microstructure and superconducting properties of the NbN films as well as the challenges involved in depositing ML SIS films onto copper substrates are reported on. A maximum Tc = 16.1 K has been achieved for single layer NbN films, as determined by AC susceptometry measurements. Initial results for the SIS film structures, deposited onto copper in the form of Nb/AlN/NbN, have also shown an enhancement in the first magnetic flux entry field value above that achieved by a single Nb layer. This enhancement has been found to be highly dependent on the quality of the SIS film coatings.

At the KEK compact energy recovery linac (cERL), a new beam line to produce radioisotopes (RIs) was constructed. To increase the beam energy for RI production, we proposed to switch the operation mode from continuous-wave mode to pulse mode. The first commissioning test of the cERL superconducting radio frequency (RF) cavities operated in pulse mode was therefore carried out. The RF pulse pattern for each cavity was carefully selected according to the corresponding cavity’s loaded Q and the capabilities of the power sources. A modified feedforward table was applied to reduce the peak reflected power at the power coupler. In addition, a filling on resonance approach that aims to reduce the required incident power during the filling time due to the existence of the Lorentz force detuning was adopted. The slow drift of the pulse-to-pulse detuning on the flat top of an RF pulse was successfully compensated for by a piezo feedback system. In this paper, we present the results related to the cERL pulse mode operation. The measured RF stabilities and beam energy stability in the cERL beam commissioning are presented as well.

Bulk Nb superconducting radio-frequency (SRF) cavities are widely used in accelerators, and their accelerating gradient and general performance are limited by the superheating field (B sh). To push the theoretical limit of the B sh, new multilayer structures are required. We fabricated FeSe-coated Nb films using pulsed laser deposition, performed structural characterizations, and measured the transport and magnetic properties for this superconductor-superconductor bilayer structure with smooth surface. Additionally, the measured B c1 of FeSe-coated Nb film is greatly enhanced, while the B sh of the FeSe layer is expected to be higher than that of bulk Nb, yet the superconducting transition temperature (T c) is less than 5 K. This work presents the first fabrication of a new coating layer: FeSe deposited on Nb, showing the possibility of using iron-based materials for multilayer structures in SRF cavities.

Nb₃Sn thin films are promising candidates for future application in superconducting radio frequency cavities due to their low surface resistivity, high critical temperature, and critical field, as compared to bulk niobium, which is the current state of the art. In this paper, we report the deposition of Nb₃Sn thin films by magnetron co-sputtering at the extremely low temperature of 435°C. These thin films show a critical temperature of 16.3 K, a high critical current density of 1.60×10⁵A/cm², and a strong shielding effect. The key to achieving low-temperature growth is the independent kinetic control of Nb and Sn species in the sputtering process. From a technological viewpoint, the low-temperature approach paves the way for the use of Nb₃Sn as a coating in cryogenic efficient copper based cavities, thereby avoiding the detrimental interdiffusion of Cu.

An ion accelerator, RAON, is going to be built in Daejeon, Korea, by the Rare Isotope Science Project (RISP) team in the Institute of Basic Science (IBS). In this accelerator, various ions, such as uranium, proton, carbon and calcium, will be accelerated and fed to researchers for experiments. To produce the ion beams and to satisfy the requirements, many devices such as accelerating structures, RF power supplies and ion sources will be installed and controlled according to the predefined control logic. For correct control of such devices, time and clock synchronization is essential. A time synchronization test bed is planned for the RAON superconducting radio-frequency (SRF) test facility, and a flexible, low-cost gps-based time server was developed for that purpose. In this paper, detailed information on the developed GPS server and the test results will be described.

We have investigated the possible influence of surface oxides on the optical properties of a high-purity niobium (Nb) material for fabrication of superconducting radio frequency (SCRF) cavities. Various peaks in the infrared region were identified using Fourier transform infrared and Raman spectroscopy. Optical response functions such as complex refractive index, dielectric and conductivity of niobium were compared with the existing results on oxides free Nb and Cu. It was observed that the presence of a mixture of niobium-oxides, and probably near other surface impurities, appreciably influence the conducting properties of the material causing deviation from the typical metallic characteristics. In this way, the key result of this work is the observation, identification of vibrational modes of some of surface complexes and study of its influences on the optical responses of materials. This method of spectroscopic investigation will help in understanding the origin of degradation of performance of SCRF cavities.

