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At the ELBE Center for High-Power Radiation Sources, electrons are accelerated to an energy of up to 40 MeV in a superconducting linear accelerator that is operated in continuous wave (CW) mode. The acceleration is achieved using superconducting radio frequency (RF) cavities, which are driven by an analog low-level RF (aLLRF) system and solid-state-based RF amplifiers. The analog aLLRF system was transformed to a digital system based on the MicroTCA standard. It is in user operation since 2020. Here, the new digital system and its integration in the ELBE control system are described. Furthermore, the system is characterized by noise measurements and shows rms field stability of the digital LLRF (dLLRF) system of 0.01° in phase and 0.005% in amplitude. In addition, an algorithm for compensating long-term drifts is presented and characterized.

We report on the magnetic levitation of a millimeter-sized neodymium permanent magnet within the interior of a superconducting radio frequency (SRF) cavity. To the best of our knowledge, this is the first experimental work on levitating a magnet within an SRF cavity. The cavity is a coaxial quarter-wave microwave resonator made from 6061 aluminum, having a resonance frequency of 10 GHz and a loaded Q of 1400. The cylindrical magnet (N50) has a height of 1 mm, a diameter of 0.75 mm, a mass of 4 mg, and a remanence of 1.44 T. This produces a peak magnetic field 140 times greater than the critical field of aluminum. The magnet is placed either on the top of the coaxial portion of the cavity or on the cavity floor before cooling it below the superconducting transition temperature of aluminum. The coaxial mode's resonance frequency shifts as a function of the levitation height of the magnet and gives an idea of the magnet's position and mechanical motion. We observe a transition at a temperature of 650 mK where the Meissner effect levitates the magnet as the material beneath the magnet becomes superconducting. The magnet is levitated to a height of 2.5 mm above the surface of the cavity stub, which is a sufficient separation for the field strength of the magnet at the surface of the stub to be less than the critical field strength of the superconducting aluminum. We measure a 120 MHz upshift in the cavity resonance as the magnet is levitated from the top of the stub and 15 MHz downshift as it levitates from the floor of the cavity. Our measurements are consistent over several heating and cooling cycles. Our work provides a path towards a novel optomechanical system.

As a low operating cost microwave source, magnetron has been fully considered for superconducting radio frequency (SRF) accelerator. A 1497-MHz, 13.5-kW continuous-wave (CW) magnetron for SRF accelerator is designed and manufactured, and the test results meet the design requirements. Two simulation steps of designing magnetron with Computer Simulation technology (CST) Studio Suite are presented. First, the Microwave Studio helped to simulate the microwave coupled output structure and the resonant cavity. Then, the output performance of magnetron was simulated with particle-in-cell (PIC) solver. The cold test results show that the circuit efficiency is 93.14 %. A magnetron hot test platform was used to measure an output power of 14.23 kW and an electronic efficiency of 84.23 %. The injection locking experiment is completed at 4.5 kW. The frequency of the magnetron is adjustable, with a tunable bandwidth of 7 MHz through a mechanical tuning rod. The problems and solutions in the process of design and manufacture are summarized, which can provide a reference for the development of high-power magnetron with different frequency. The development of 1497-MHz magnetron broadens the selectable frequency of high-power magnetron, which is beneficial to its industrial applications.

Surface roughness represents the measure of the irregularities on the surface contributing to the local field enhancement. The traditional Fowler-Nordheim equation established for perfectly planar surfaces is not suitable for describing emission from rough surfaces. Instead, it is more appropriate to use the equation that accounts for the field enhancement factor describing the effect of the surface morphology. In superconducting radio frequency cavities, field emission may occur in the irises while the tips on the cavity surface may act as an emitter leading to the high electric field. For this study, calculations for hemispherical, cylindrical, and conical tips have been performed using a Multiphysics software package COMSOL. The focus was put on the dependence of the field enhancement factor on the shape and the radius of the protrusions. The electric field strength and the current density increase with increasing the root mean square average of the profile heights due to field enhancement at the cavity irises. The lowest value of the electric field has been achieved for the hemisphere. The calculated values for the field enhancement factors are consistent with the data from the literature, in which case the protrusion may represent a small local bump on the surface of a superconducting cavity. Based on the fit of the results, presented here, the relation between the enhancement factor and the radius has been suggested. The accurate estimation of the field emission may play a crucial role in the design of accelerators and other technological applications with requirements of extremely high precision.

