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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.

An optimal electron-beam welding operating regime for ultra-pure sheet niobium has been developed for use in a superconducting resonator for the International Linear Collider (ILC). The formation of weld joints is studied and their microstructure and microhardness are investigated taking the required geometry of the weld seams into account. Low-temperature electrical measurements in magnetic fields up to 2 T are used to determine the critical parameters of the superconducting transition in the weld area. From the standpoint of the superconducting properties of the resonator, the slight degradation in the characteristics of sheet niobium observed in the thermally affected area (about 10% on average) is not of fundamental importance.

A superconducting solenoid-based focusing lens was designed and built for use in the SSR1 cryomodule of PXIE test facility at FNAL. As the cryomodule contains superconducting spoke-type cavities, one of main goals during design stage was minimization of magnetic field on walls of the cavities. The design also attempted minimization of the uncertainty of the magnetic axis position in the lens. This report describes main features of the design and summarizes results of performance tests and magnetic axis position measurements.

There are very strong requirements for the magnets with very high magnetic field for experimental facilities used in the new generations of special material processing devices. A high magnetic field superconducting magnet has been designed with the center field of 8 T, warm bore of 100 mm, and warm split-gap of 100 mm. The maximum magnetic field in the superconducting winding is about 11 T. Two GM cryocoolers are employed to cool down the system. The magnet consists of two high temperature superconducting coils with commercially available Bi-2223 tapes and six NbTi superconducting coils. There is very strong attractive force between the split-pairs coils. The design, fabrication and test of the superconducting magnet are reported in this paper.

This work examines the design and operation of a longitudinal resonant cavity, paired with monopole send and reciprocal patch receive antennae, that couples radio-frequency energy to a superconducting thin film carrying high current densities (~10 5 A/cm 2 ). The dielectric substrate supporting the film penetrates the waveguide, which operates in an evanescent mode below the design cutoff frequency of 18 GHz. Oscillatory vortex motion in the thin film is found to produce a small (~0.1 mV) dc voltage. When the niobium film is patterned to form an aperture that permits resonant conditions within the waveguide volume, the measured voltage increases by an order of magnitude. The increase is explained in the framework of the Larkin-Ovchinnikov model for quasiparticle behavior inside a moving normal vortex core. Operated near the superconducting transition, this device is useful for materials characterization, including the possibility to extract parameters including the pinning force. The authors suggest that the device could be used to characterize the pinning potential or to explore quasiparticle dynamics in superconducting thin films.

Superconducting magnets are key components to store energy for a superconducting magnet energy storage system (SMES). The quench of superconducting magnets will affect the normal operation of SMES, and even damage the device. So a quick and precise quench detection is needed to ensure the safety of SMES. During the dynamic power compensation, the current flowing in the superconducting magnet is not constant, and shows high frequency components due to the switching of the converter. This paper proposes a flux induction based quench detection method that can compensate nearly all the inductive voltage and electromagnetic noise. Furthermore, it can be used to locate the exact quench position. The principle of this new method is analyzed, and a quench detection scheme for a 35 kJ HTS SMES is designed. The effectiveness of the proposed method was evaluated by a test experiment.

The first long shutdown (LS1) of the LHC machine at CERN started in February 2013. The main trigger for LS1 was the consolidation of the bus bar joints in the 13-kA superconducting magnet circuits. Before LS1, the copper continuity of these joints was not sufficiently good to ensure a safe passage of the current in case of a quench in the superconducting cable at currents larger than 7-8 kA. The consolidation mainly consists in adding shunts made of high conductivity copper across each joint connecting the bus bars of the neighboring magnets in order to ensure the copper continuity. After LS1, the LHC can then be operated up to the original nominal conditions, i.e., a collision energy of 14 TeV. There are 1695 interconnects, each containing six 13-kA joints, of which two belong to the main dipole circuit and four to the main quadrupole circuits. This paper describes the different steps of the consolidation work performed on the 10 170 magnet-to-magnet joints. The work on the splices proper has been organized in the form of a train made of six production teams, all together consisting of about 90 persons, including operators, supervision, and quality assurance. The work flow is managed with the help of a web-based tool called WISH. This paper describes as well the quality control and quality assurance methods put in place to ensure the quality of the work throughout the project, and it summarizes the benefits of the consolidation work in terms of copper continuity, operation margin, and robustness of the new insulation system.

Pb thin films were prepared by the nanosecond pulsed laser deposition technique on Si (100) and polycrystalline Nb substrates for photocathode application. As the photoemission performances of a cathode are strongly affected by its surface characteristics, the Pb films were grown at different substrate temperatures with the aim of modifying the morphology and structure of thin films. An evident morphological modification in the deposited films with the formation of spherical grains at higher temperatures has been observed. X-ray diffraction measurements showed that a preferred orientation of Pb (111) normal to the substrate was achieved at 30 °C while the Pb (200) plane became strongly pronounced with the increase in the substrate temperature. Finally, a Pb thin film deposited on Nb substrate at 30 °C and tested as the photocathode showed interesting results for the application of such a device in superconducting radio frequency guns.

Equipment, such as a vacuum furnace, high pressure rinse (HPR), eddy current test (ECT) and buffered chemical polishing (BCP), are installed in the superconducting radio frequency (SRF) test facility. Three different sizes of cryostats (diameters of 600 mm for a quarter wave resonator (QWR), 900 mm for a half wave resonator (HWR), and 1200 mm for single spoke resonator 1&2 (SSR 1&2)) for vertical RF tests are installed for testing cavities. We confirmed that as-received niobium sheets (ASTM B393, RRR300) good electrical properties because they showed average residual resistance ratio (RRR) values higher than 300. However, serious RRR degradation occurred after joining two pieces of Nb by e-beam welding because the average RRR values of the samples were ∼179, which was only ∼60% of as-received RRR value. From various e-beam welding experiments in which the welding current and a speed at a fixed welding voltage were changed, we confirmed that good welding results were obtained at a 53 mA welding current and a 20-mm/s welding speed at a fixed welding voltage of 150 kV.