This paper presents a simplification method for finite element models for the calculation of AC losses in stacks of superconducting tapes. A fast and accurate model of these losses can be exploited in the design of large-scale superconducting applications, where the coils are represented as stacks of tapes in 2D models. The simplification method aggregates the superconducting, substrate, and copper layers of several windings of the superconducting tape to form bulk elements. The resulting model is able to evaluate the AC losses in the superconducting layer faster than the full model. The accuracy of the proposed method and decrease in computation time are shown.

Superconducting radio frequency accelerators are used to produce terahertz (THz) radiation pulses with a high repetition rate. In this study, a compact high repetition rate THz radiation source has been developed based on a DC-SRF photo-injector through velocity bunching at Peking University. This compact THz source can theoretically generate approximately 1 W of superradiant THz radiation, with a repetition rate of 16.25 MHz and a frequency that can be tuned from 0.24 THz to 0.42 THz by varying the electron beam energy from 2.4 MeV to 3.1 MeV. Simulation results indicate that the asymmetrical longitudinal distribution of electrons in each bunch caused by velocity bunching increases the THz power by about 2 orders at wavelength within 400–700 μm. Experimental measurements are consistent with the calculation results when propagation loss is considered. This paper presents the system description, simulation, and experiments of the high repetition rate THz source.

The power supply based on the full-wave superconducting rectifier is used for excitation of the main superconducting solenoid of the KEDR universal magnetic detector at the VEPP-4 collider. The rectifier consists of the air-core superconducting transformer and two thermally controlled superconducting switches. Several superconducting switch designs were developed, in order to achieve appropriate parameters of the rectifier operation. The present version of the switches was designed and tested in November 2013. The rectifier is cooled with a liquid helium bath. The primary winding of the superconducting transformer is excited with an ac current amplitude of 28 A at a voltage amplitude of 5 V. The heat power used for a superconducting switch control is 2.7 W. The power supply provides rising the magnetic field inside the KEDR main solenoid, long-term stabilization, and lowering the field to zero. The inductance of the solenoid is 1.2 H. The usual operating magnetic field is 0.6 T, which corresponds to the 2.4-kA current in the solenoid winding and 3.46 MJ of stored energy. The power supply pumps the field up to 0.6 T, within 110 h, and can keep it constant for several months. In this paper, the design, parameters, and performance of the superconducting rectifier are discussed.

The High-Intensity Heavy-Ion Accelerator Facility is a new major engineering project in the Institute of Modern Physics. The dipole magnets of the booster ring are conceived as fast-cycled superconducting magnets with a high magnetic field and a large gap, the warm iron and superconducting coil structure (superferric) is adopted. The coil is wound with a Nuclotron-type superconducting cable to reduce the ac loss. The magnet with a large operation current and a low inductance assures the fast-pulsed operation. Based on the superconducting cable with large operation current, the design of a superconducting coil prototype is shown in this paper. The magnetic field analysis based on 3-D-OPERA is carried out to obtain the basic dimension of coils. Then, the design of the Nuclotron-type superconducting cable, the coil, and coil case are described in detail. In addition, fabrication of the superconducting cable, the coil winding, and other components are also introduced in this paper.

The Superconducting Link project at CERN aims at developing helium-gas-cooled high-current electrical transfer lines for the powering of the superconducting magnets, which are being developed in the framework of the High-Luminosity upgrade of the Large Hadron Collider. A candidate conductor for the cables is magnesium diboride (MgB 2 ) round wire operated at temperatures of up to 25 K. One of the most important factors to be taken into account during the design phase is quench protection. In this work, we investigate the quench behavior of high-current cables made of MgB 2 . The wires were produced by Columbus Superconductors. The cables are operated in the Superconducting Link Test Station at CERN, which are cooled by helium gas at temperatures between 10 and 30 K. The propagation of the quench front is monitored by means of voltage and temperature sensors installed on the cables. Results of the measurements are compared to numerical simulations, in order to validate the models and quantify the parameters related to quench propagation and protection. The ultimate goal is to devise a quench protection strategy suitable for the safe and reliable operation of the Superconducting Link circuits.

This article attempts to give a historical account and review of technological developments and innovations in radio frequency (RF) systems for particle accelerators. The evolution from electrostatic field to the use of RF voltage suggested by R. Wideroe made it possible to overcome the shortcomings of electrostatic accelerators, which limited the maximum achievable electric field due to voltage breakdown. After an introduction, we will provide reviews of technological developments of RF systems for particle accelerators.

