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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.

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.

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.

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.

Superconducting radio-frequency cavities are commonly used in modern particle accelerators for applied and fundamental research. Such cavities are typically made of high-purity, bulk Nb and with cooling by a liquid helium bath at a temperature of ~2 K. The size, cost and complexity of operating a particle accelerator with a liquid helium refrigerator makes the current cavity technology not favorable for use in industrial-type accelerators. We developed a multi-metallic 1.495 GHz elliptical cavity conductively cooled by a cryocooler. The cavity has a ~2 μm thick layer of Nb3Sn on the inner surface, exposed to the rf field, deposited on a ~3 mm thick bulk Nb shell and a bulk Cu shell, of thickness ≥ 5 mm deposited on the outer surface by electroplating. A bolt-on Cu plate 1.27 cm thick was used to thermally connect the cavity equator to the second stage of a Gifford-McMahon cryocooler with a nominal capacity of 2 W at 4.2 K. The cavity was tested initially in liquid helium at 4.3 K and reached a peak surface magnetic field of ~36 mT with a quality factor of 2 × 109. The cavity cooled by the crycooler achieved a peak surface magnetic field of ~29 mT, equivalent to an accelerating gradient of 6.5 MV/m. The conduction-cooled cavity could be operated in continuous-wave with as high as 5 W dissipation in the cavity for 1 h without any thermal breakdown, because of the Cu outer layer with high thermal conductivity. This result represents a paradigm shift in the technology of superconducting accelerator cavities.

We tested the prototype DQW cavity in GERSEMI vertical test stand in FREIA. The performance metthe spec ification , and our experimental procedure and infrastructure are qualified for testing crab cavitiesi n the HL LHC project. This was the critic al milestone in the project and it also opens a newopportunity for testing superconducting cavities in FREIA.

We report on an effort to improve the performance of superconducting radiofrequency cavities by the use of heat treatment in a temperature range sufficient to dissociate the natural surface oxide. We find that the residual resistance is significantly decreased, and we find an unexpected reduction in the BCS resistance. Together these result in extremely high quality factor values at relatively large accelerating fields Eacc ~20 MV/m: Q0 of 3-4x10^11 at, 9 pages, 9 figures

To improve RHIC luminosity for heavy ion beam energies below $10\text{ }\text{ }\mathrm{GeV}/\text{nucleon}$, the Low Energy RHIC electron Cooler (LEReC) is currently under commissioning at BNL. The linac of LEReC is designed to deliver a 1.6--2.6 MeV electron beam, with rms $dp/p$ less than $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$. A 704 MHz superconducting radio frequency booster cavity in this linac provides up to 2.2 MeV accelerating voltage. With such a low energy and very demanding energy spread requirement, control of higher order modes (HOMs) in the cavities becomes critical and needs to be carefully evaluated to ensure minimum impact on the beam. In this paper, we report the multiphysics design of the HOM damper for this cavity to meet the energy spread requirement, as well as experimental results of the cavity with and without the HOM damper.

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.

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.

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 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

Superconducting Radio Frequency (SRF) cavities are used in modern accelerators to efficiently accelerate particles. When cavity is supplied with pulsed RF field it undergoes a mechanical strain due to the Lorentz force. The resulting deformation causes dynamic detuning whose amplitude depends on mechanical properties of the cavity, RF pulse rate and their profile. This effect causes considerable loss of acceleration performance. Therefore, it is usually actively compensated, most commonly with fast piezoelectric actuators. MicroTCA.4 standard was developed to accommodate control and data acquisition electronic systems of large-scale physics applications. The paper presents a design of high-power amplifier implemented using the MicroTCA.4 technology. The design of the driver was optimized for driving large-capacitance piezo actuators. Several possible architectures of the driver are presented and compared, taking into consideration the power and cooling limitations of MicroTCA.4. The design of a two-channel piezo driver and its initial laboratory test results are also discussed., 2 pages, 3 figures, short form for Real-Time 2018 Conference