A reliable and repeatable post-processing technology of improving the performance of 1.3 GHz superconducting radio frequency (SRF) cavities is one of the critical technologies for the ILC and XFEL and ERL projects. Three 1.3 GHz single-cell cavities were fabricated and received a baking in temperature 330 °C, while the interior of the cavity stayed in ultra-high vacuum (UHV). The cavities were also vertical-tested after electropolishing (EP) with 120 °C 48-h baking and with nitrogen doping separately for a comparison. The Q0 of 1.3 GHz single cavity after medium-temperature baking can be 2–3 × 1010 in the accelerating gradient range of 2–35 MV/m in the 2 K vertical test in IHEP. Meanwhile, the outer surface oxidation of niobium cavity caused by baking will decrease the performance of the SRF cavity. Medium-temperature (250–400 °C) baking on the 1.3 GHz single-cell cavity will improve its Q0 in 2 K vertical test compared with EP followed by 120 °C 48-h baking baseline and reach a similar level of nitrogen doping, and the quench field will lower to a typical range of 20–30 MV/m. Meanwhile, the cavity performance is sensitive to the baking time and temperature, which indicates that a tremendous improvement can be made on the current treatment.

Magnesium diboride (MgB2) is considered a potential material for superconducting radio frequency cavities. MgB2-coated Cu cavity will allow for a higher operational temperature than a bulk Nb cavity because of the higher transition temperature of MgB2 and the high thermal conductivity of Cu. Using the hybrid physical chemical vapor deposition technique, MgB2 coatings were successfully achieved on the inner wall of 3-GHz Cu cavities. The surface and superconducting properties of the coatings were characterized using small samples on Cu plugs mounted at different locations of mock cavities. RF measurement of a MgB2-coated single-cell 3-GHz test cavity was carried out, and it showed superconducting transition at 36 K. The quality factor of this test cavity was lower than expected due to poor connectivity and inclusion of Mg–Cu alloy in the MgB2 coating.

Superconducting radio frequency (SRF) cavities are adopted to upgrade the CSNS Linac energy from 80 to 300 MeV in phase II. A 648 MHz single spoke resonator (SSR) prototype has a compact size and low cost, which is a good pre-research cavity for CSNS to learn SRF technology. In this paper, the sequential approach of analysis and optimization performed for the structural design of 648 MHz SSR prototype are presented. The COMSOL Multiphysics is used to perform the multiphysics analysis of jacketed cavity. Firstly, the mechanical performance of bare cavity is estimated, and then, several stiffeners are added to strengthen the cavity. After that, the Lorentz coefficient and helium fluctuation df/dp of jacketed cavity are optimized. At last, the tuning sensitivity and mechanical performance of maximum allowable working pressure are calculated. The final results of Lorentz coefficient and df/dp are − 5.06 Hz/(MV/m)2 and 0.3 Hz/mbar, respectively, and the tuning range of jacketed cavity is larger than ± 1000 kHz at 2 K. The structural design of a 648 MHz SSR prototype is performed in this paper. The Lorentz coefficient and df/dp are carefully optimized. The simulation result shows all mechanical performance satisfies the design goals very well.

An ion accelerator, RAON is under construction in Daejeon, Korea, by the Rare Isotope Science Project (RISP) in the Institute of Basic Science (IBS). In this accelerator various ions, such as uranium, proton, carbon, calcium, etc. are planned to be accelerated by using four kinds of superconducting cavities. The four kinds of cavities are the quarter wave resonator (QWR), half wave resonator (HWR), and two kinds of single spoke resonators (SSRs) with different betas. Recently, the upgrade and the performance tests with newly developed field-programmable gate array (FPGA)-based digital low level radio frequency (LLRF) and superconducting cavities were conducted at the superconducting radio frequency (SRF) facility. In this paper, the status and the test result of the RAON LLRF will be described.