An in situ plasma processing technique using chemically reactive oxygen plasma to remove hydrocarbons from superconducting radio frequency cavity surfaces at room temperature has been developed at the spallation neutron source, at Oak Ridge National Laboratory. To understand better the interaction between the plasma and niobium surface, surface studies on small samples were performed. In this article, we report the results from those surface studies. The results show that plasma processing removes hydrocarbons from top surface and improves the surface work function by 0.5–1.0 eV. Improving the work function of RF surface of cavities can help to improve their operational performance.

Fermilab has proposed an upgrade of its proton accelerator complex based on construction of a new superconducting radio frequency linac. The plan is structured to deliver, in a cost effective manner, more than 1 MW of beam power to the neutrino production target at the initiation of the Long Baseline Neutrino Facility, while simultaneously creating a flexible platform for longer-term development of the Fermilab complex to multi-MW capabilities in support of a broader research program.

A new in-situ plasma processing technique is being developed at the Spallation Neutron Source (SNS) to improve the performance of the cavities in operation. The technique utilizes a low-density reactive oxygen plasma at room temperature to remove top surface hydrocarbons. The plasma processing technique increases the work function of the cavity surface and reduces the overall amount of vacuum and electron activity during cavity operation; in particular it increases the field emission onset, which enables cavity operation at higher accelerating gradients. Experimental evidence also suggests that the SEY of the Nb surface decreases after plasma processing which helps mitigating multipacting issues. In this article, the main developments and results from the plasma processing R&D are presented and experimental results for in-situ plasma processing of dressed cavities in the SNS horizontal test apparatus are discussed.

The eddy current test (ECT) is frequently used as a non-destructive method to check for the defects of high purity niobium (RRR300, Residual Resistivity Ratio) in a superconducting radio frequency (SRF) cavity. Determining an optimal frequency corresponding to specific material properties and probe specification is a very important step. The ECT experiments for high purity Nb were performed to determine the optimal frequency using the standard sample of high purity Nb having artificial defects. The target depth was considered with the treatment step that the niobium receives as the SRF cavity material. The results were analysed via the selectivity that led to a specific result, depending on the size of the defects. According to the results, the optimal frequency was determined to be 200 kHz, and a few features of the ECT for the high purity Nb were observed.

The heat treatment, tensile testing, and bulge testing of Cu and Nb tubes has been carried out to gain experience for the subsequent hydroforming of Nb tube into seamless superconducting radio frequency (SRF) cavities for high energy particle acceleration. In the experimental part of the study samples removed from representative tubes were prepared for heat treatment, tensile testing, residual resistance ratio measurement, and orientation imaging electron microscopy (OIM). After being optimally heat treated Cu and Nb tubes were subjected to hydraulic bulge testing and the results analyzed. In the final part of the study finite-element models (FEM) incorporating constitutive (stress–strain) relationships analytically derived from the tensile and bulge tests, respectively, were used to replicate the bulge test. As expected, agreement was obtained between the experimental bulge parameters and the FEM model based on the bulge-derived constitutive relationship. Not so for the FEM model based on tensile-test data. It is concluded that a constitutive relationship based on bulge testing is necessary to predict a material's performance under hydraulic deformation.

To obtain low-emittance electron beams with high average current, superconducting radio frequency photocathode guns have been developed by several laboratories worldwide. A DC-SRF injector, which combines a DC pierce gun and a superconducting cavity, has been developed at Peking University. Recently, stable operation of the DC-SRF photoinjector has produced encouraging results. The average beam current of 1 mA in macropulses of 7 ms with a repetition rate of 10 Hz has been reached and can be kept at about 0.5 mA for routine operation. In this paper, we describe the DC-SRF photoinjector and will discuss the experiments and the test results.

In current superconducting qubit circuit development, coherence time is limited by low-frequency flux noise introduced from localized two-level systems (TLSs) in the fabricated superconducting circuit, including superconducting materials, the interface between thin films, and the substrate. As a part of the effect to tackle this issue from a substrate standpoint, we succeed in fabricating NbN (200) oriented coplanar waveguide resonator (CPWR) on a thin titanium nitride (TiN) (200) buffered Si (100) substrate. The internal loss is close to 3 × 10 -5 near ~1, improved from ~2 × 10 -4 in NbN (200) CPWR fabricated on MgO (100). For NbN (200) resonators grown on Si (100), the internal loss shows microwave energy independent. To further clarity if this energy independence is due to the change of substrate material, we also fabricated TiN (200) CPWR on Si (100). In contrast to NbN (200) CPWR, the loss of TiN (200) resonator shows a strong microwave dependent, an indication of TLSs originated from the interface and the surface. We suggest TLSs in NbN (200) CPWR are dominated from different mechanism other than arising from the interface and surface.