A compact Inverse Compton Light Source (ICLS) design is presented, with flux and brilliance orders of magnitude beyond conventional laboratory-scale sources and other compact ICLS designs. This design utilizes the physics of inverse Compton scattering of an extremely low emittance electron beam by a laser pulse of rms length of approximately two-thirds of a picosecond (2/3 ps). The accelerator is composed of a superconducting radiofrequency (SRF) reentrant gun followed by four double-spoke SRF cavities. After the linac are three quadrupole magnets to focus the electron beam to the interaction point (IP). The distance from cathode surface to IP is less than 6 meters, with the cathode producing electron bunches with a bunch charge of 10 pC and a few picoseconds in length. The incident laser has 1 MW circulating power, a 1 micron wavelength, and a spot size of 3.2 microns at the IP. The repetition rate of this source is 100 MHz, in order to achieve a high flux despite the low bunch charge. The anticipated X-ray source parameters include an energy of 12 keV, with a total flux of $1.4\times10^{14}$ ph/s, the flux into a 0.1% bandwidth of $2.1\times10^{11}$ ph/(s-0.1%BW), and the average brilliance of $2.2\times10^{15}$ ph/(s-mm$^2$-mrad$^2$-0.1%BW)., 19 pages, 18 figures; Submitted to Phys. Rev. Accel. Beams

The niobium is currently used for the construction of the superconducting radio frequency (RF) Crab Cavity for the particle accelerator LHC at CERN in Geneva. An alternative technique to traditional forming methods is the electrohydraulic forming (EHF), in which ultrahigh-speed deformation of blank sheets is performed by using shockwaves electrically induced in water. A big effort is made for the analysis of the forming processes by FEM simulations, which require the definition of an appropriate flow stress material model. With this aim, in the present work, a testing campaign was performed in tension on sheet specimens with a rectangular cross-section at different strain-rates, up to 103 s-1. The obtained results showed the material is strongly sensitive to strain-rate, as expected for a pure BCC metal. The data, were processed via a reverse engineering procedure, based on finite element simulations of the experimental tests. This methodology allowed the identification of a tabular flow stress model (MAT_224 implemented in LSDYNA) for the prediction of the material behaviour as a function of the plastic strain, strain-rate and temperature.

We report the rf performance of a single cell superconducting radiofrequency cavity after low temperature baking in a nitrogen environment. A significant increase in quality factor has been observed when the cavity was heat treated in the temperature range of $120--160\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ with a nitrogen partial pressure of $\ensuremath{\sim}25\text{ }\text{ }\mathrm{m}\text{ }\mathrm{Torr}$. This increase in quality factor as well as the $Q$-rise phenomenon (anti-$Q$-slope) is similar to those previously obtained with high temperature nitrogen doping as well as titanium doping. In this study, a cavity ${\mathrm{N}}_{2}$-treated at $120\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ and at $140\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ showed no degradation in accelerating gradient, however the accelerating gradient was reduced by $\ensuremath{\sim}25%$ with a $160\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ ${\mathrm{N}}_{2}$ treatment, compared to the baseline tests after electropolishing. Sample coupons treated in the same conditions as the cavity were analyzed by scanning electron microscope, x-ray photoelectron spectroscopy and secondary ion mass spectroscopy revealed a complex surface composition of ${\mathrm{Nb}}_{2}{\mathrm{O}}_{5}$, NbO and ${\mathrm{NbN}}_{(1\ensuremath{-}x)}{\mathrm{O}}_{x}$ within the rf penetration depth. Furthermore, magnetization measurements showed no significant change on bulk superconducting properties.