Superconducting radio frequency (SRF) cavities are often limited by thermal quench, which is an excessive electromagnetic heating that occurs in the high magnetic field area and expands thereafter forcing the cavity to lose its superconducting state. In this article, we demonstrate how the quench phenomena can be modeled using a coupled electromagnetic thermal analysis. The proposed model takes into account the nonlinearity of the material properties at cryogenic temperature and the effect of Kapitza resistance. The proposed approach is used to compute the thermal quench field of a 3.9-GHz 9-cell accelerating cavity, a 2.815-GHz deflecting cavity, and a 1.3-GHz 9-cell accelerating cavity. The computed values of quench field are in good agreement with the measured ones observed during vertical testing at 2 K. Without loss of generality, the proposed methodology can be applied to other cavity geometries.

High-Q operation of a superconducting radio frequency cavity can reduce the power loss at the surface and is desirable for continuous-wave operation. To realize the high-Q operation, the surface resistance, which is a sum of the Barden-Cooper-Schrieffer resistance and residual resistance, needs to be reduced. The residual resistance has been found to originate primarily from the magnetic flux trapping in defects in the cavity during the cooling down process. A magnetometer called the “flux gate sensor” has been used to measure the ambient magnetic field; however, this is larger in comparison with the cavity and it is very expensive. Sensors based on the magneto-resistance effect, such as the Anisotropic-Magneto-Resistance (AMR) sensor, are smaller and much less expensive than a flux gate sensor. We examined the characteristics of the magneto-resistance sensors at room temperature and liquid nitrogen temperature. The results are reported in this paper.

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.

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.

A combination of X-ray computed tomography (CT) and focused ion beam - scanning electron microscopy (FIB-SEM) has been employed to investigate substrate and related surface defects in a niobium coated superconducting radio frequency (SRF) copper cavity. The cavity was manufactured by spinning, with subsequent application of a sputtering-deposited niobium coating (≈40 μm thick) on the internal surface. Before coating, the copper surface was pre-treated in several stages, ending with chemical polishing. CT and FIB-SEM identified furrow defects (≈20 μm deep) in the copper beneath the coating, which originated from the spinning process. The furrows were filled with niobium and contained voids at the Nb/Cu interface that extended a few microns into the niobium coating. The presence of the defects led to similar furrows at the niobium surface. The study revealed the importance of pre-treatment of the cavity internal surface and control of the spinning process to avoid defects that may have deleterious influence on the Q slope and durability of the niobium coating.

Test cavities to characterize superconductor samples are of great interest for the development of materials suitable for superconducting radio frequency (SRF) accelerator systems. They can be used to investigate fundamental SRF loss mechanisms and to study the material limitations for accelerator applications. Worldwide, this research is based on only few systems that differ in operating frequency, sample size and shape, and the accessible parameter space of frequency, temperature, and RF field strength. For useful performance predictions in future accelerators, it is important that the operating parameter range is close to that employed in accelerating systems. Since 2014, the Helmholtz-Zentrum Berlin has operated such a system built around a redesigned Quadrupole Resonator (QPR). It is based on a system originally developed at CERN. Important new design modifications were developed, along with new measurement techniques and insight into their limitations. In the meantime, an increasing number of laboratories are adopting the QPR for their measurement campaigns. This paper provides a comprehensive overview of the state-of-the-art, the wide spectrum of measurement capabilities, and a detailed analysis of measurement uncertainties, as well as the limitations one should be aware of to maximize the effectiveness of the system. In the process, we provide examples of measurements performed with Nb3Sn and bulk niobium.