Superconducting radio frequency niobium cavities are the building blocks of modern accelerators for scientific applications. Lower surface resistance, higher fields, and high operating temperatures advance the reach of the future accelerators for scientific discovery as well as potentially enabling cost-effective industrial solutions. We describe the design and performance of an Nb3Sn coating system that converts the inner surface of niobium cavities to an Nb3Sn film. The niobium surface, heated by radiation from the niobium retort, is exposed to Sn and SnCl2 vapor during the heat cycle, which results in about 2 μm Nb3Sn film on the niobium surface. Film composition and structure as well as radio frequency properties with 1-cell R&D cavities and 5-cell practical accelerator cavities are presented.

Niobium superconducting radio frequency (SRF) cavities enable the operation of modern superconducting accelerator facilities. These cavities do not approach the theoretical performance limits of Nb due to the deleterious effects of surface defects and chemical inhomogeneities such as Nb hydrides. Nitrogen doping is known to consistently increase the cavity performance and inhibit Nb hydride growth, but a comprehensive understanding of Nb hydride growth and suppression is not yet realized. Scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and density functional theory (DFT) calculations presented herein elucidate the real-time, nanoscale structural and electronic evolution of undoped, hydrogen doped, and hydrogen and nitrogen doped Nb(100) due to the growth and suppression of Nb nano-hydrides. DFT calculations in agreement with the experimental data found unique near-surface phases stabilized upon dopant incorporation. The experimental STM and STS results and DFT calculations reported herein provide the first in situ and real-time nanoscale visualization and characterization of the effects of nitrogen doping on Nb hydride suppression and growth. Such information allows for further optimization of nitrogen doping procedures and advances in the performance of SRF materials for next-generation SRF-based accelerators and free electron lasers.

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.

Digital low-level radio-frequency (LLRF) systems are used at the compact energy recovery linac (cERL) test facility to stabilize the accelerating field inside the radio frequency (RF) cavities. Proportional and integral feedback controllers are implemented in the LLRF system to suppress the various disturbances in the cavities. At cERL, the typical disturbances include microphonics detuning and high-voltage power supply (HVPS) ripples. These two types of disturbances are reflected in the sum of sinusoidal fluctuations in the RF phase. Recently, a real-time narrow-band active noise control (ANC) approach which aims to reject the microphonics detuning of less than several dozen Hz, was proved to be effective in cavity resonance control. We extend this ANC method to the LLRF field control to suppress the RF ripples caused by the HVPS. In this paper, we present our experience of applying the ANC algorithm to the LLRF system of the cERL facility at KEK. We confirmed that high frequency ripples of up to 20 kHz can be well compensated by the ANC method during the cERL beam-commissioning.

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.

The Shanghai Synchrotron Radiation Facility (SSRF) is an intermediate energy light source built at Zhang-Jiang Hi-Tech Park in Shanghai, China. The RF power and voltage required for storing the electron beam are provided by means of three SC (superconducting) cryomodules, each containing one 500 MHz superconducting cavity. A cryogenic plant with cooling capacity of 650 W at 4,5 K (herein called SSRF-I cryoplant) supplied by Air Liquide advanced Technologies has been in operation since August of 2008 to provide cooling for the three superconducting cavities. In order to further improve the performance of SSRF, the following SC devices are to be applied for the SSRF upgrade (SSRF-II): 1) 2 harmonic SRF (superconducting radio frequency) cavities with 1,5 GHz will run alternatively at 2 K (31 mbar). 2) One superconducting wiggler is to be used for one of the new-built beam lines, ultra-hard multi-functional beam line. The SC wiggler will be cooled by cryocoolers at 4,2 K region. To support the operation of the new cavities, AL-aT (Air Liquide advanced Technologies) has designed, manufactured and started a new cryogenic system including a 2K system with equivalent cooling capacity of at least 650 W at 4,5 K and 60 W at 2K. The system is mainly constituted of a refrigerator HELIAL MF, an auxiliary 2 K cold box and of a 2 K warm pumping station. This cryogenic unit has been started successfully at the end of 2018 and has proven performances both at 4,5K and at 2K.

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.