Superconductor magnesium diboride is considered one of the viable materials to substitute bulk niobium for superconducting radio frequency cavities. Utilizing a ${\mathrm{MgB}}_{2}$ coating on the inner wall of a copper cavity will allow operation at higher temperatures (20--25 K) than Nb cavities due to the high transition temperature of ${\mathrm{MgB}}_{2}$ (39 K) and the high thermal conductivity of Cu. In this paper, we present results of ${\mathrm{MgB}}_{2}$ coating on Cu tubes with similar dimensions to a 3 GHz cavity, as the first step towards coating the actual cavity, using the hybrid physical chemical vapor deposition technique. The results show successful coating of a uniform ${\mathrm{MgB}}_{2}$ layer on the inner wall of the Cu tubes with ${T}_{c}$ as high as 37 K.

The inner surface of superconducting cavities plays a crucial role to achieve highest accelerating fields and low losses. For an investigation of this inner surface of more than 100 cavities within the cavity fabrication for the European XFEL and the ILC HiGrade Research Project, an optical inspection robot OBACHT was constructed. To analyze up to 2325 images per cavity, an image processing and analysis code was developed and new variables to describe the cavity surface were obtained. The accuracy of this code is up to 97% and the PPV 99% within the resolution of 15.63 $\mu \mathrm{m}$. The optical obtained surface roughness is in agreement with standard profilometric methods. The image analysis algorithm identified and quantified vendor specific fabrication properties as the electron beam welding speed and the different surface roughness due to the different chemical treatments. In addition, a correlation of $\rho = -0.93$ with a significance of $6\,\sigma$ between an obtained surface variable and the maximal accelerating field was found.

Thermal stability is an important parameter for the operation of the superconducting radio frequency (SRF) cavities used in particle accelerators. The rf power dissipated on the inner surface of the cavities is conducted to the helium bath cooling the outer cavity surface and the equilibrium temperature of the inner surface depends on the thermal resistance. In this manuscript, we present the results of direct measurements of thermal resistance on 1.3 GHz single cell SRF cavities made from high purity large-grain and fine-grain niobium as well as their rf performance for different treatments applied to outer cavity surface in order to investigate the role of the Kapitza resistance to the overall thermal resistance and to the SRF cavity performance. The results show no significant impact of the thermal resistance to the SRF cavity performance after chemical polishing, mechanical polishing or anodization of the outer cavity surface. Temperature maps taken during the rf test show nonuniform heating of the surface at medium rf fields. Calculations of ${Q}_{0}({B}_{p})$ curves using the thermal feedback model show good agreement with experimental data at 2 and 1.8 K when a pair-braking term is included in the calculation of the Bardeen-Cooper-Schrieffer surface resistance. These results indicate local intrinsic nonlinearities of the surface resistance, rather than purely thermal effects, to be the main cause for the observed field dependence of ${Q}_{0}({B}_{p})$.

Elliptic cavities at medium- and high-$\ensuremath{\beta}$ range are receiving broader use in the particle accelerator applications. Optimizing the shape of these cavities is a complex and demanding process. In this paper we propose an optimization approach to minimize the ratio of peak magnetic field to the acceleration field ${H}_{\text{pk}}/{E}_{\text{acc}}$ while keeping the ratio of peak surface electric field to the accelerating field ${E}_{\text{pk}}/{E}_{\mathrm{acc}}$, aperture radius and wall slope angle $\ensuremath{\alpha}$ at some permitted values. We show that it is possible to substantially vary the cavity geometry without violating the constraints or deteriorating the objective of the optimization. This gives us freedom in designing the geometry to overcome problems such as multipactor while maintaining the minimal ${H}_{\text{pk}}/{E}_{\mathrm{acc}}$. The optimization is then performed to find a set of optimized geometries with minimum ${H}_{\text{pk}}/{E}_{\mathrm{acc}}$ for different $\ensuremath{\beta}$'s ranging from 0.4 to 1, different peak surface electric fields, wall slope angles and aperture radii. These data could be generally used as a suitable starting point in designing elliptic cavities.