We propose an experimental setup to search for Axion-like particles (ALPs) using two superconducting radio-frequency cavities. In this light-shining-through-wall setup the axion is sourced by two modes with large fields and nonzero $\vec E\cdot \vec B$ in an emitter cavity. In a nearby identical cavity only one of these modes, the spectator, is populated while the other is a quiet signal mode. Axions can up-convert off the spectator mode into signal photons. We discuss the physics reach of this setup finding potential to explore new ALP parameter space. Enhanced sensitivity can be achieved if high-level modes can be used, thanks to improved phase matching between the excited modes and the generated axion field. We also discuss the potential leakage noise effects and their mitigation, which is aided by O(GHz) separation between the spectator and signal frequencies., Comment: 16 pages, 6 figures

The development of iterative learning control combined with disturbance-observer-based (DOB) control for the digital low-level radio frequency (LLRF) system of the International Linear Collider project is presented. The objective of this study is to compensate for both repetitive (or predictable) and unpredictable disturbances in a radio frequency system (e.g., beam loading, Lorentz force detuning, and microphonics). The DOB control approach was verified using the LLRF system at the Superconducting Test Facility (STF) at KEK in the absence of a beam. The method comprising DOB control combined with an iterative learning control algorithm was then demonstrated in a cavity-simulator-based test bench, where a simulated beam was available. The results showed that the performance of the LLRF system was improved, as expected by this combined control approach. We plan to further generalize this approach to LLRF systems at the STF and the future International Linear Collider project.

The Residual Resistivity Ratio (RRR) is an important parameter which is used to determine the purity and thermal conductivity of Niobium (Nb) utilized for fabrication of Superconducting Radio Frequency (SRF) cavities. The usual 4-probe electrical resistance measurement method is both destructive in nature and produces a non-local (average) measurement of the electrical conductivity. The present work utilizes Printed Circuit Board (PCB) based planar inductor as sensing element. The impedance variation of the sensing element in the presence of a conducting metal (Nb), is used to determine the conductivity of the Nb target. A generalized equation for RRR is proposed in terms of parameters ‘a’ and ‘b’ associated with the sensing coil. The measurement method is experimentally verified by utilizing samples of Nb of known RRR values. The sensor parameters (‘a’ and ‘b’) are also established for cryogenic conditions. Errors in measurement, accuracy and repeatability of measurements down to 4.2 K are reported.

The ever-present native oxide layer on niobium plays a fundamental role in the performance of Nb superconducting radio frequency (SRF) cavities for particle accelerators and light sources. Using Nb(111) and Nb(100) as model systems, oxygen dissolution and surface structural evolution as a function of thermal treatments in ultrahigh vacuum (UHV) were studied using combined Auger electron spectroscopy (AES) and scanning tunneling microscopy (STM), providing novel, real space information regarding the complex evolution of the Nb surface oxide. The surface crystallographic orientation was shown to influence oxide surface structure; Nb(111) displayed disordered surface oxide domains while Nb(100) was comprised of well-ordered, (n × 1)-O domains. The temperature-dependent dissolution of the native oxide layer on Nb(100) was characterized to ascertain the energetics for oxygen dissolution, and evolution of the surface oxide superlattice structure following thermal annealing was observed with STM. By understanding the kinetic behavior of oxygen dissolution during thermal annealing and subsequent structural evolution of the Nb(n × 1)-O superlattice on Nb(100), a more thorough understanding of the complex interactions driving chemical composition and cavity surface structure under Nb SRF cavity processing and operating conditions may be understood.