The LINAC Coherent Light Source II (LCLS-II) located at SLAC National Accelerator Laboratory (SLAC) in Menlo Park, CA, is a U.S. Department of Energy project tasked to design and build a world-class x-ray free-electron laser facility for scientific research. The Linac has superconducting radio frequency cryomodules that are connected to the cryogenic plant by the Cryogenic Distribution System (CDS), which consists of distribution boxes with heat exchangers and reliefs, feed caps, end caps, and surface, vertical, and bypass transfer lines. The CDS components were designed and built to specification by industry. The components have been delivered and their installation at SLAC will be discussed. The as-built relief system design will be presented, showing minimization of relief inlet pressure drops while meeting capacity requirements. The relieving flow pressure drops along the lengths of the CDS to centrally located reliefs at the distribution box were analysed to satisfy Pressure Vessel and Process Piping Code criteria to ensure relief performance. The sub-atmospheric 2 K circuit relieving approach will be discussed, which includes a three-way diverter isolation valve suitable for sub-atmospheric service. The component anchoring load design approach and installation into concrete floor will be discussed. This addresses loading, including seismic loading, along the component’s load path to floor anchoring system.

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.

Superconducting radio frequency cavities meet the demanding performance requirements of modern accelerators and high-brilliance light sources. Their design requires a precise knowledge of their electromagnetic resonances. A numerical solution of Maxwell’s equations is required to compute the resonant eigenmodes, their frequencies and losses due to the complex cavity shape. The consideration of resonances damped by external losses leads to a nonlinear eigenvalue problem. Previous work showed that, using State-Space Concatenation to construct a reduced order model and Newton iteration to solve the arising eigenvalue problem, solutions can be obtained on workstation computers even for large-scale problems without extensive simplification of the structure itself. In this paper, we augment the solution workflow by Beyn’s contour integral algorithm to increase the number of found eigenmodes. Numerical experiments are presented for one academic and two real-life superconducting cavities and partially compared to measurements.

We use a model of vortex dynamics and collective weak pinning theory to study the residual dissipation due to trapped magnetic flux in a dirty superconductor. Using simple estimates, approximate analytical calculations, and numerical simulations, we make predictions and comparisons with experiments performed in CERN and Cornell on resonant superconducting radio-frequency NbCu, doped-Nb and Nb$_3$Sn cavities. We invoke hysteretic losses originating in a rugged pinning potential landscape to explain the linear behavior of the sensitivity of the residual resistance to trapped magnetic flux as a function of the amplitude of the radio-frequency field. Our calculations also predict and describe the crossover from hysteretic-dominated to viscous-dominated regimes of dissipation. We propose simple formulas describing power losses and crossover behavior, which can be used to guide the tuning of material parameters to optimize cavity performance., 15 pages, 8 figures

Journal of materials science 53(14), 10411-10422 (2018). doi:10.1007/s10853-018-2310-8, (100) Oriented niobium (Nb) crystals annealed in the vacuum conditions close to that used in mass production of 1.3 GHz superconducting radio frequency cavities for linear accelerators and treated in nitrogen at a partial pressure of 0.04 mbar at temperatures of 800 and 900 °C have been studied. The surfaces of the nitrogen-treated samples were investigated by means of various surface-sensitive techniques, including grazing-incidence X-ray diffraction, X-ray photoemission spectroscopy, and scanning electron microscopy with energy-dispersive X-ray spectroscopy in planar view and on cross-sections prepared by a focused ion beam. The appearance of a dense layer of epitaxial rectangular precipitates has been observed for the Niobium nitrided at 900 °C. Increased nitrogen concentration in the near-surface region was detected by glow-discharge optical-emission spectroscopy, focused ion-beam cross-sectional images and X-ray photoelectron spectroscopy. Crystalline phases of NbO and β-Nb2N were identified by X-ray diffraction. This information was confirmed by X-ray photoelectron measurements, which in addition revealed the presence of Nb2O5, NbON, NbN, and NbN x O y components on the surface. These results establish the near-surface Nb phase composition after high-temperature nitrogen treatment, which is important for obtaining a better understanding of the improved RF cavity performance., Published by Springer Science + Business Media B.V, Dordrecht [u.a.]

A superconducting electron linac (e-Linac) photo-fission driver is under construction at VECC for the upcoming rare isotope beam facility called ANURIB. The first stage consists of an injector linac based on 1.3 GHz superconducting radio frequency technology designed for accelerating 10 MeV, 2 mA CW electron beam. An rf modulated thermionic electron gun (E-Gun) has been developed and tested for this injector linac. The E-Gun will allow continuous beam operation up to 300 keV with around 3 pC charge per pulse at 650 MHz. Electron beam with RF modulation is extracted from the gun and intensity, energy, size of the DC beam as well as trans-conductance of the electron source are measured. The design of some major components for the electron gun and low energy beam transport line is discussed and results of beam tests are presented.

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 10 5 to 10 9. 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.