Recent studies by Romanenko et al. revealed that cooling down a superconducting cavity under a large spatial temperature gradient decreases the amount of trapped flux and leads to reduction of the residual surface resistance. In the present paper, the flux expulsion ratio and the trapped-flux-induced surface resistance of a large-grain cavity cooled down under a spatial temperature gradient up to $80\text{ }\text{ }\mathrm{K}/\mathrm{m}$ are studied under various applied magnetic fields from 5 to $20\text{ }\text{ }\ensuremath{\mu}\mathrm{T}$. We show the flux expulsion ratio improves as the spatial temperature gradient increases, independent of the applied magnetic field: our results support and enforce the previous studies. We then analyze all rf measurement results obtained under different applied magnetic fields together by plotting the trapped-flux-induced surface resistance normalized by the applied magnetic field as a function of the spatial temperature gradient. All the data can be fitted by a single curve, which defines an empirical formula for the trapped-flux-induced surface resistance as a function of the spatial temperature gradient and applied magnetic field. The formula can fit not only the present results but also those obtained by Romanenko et al. previously. The sensitivity ${r}_{\mathrm{fl}}$ of surface resistance from trapped magnetic flux of fine-grain and large-grain niobium cavities and the origin of $dT/ds$ dependence of ${R}_{\mathrm{fl}}/{B}_{a}$ are also discussed.

Theory of the superconductor-insulator-superconductor (S-I-S) multilayer structure in superconducting accelerating cavity application is reviewed. The theoretical field limit, optimum layer thicknesses and material combination, and surface resistance are discussed. Those for the S-S bilayer structure are also reviewed., 38 pages, 21 figures

We present a new concept for a two-pole superconducting synchronous machine. This electrical motor is composed of a classical stator yoke, and an original superconducting inductor. The inductor is composed of two coaxial circular superconducting coils separated by superconducting bulks. Both solenoids are fed by a direct current and each of them produces a magnetic field in the same direction. An inclined superconducting bulk is placed between the two solenoids. It is used as superconducting magnetic barrier. The diamagnetic behavior of a superconducting bulk cooled under zero-field is used to obtain a variation of the magnetic flux density in the air-gap. The inductor is made of low-temperature superconducting wires for the coils, NbTi, and a high-temperature superconducting YBCO bulk, cooled down to 4.2K using liquid helium for the flux barrier. The armature is wound with copper wires as a classical one. We present design and tests of this superconducting motor. We made no load, short-circuit and full load tests.

During the fabrication of niobium superconducting radio frequency (SRF) particle accelerator cavities procedures are used that chemically or mechanically remove the passivating surface film of niobium pentoxide (Nb2O5). Removal of this film will expose the underlying niobium metal and allow it to react with the processing environment. If these reactions produce hydrogen at sufficient concentrations and rates, then hydrogen will be absorbed and diffuse into the metal. High hydrogen activities could result in supersaturation and the nucleation of hydride phases. If the metal repassivates at the conclusion of the processing step and the passive film blocks hydrogen egress, then the absorbed hydrogen or hydrides could be retained and alter the performance of the metal during subsequent processing steps or in-service. This report examines the feasibility of this hypothesis by first identifying the postulated events, conditions, and reactions and then determining if each is consistent with accepted scientific principles, literature, and data. Established precedent for similar events in other systems was found in the scientific literature and thermodynamic analysis found that the postulated reactions were not only energetically favorable, but produced large driving forces. The hydrogen activity or fugacity required for the reactions to be at equilibrium was determined to indicate the propensity for hydrogen evolution, absorption, and hydride nucleation. The influence of processing conditions and kinetics on the proximity of hydrogen surface coverage to these theoretical values is discussed. This examination found that the hypothesis of hydrogen absorption during SRF processing is consistent with published scientific literature and thermodynamic principles.