Superconducting radio frequency (SRF) cavities with quality factors ∼1010 near 4 K have potential to be cooled using regenerative-cycle cryocoolers. Contrary to using liquid helium, cryogen-free operation can be realized by conductively linking a cryocooler to a cavity for extracting the cavity RF dissipation. The cavity-cryocooler thermal link needs careful design as its thermal conductance will control the temperatures of the cavity and the cryocooler. We present a thermal analysis of a conduction-cooled SRF cavity that identifies the link thermal conductance requirement. The analysis uses published or expected RF dissipation characteristics of an Nb3Sn coated niobium cavity and measured cooling capacity of a pulse tube cryocooler. We describe the mechanical design of a link that is constructed using commercial high-purity aluminum and facilitates bolted-connection to elliptical SRF cavities. The thermal performance of the link is assessed by finite element simulations, taking into account temperature dependent thermal conductivities and measured thermal contact resistance of aluminum and niobium. The link is shown to support operation at an accelerating gradient of 10 MV/m with the lowest-known ‘perfect’ Nb3Sn residual surface resistance (∼10 nΩ) and also under non-ideal cases that assume certain static heat leak into the system and non-perfect Nb3Sn coatings.

Superconducting radio frequency cavities are used as main accelerating cavities and harmonic cavities in modern light sources. The addition of a harmonic cavity provides the possibility to increase the electron bunch length and therefore to improve the beam life time. The effects on instabilities and the bunch lengthening factor by harmonic cavities can be obtained from theoretical calculations. However, these calculations ignore phase transient effects and may overestimate the desired performance of the harmonic cavities. A multibunch multiparticle tracking method was developed to simulate the performance of harmonic cavities for non-uniform filling patterns, which take into account phase transient effects. Performance simulations using this tracking method for a prototype 1.5-GHz superconducting harmonic cavity for the Taiwan photon source are presented here.

Superconducting magnesium diboride is a possible candidate for replacing Nb in superconducting radio frequency (SRF) cavities because of its higher T c and potentially higher breakdown field. However, the existence of two energy gaps, the σ gap at 7.2 eV and the π gap at 2.2 eV, could impact the RF properties of MgB 2 . It has been shown that chemical doping and disorder can increase the interband scattering, thus modifying the two gaps. In this work, we performed Al ion implantation in MgB 2 thin films to introduce Al doping in the films. The Al ion implanted films showed decreases in transition temperature, from 40.4 K in the pristine films to non-superconducting above the dose of 2 × 10 14 atoms/cm 2 . An expansion in the c-axis lattice parameter with increasing dose was also observed. These results suggest that the degradation of superconductivity in the Al ion implanted films is mainly due to the ion beam damage rather than Al doping.

A vertical test for a 500-MHz superconducting radio frequency (SRF) cavity succeeded in the RF laboratory of the National Synchrotron Radiation Research Center. The test system, consisting of a vertical test cryostat, an RF power amplifier, a tunable coupler, and a low-level RF control system is integrated with the existing long-distance cryogenic transfer system and radiation shield. Relying neither on a gasbag nor a purifier, the vertical test cryostat is directly connected to the 400-W cryogenic plant that simultaneously provides liquid helium to the SRF module in the Taiwan Light Source. Applying special cleaning, drying, and purging processes, the vaporized helium gas suffers no pollution during the vertical tests. The valve on the return line for the gaseous helium regulates the helium pressure of the vertical test cryostat at an acceptable fluctuation of +/– 100 Pa. The measurement results for a KEKB-type 500-MHz SRF cavity are illustrated herein to demonstrate the integrated performance of this measurement system.

A vertical test system (VTS) for superconducting radio frequency cavities had been designed and constructed at Peking University. During the test, field emission electrons in the cavity will be accelerated and some of them will hit the flange of cavity with high-energy. These high-energy electrons will have a significant impact on the radiation safety of the system and environment. In this paper, the Particle-in-Cell solver in CST software was used to analyze the behavior of electrons emitted from different field emission positions and found the worst field emission site which deposited the most energy on the cavity wall. The impact information of the electrons emitted from the worst field emission site was utilized to character the source term. Prompt dose equivalent rate distribution and residual radioactivity was predicted by using the Monte Carlo simulation program FLUKA. Further vertical test results are consistent with the simulation results.