An apparatus and a method are described for plasma etching of the inner surface of superconducting radio frequency (SRF) cavities. Accelerator SRF cavities are formed into a variable-diameter cylindrical structure made of bulk niobium, for resonant generation of the particle accelerating field. The etch rate non-uniformity due to depletion of the radicals has been overcome by the simultaneous movement of the gas flow inlet and the inner electrode. An effective shape of the inner electrode to reduce the plasma asymmetry for the coaxial cylindrical rf plasma reactor is determined and implemented in the cavity processing method. The processing was accomplished by moving axially the inner electrode and the gas flow inlet in a step-wise way to establish segmented plasma columns. The test structure was a pillbox cavity made of steel of similar dimension to the standard SRF cavity. This was adopted to experimentally verify the plasma surface reaction on cylindrical structures with variable diameter using the segmented plasma generation approach. The pill box cavity is filled with niobium ring- and disk-type samples and the etch rate of these samples was measured., 8 pages, 10 figures

In this study, plasma based surface modification of niobium is a promising alternative to wet etching of superconducting radio frequency (SRF) cavities. The development of the technology based on Cl2/Ar plasma etching has to address several crucial parameters which influence the etching rate and surface roughness, and eventually, determine cavity performance. This includes dependence of the process on the frequency of the RF generator, gas pressure, power level, the driven (inner) electrode configuration, and the chlorine concentration in the gas mixture during plasma processing. To demonstrate surface layer removal in the asymmetric non-planar geometry, we are using a simple cylindrical cavity with 8 ports symmetrically distributed over the cylinder. The ports are used for diagnosing the plasma parameters and as holders for the samples to be etched. The etching rate is highly correlated with the shape of the inner electrode, radio-frequency (RF) circuit elements, chlorine concentration in the Cl2/Ar gas mixtures, residence time of reactive species and temperature of the cavity. Using cylindrical electrodes with variable radius, large-surface ring-shaped samples and d.c. bias implementation in the external circuit we have demonstrated substantial average etching rates and outlined the possibility to optimize plasma properties with respect to maximum surface processing effect.

Multipacting (MP) in higher-order-mode (HOM) couplers of the International Linear Collider (ILC) baseline cavity and the Continuous Electron Beam Accelerator Facility (CEBAF) 12 GeV upgrade cavity is studied by using the ACE3P suites, developed by the Advanced Computations Department at SLAC. For the ILC cavity HOM coupler, the simulation results show that resonant trajectories exist in three zones, corresponding to an accelerating gradient range of $0.6--1.6\text{ }\text{ }\mathrm{MV}/\mathrm{m}$, $21--34\text{ }\text{ }\text{ }\mathrm{MV}/\mathrm{m}$, $32--35\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ and $g40\text{ }\text{ }\mathrm{MV}/\mathrm{m}$, respectively. For the CEBAF 12 GeV upgrade cavity HOM coupler, resonant trajectories exist in one zone, corresponding to an accelerating gradient range of $6--13\text{ }\text{ }\mathrm{MV}/\mathrm{m}$. Potential implications of these MP barriers are discussed in the context of future high-energy pulsed as well as medium-energy continuous wave accelerators based on superconducting radio frequency cavities. Frequency scaling of MP's predicted in HOM couplers of the ILC, CEBAF upgrade, Spallation Neutron Source (SNS), and Free-Electron Laser in Hamburg (FLASH) third harmonic cavity is given and found to be in good agreement with the analytical result based on the parallel plate model.

The transverse emittance is one of the most important quantities which characterize the quality of an electron source. For high quality experiments low beam emittance is required. By means of theoretical considerations and simulation calculations we have studied how the emittance of the Rossendorf superconducting radio-frequency photoelectron source (SRF gun) can be minimized. It turned out that neither a solenoid magnet nor the effect of space charge forces is needed to create a pronounced emittance minimum. The minimum appears by just adjusting the starting phase of the electron bunch with respect to the RF phase of the gun in a suitable way. Investigation of various correlations between the properties of the beam particles led to an explanation on how the minimum comes about. It is shown that the basic mechanism of minimization is the fact that the longitudinal properties of the particles (energy) are strongly influenced by the starting phase. Due to the coupling of the longitudinal and transverse degrees of freedom by the relativistic equation of motion the transverse degrees of freedom and thereby the emittance can be strongly influenced by the starting phase as well. The results obtained in this study will be applied to minimize the emittance in the commissioning phase of the SRF gun.