At the electron linac for beams with high brilliance and low emittance (ELBE) center for high-power radiation sources, the second version of a superconducting radio-frequency (SRF) photoinjector has been put into operation and has been routinely applied for user operation at the ELBE electron accelerator. SRF guns are suitable for generating a continuous wave electron beam with high average currents and high beam brightness. The SRF gun at ELBE has the goal to generate short electron pulses with bunch charges of 200–300 pC at typical repetition rates of 100 kHz for the production of superradiant, coherent terahertz radiation. The SRF gun includes a 3.5-cell, 1.3-GHz niobium cavity and a superconducting solenoid. A support system with liquid nitrogen (LN_{2}) cooling allows the operation of normal-conducting, high quantum efficiency photocathodes. We present the design and performance of the SRF gun as well as beam measurement results of the operation with Mg photocathodes at an acceleration gradient of 8 MV/m (4 MeV kinetic energy). In the last section, we discuss the SRF gun application for production of coherent terahertz radiation at the ELBE facility.

Field emission is one of the main factors that can limit the performance of superconducting radio frequency cavities. To reduce possible field emission in the Linac Coherent Light Source II (LCLS-II), we are developing plasma processing for 1.3 GHz nine-cell cavities. The ultimate goal of plasma processing will be to apply the technique in situ in the cryomodules in order to mitigate hydrocarbon-related field emission without disassembling them. Herein is presented the first systematic study of plasma processing applied to LCLS-II superconducting radio frequency cavities. Having developed a new method of plasma ignition for LCLS-II cavities, we applied plasma processing to 1.3 GHz cavities starting with a clean nitrogen doped cavity and proceeding with studying natural field emission and artificially contaminated cavities. All the cavities were cold tested before and after plasma cleaning in order to compare their performances. It was proved that this technique successfully removes carbon-based contamination from the cavity iris and that it is able to eliminate field emission in a naturally field emitting cavity. The effect of plasma processing on cavities exposed to vacuum failures was also investigated, showing positive results in some cases. This work shows how successful plasma processing is in removing hydrocarbon related contamination from the cavity surface without affecting the high Q-factors and quench fields characteristic of nitrogen doped cavities.

20 pags., 10 figs., 1 tab., Helium Atom Scattering (HAS) and Helium Spin-Echo scattering (HeSE), together helium scattering, are well established, but non-commercial surface science techniques. They are characterised by the beam inertness and very low beam energy (

The helium cryogenic system at Spallation Neutron Source (SNS) provides cooling to 81 superconducting radio frequency cavities. To support the operation of the cryogenic facility, a highly reliable control system consisting of software, hardware, and Human Machine Interface (HMI) has been developed and improved during the first fifteen years of operation. Integrating the cryogenic control system with other subsystems of the SNS complex is an important aspect to the success of the operation. The operating experience, lessons learned and recommendations to consider for future facilities will be detailed in this paper.

International audience; Controlling trapped magnetic flux in superconducting radio frequency (rf) cavities is of crucial importance in modern accelerator projects. In order to study flux trapping efficiency and sensitivity of surface resistance, dedicated experiments have been carried out on different types of low-β superconducting accelerating cavities. Even under almost full trapping conditions, we found that the measured magnetic sensitivities of these cavity geometries were significantly lower than the theoretical values predicted by commonly used models based on local material properties. This must be resolved by taking account of geometrical effects of flux trapping and flux oscillation under rf surface current in such cavity shape. In this paper, we propose a new approach to convolute the influence of geometries. We point out a puzzling contradiction between sample measurements and recent cavity experiments, which leads to two different hypotheses to simulate oscillating flux trapped in the cavity surface. A critical reconsideration of flux oscillation by the rf Lorentz force, compared with temperature mapping studies in elliptical cavities, favored the results of previous sample measurements, which suggested preferential flux trapping of normal component to the cavity inner surface. Based on this observation, we builded a new model to our experimental results and the discrepancy between old theory and data were resolved.

Applied physics letters 119(19), 194102 (2021). doi:10.1063/5.0063379, A fundamental understanding of superconducting radio frequency Nb cavity processing is necessary to achieve the desired improvement in their performance, which is needed for further upgrades of modern particle accelerators. To recognize the physical processes behind the losses in the accelerator modules, it is required to address not only the observed improvements but also the degradation occurring after different surface treatments. Here, we report on microscopic and spectroscopic studies of several cutouts from an extremely well performing cavity, which showed a systematic degradation after modified surface treatments and annealing conditions. Our results suggest that an abundance of low-angle grain boundaries surrounding the small sized grains can be related to the local superconductivity breakdown at high accelerating field gradients. Losses due to grain boundary segregated carbides are discussed to being most dominant and to leading to an anomalous Q-degradation of the whole cavity starting at low fields., Published by American Inst. of Physics, Melville, NY

A disturbance observer (DOB)-based control approach was verified using the low-level radio-frequency systems of superconducting (SC) cavities operated in a continuous wave (CW) mode in a compact energy recovery linac (cERL) at KEK. This approach compensates for both predictable and unpredictable disturbances in a radio frequency (rf) system such as beam loading, Lorentz force detuning, and microphonics. This paper generalizes the DOB method in pulsed SC rf systems in the cERL at KEK. The results show that the pulse-to-pulse stabilities of the rf system are improved as expected by using this control approach. Furthermore, this method is validated by successfully compensating long pulse beam-loading effects in cERL.

The high-energy particle accelerator uses the fundamental mode of the superconducting radio frequency cavity to accelerate the charged particle beam, and when the charged particle beam passes through the radio frequency(RF) cavity, it will excite a series of harmful higher-order modes(HOMs) besides the fundamental mode. The 3D simulation software is used for 9-cell TESLA superconducting acceleration cavity HOM simulation calculation. Analyze the mode spectrum distribution of each HOM and the performance parameters that characterize the HOM, and identify harmful HOMs.

We address production of massive axion-like particles by two electromagnetic modes inside a superconducting radio-frequency (SRF) cylindrical cavity. We discuss in detail the choice of pump modes and cavity design. We numerically compute time-averaged energy density of produced axion field for various cavity modes and wide range of axion masses. This allows us to estimate optimal conditions for axion production within a cavity. In addition, we consider photon regeneration process initiated by produced axion field in a screened radio-frequency cavity and derive constraints in parameter space $(g_{a\gamma\gamma},\ m_a)$ for different choice of pump modes., Comment: 22 pages, 6 figures, 1 table

We report on the development of machine learning models for classifying C100 superconducting radio-frequency (SRF) cavity faults in the Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson Lab. CEBAF is a continuous-wave recirculating linac utilizing 418 SRF cavities to accelerate electrons up to 12 GeV through 5-passes. Of these, 96 cavities (12 cryomodules) are designed with a digital low-level RF system configured such that a cavity fault triggers waveform recordings of 17 RF signals for each of the 8 cavities in the cryomodule. Subject matter experts (SME) are able to analyze the collected time-series data and identify which of the eight cavities faulted first and classify the type of fault. This information is used to find trends and strategically deploy mitigations to problematic cryomodules. However manually labeling the data is laborious and time-consuming. By leveraging machine learning, near real-time (rather than post-mortem) identification of the offending cavity and classification of the fault type has been implemented. We discuss performance of the ML models during a recent physics run. Results show the cavity identification and fault classification models have accuracies of 84.9% and 78.2%, respectively., 20 pages, 10 figures submitted to Physical Review Accelerators and Beams

The efficiency of the superconducting radio frequency cavities composed of Nb required the deposition of thickness-controlled multilayer coatings of superconductor-insulator-superconductor (S-I-S) on the internal surfaces of the cavities. Herein, we report the plasma-enhanced atomic layer deposition of carbon-free NbN (50 µm thick), followed by a thermal treatment, to obtain the superconducting layer in the S-I-S structure. Using (tert-butylimido)-tris(diethylamino)-niobium as the niobium precursor and H2 and NH3 plasma as reactive gasses, the deposition and annealing parameters were optimized by studying their effects on the film properties (crystallinity, density, and composition). We demonstrated that the superconducting critical temperature (Tc) can be improved after thermal annealing up to 13.8 K, a value compatible with the targeted application. In addition to the expected densified layers and increased grain size, we observed a partial transformation of the oxide present in the as-deposited layer into niobium oxynitride, which could indicate the origin of the improvement of the superconducting properties. With low carbon and oxygen impurity concentrations in the films, this study contributes to the understanding of the relationship between the structure, composition, and superconductivity in NbN.