As in 2007 the first 3.5 cell superconducting radio frequency (SRF) gun was taken into operation at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), it turned out that the specified performance to realize an electron energy gain of 9.4 MeV (E_peak=50 MV/m @ Q_0=1e10) has not been achieved. Instead, the resonator of the gun was limited by field emission to about one third of these values and the measured beam parameters remained significantly below the expectations. However, to demonstrate the full potential of this new electron source for the ELBE LINAC, a second and slightly modified SRF gun was developed and built in collaboration with Thomas Jefferson National Accelerator Facility (TJNAF). We will report on commissioning and first results of this new SRF gun. This includes in particular the characterization of the most important RF properties of the cavity as well as their comparison with previous vertical test results.

In January 2012 the 10 kW klystrons of the CW LINAC ELBE were replaced by pairs of 10kW solid state power amplifiers (SSPA). The talk gives an overview on the SSPA based ELBE RF system. The experience gained during the first two years of operation is reviewed.

Superconducting RF particle sources combine the advantages of normal conducting RF sources and high duty cycle non-RF sources. The Rossendorf SRF gun was the first to demonstrate this injecting electrons into the ELBE accelerator at 13 MHz. Recently, a new 3-1/2-gun cavity has been prepared at Jefferson Lab for its use in an updated injector which is expected to increase the electron energy from 2.4 to 7.5 MeV. Along with this new cavity, a new gun cryostat has been introduced. It combines several minor updates to the setup with the installation of a superconducting solenoid right at the exit of the gun, compensating the emittance growth of the electron bunch at an early stage. The poster is going to conclude the results of the commissioning of the new cryostat including the solenoid and compare it to the prior concept using a normal conducting solenoid outside the cryostat. As it is of great importance to this subject, studies of the magnetic shielding are going to be presented as well., Proceedings of the 5th Int. Particle Accelerator Conf., IPAC2014, Dresden, Germany

The authors have developed a 6 kV-200 A class superconducting fault current limiter (SC-FCL). The SC-FCL has eight co-axial coils connected in order to minimize total inductance. In the present paper, taking consideration of the self-magnetic field in the SC-FCL, they discussed the position of quench initiation. They calculated spatial profiles of the strength and the direction of the self-magnetic field in the SC-FCL. As a result, the self-magnetic field is applied to sections of the superconducting cable (SC cable) in the transverse direction (maximum value: 6.42/spl times/10/sup -4/ T/A). The connective section between coils, however, suffers a magnetic field with a longitudinal component of 3.34/spl times/10/sup -4/ T/A. In the case of a short sample of the SC cable which was used for the SC-FCL, the quench current levels were found to decrease at the rate of 40 A per 0.1 T in a transverse magnetic field and 100 A per 0.1 T in a longitudinal field. Taking these results into consideration, the authors pointed out that the quench may be initiated in the connective section between coils, where the self-magnetic field is applied in the longitudinal direction.

We investigate "hot" regions with anomalous high field dissipation in bulk niobium superconducting radio frequency cavities for particle accelerators by using low energy muon spin rotation (LE-$\mu$SR) on corresponding cavity cutouts. We demonstrate that superconducting properties at the hot region are well described by the non-local Pippard/BCS model for niobium in the clean limit with a London penetration depth $\lambda_\mathrm{L} = 23 \pm 2$ nm. In contrast, a cutout sample from the 120$^\circ$C baked cavity shows a much larger $\lambda > 100$ nm and a depth dependent mean free path, likely due to gradient in vacancy concentration. We suggest that these vacancies can efficiently trap hydrogen and hence prevent the formation of hydrides responsible for rf losses in hot regions.