High brightness, high charge electron beams are critical for a number of advanced accelerator applications. The initial emittance of the electron beam, which is determined by the mean transverse energy (MTE) and laser spot size, is one of the most important parameters determining the beam quality. The bialkali photocathodes illuminated by a visible laser have the advantages of high quantum efficiency (QE) and low MTE. Furthermore, Superconducting Radio Frequency (SRF) guns can operate in the continuous wave (CW) mode at high accelerating gradients, e.g. with significant reduction of the laser spot size at the photocathode. Combining the bialkali photocathode with the SRF gun enables generation of high charge, high brightness, and possibly high average current electron beams. However, integrating the high QE semiconductor photocathode into the SRF guns has been challenging. In this article, we report on the development of bialkali photocathodes for successful operation in the SRF gun with months-long lifetime while delivering CW beams with nano-coulomb charge per bunch. This achievement opens a new era for high charge, high brightness CW electron beams.

Energy recovery linear accelerators (ERLs) rely upon single-axis superconducting radio frequency (SRF) cavities to be an efficient source of relativistic electrons for high energy and nuclear physics. SRF cavities are also considered relevant for next-generation photon factories and radio-isotope production facilities. The ultimate energy recovery capability for the accelerator would be the ability to operate with a sufficiently spent beam and decrease the energy of the beam before the beam dump to a value lower than the beam injection energy. This is especially important for high current accelerators, where the beam injection energy could be as high as several MeV, and systems where partial beam loss can be expected. The efficient operation of energy recovery in linear accelerators is adversely affected by the typical degradation of the beam quality and current loss. This hinders the application of the ERLs in research and industry, and enabling the use of spent (partially lost current), degraded beams should broaden their application. We suggest that the use of the asymmetric operating mode observed in the dual-axis asymmetric cavities will enable such ultimate capabilities. We discuss the advantages of the application of fields of different amplitude along the cavity accelerating and decelerating axes and demonstrate that the fields can be tuned separately in each axis of the dual-axis cavity. The design of such a cavity and ways to optimize the energy recovery of a spent electron beam by tuning the dual-axis asymmetric SRF cavity are discussed.

Continuous-wave photoinjectors operating at high accelerating gradients promise to revolutionize many areas of science and applications. They can establish the basis for a new generation of monochromatic x-ray free electron lasers, high-brightness hadron beams, or a new generation of microchip production. In this Letter we report on the record-performing superconducting rf electron gun with CsK_{2}Sb photocathode. The gun is generating high charge electron bunches (up to 10 nC/bunch) and low transverse emittances, while operating for months with a single photocathode. This achievement opens a new era in generating high-power beams with a very high average brightness.

This paper consists of two main parts regarding coherent synchrotron radiation (CSR). The first part extends the CSR theory of two particle interaction from a system of one bending magnet to two bending magnets, where the wake can leak from the first to the second. The new theory agrees well with the established simulation code Bmad. The second part of the paper presents the CSR simulation results on CBETA, the Cornell BNL Energy-Recovery-Linac (ERL) Test Accelerator, in which the magnets are so close to each other and the new extended theory becomes important. CBETA is the first multiturn ERL with superconducting radio frequency (SRF) accelerating cavities and a Fixed Field Alternating gradient (FFA) beamline. Simulations show that CSR causes phase space dilution that becomes more significant as the bunch charge and the number of recirculation passes increase. Potential ways to mitigate the CSR effects, including adding vacuum chamber shielding and increasing bunch length, are being investigated.