Evaluations for possible using of superconducting materials in high power microwave generators based on the method of the resonant microwave compression were made. As an example the relationship between the power amplification factor and frequency was determined for superconducting rectangular cavities. Capabilities of superconducting cavities as energy accumulators were analyzed. Microwave power value required for feeding superconducting accumulator is estimated. Experimental results of storing and extracting the energy in superconducting cavities are presented.

LCLS-II is a proposed upgrade of the most powerful X-ray laser in the world built at SLAC National Accelerator Laboratory. When complete, multiple free electron laser (FEL) sections will be powered by a continuous wave superconducting radio frequency (SRF) electron linac. The vital parameters for FEL radiation quality are beam emittance and beam transverse position stability. Excitation of high order modes (HOM) in SRF niobium cavities leads to additional beam power dissipation through incoherent and coherent losses. Energy stored in HOM may cause cumulative effects — transverse and longitudinal position displacement of the center of bunches, which leads to transverse and longitudinal emittance dilution. Cumulative effects due to dipole HOM excitation in LCLS-II are analyzed. Transition HOM effects are caused by sudden changes (periodic or single case) in the beam bunch structure. Bunch center position disturbance due to transition HOM effects is estimated.

In this paper, we track both center frequency and quality factor of a superconducting microwave resonator with very high precision using a modified Pound-Drever-Hall frequency locking scheme. The quality factor measurements can be done with high enough bandwidth to study different processes in a superconducting resonator in real time. With this added functionality, the frequency readout remains unaffected, with typical accuracy around 10-30 Hz/√(Hz), whereas the cavity loss rate can be determined to within 0.1% on a millisecond time scale (34 dB/√(Hz)). As we demonstrate, this can be particularly useful for electron spin resonance measurements, where we achieve a minimum number of detectable spins coupled to the resonator N = 5 ·105 spins/√(Hz), based on quality factor measurements. It also enables the study of fast dynamic processes, for example, vortex dynamics, in these devices on a short time scale.

We present calculations for an electromagnetic design of a β_g = 0.61 multicell superconducting radio frequency cavity for the Indian Spallation Neutron Source project. The geometry parameters of the midcells are optimized using a step-by-step 1-D optimization technique. This is followed by the optimization of end cells, which is done to achieve the required field flatness and to avoid trapping of higher order modes. Calculations of the threshold beam current for the excitation of regenerative beam break-up instability excited by the dipole modes are also presented, which is followed by wakefield calculations and estimation of related effects. Specific aspects in these calculations, which are relevant for medium β_g cavities, are highlighted. Finally, studies are performed for the static and dynamic Lorentz force detuning, based on which the stiffness design of the cavity is optimized.

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.

Brookhaven National Laboratory (BNL) has proposed to build an electron ion collider (EIC) as an upgrade to the existing Relativistic Heavy Ion Collider (RHIC). A part of the new design is to use superconducting radio frequency (SRF) cavities for acceleration, which sit in a bath of superfluid helium at a temperature of 2 K. SRF cavities designed for the BNL EIC create a standing electromagnetic wave, oscillating at a fundamental frequency of 647 MHz. Interaction of the charged particle beam with the EM field in the cavity creates higher order modes (HOM) of oscillation which have adverse effects on the beam when allowed to propagate down the beam tube. HOM waveguides are thus designed to remove this excess energy which is then damped at room temperature. As a result, these waveguides provide a direct thermal link between room temperature and the superconducting cavities adding a static thermal load. The EM wave propagating through the warmer sections of the waveguide creates an additional dynamic thermal load. This study calculates these thermal loads, concluding that the dynamic load is small in comparison to the static load. Temperature distributions are mapped on the waveguide and the number of heat intercepts required to efficiently manage thermal loads have been determined. In addition, a thermal radiation study has been performed and it is found that this contribution is around three orders of magnitude smaller than the static conduction and dynamic loads.

In superconducting films, the dissipative flux flow state plays a crucial role in the stability of those devices that operate under bias current whose value is very close to the critical current value, such as superconducting electronics and superconducting single particle/photon detectors. Therefore, we study the voltage stability versus electrical current transport by proper current-voltage measurement modes in superconducting films, investigating the effects of ion irradiation on the relevant current parameters.

In niobium superconducting radio frequency (SRF) accelerating cavities a decrease of the quality factor at lower fields - a so called \emph{low field Q slope or LFQS} - has been a long-standing unexplained effect. By extending the high $Q$ measurement techniques to ultralow fields we discover two previously unknown features of the effect: i) saturation at rf fields lower than $E_\mathrm{acc} \sim 0.1$~MV/m; ii) strong degradation enhancement by growing thicker niobium pentoxide. Our findings suggest that the LFQS may be caused by the two level systems in the natural niobium oxide on the inner cavity surface, thereby identifying a new source of residual resistance and providing guidance for potential non-accelerator low field applications of SRF cavities., final version

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 ${Q}_{0}({B}_{p})$.

We study quench detection in superconducting accelerator cavities cooled with He-II. A rigorous mathematical formula is derived to localize the quench position from dynamical data over a finite time interval at a second sound detector., Comment: 18 pages. Typos fixed; clarifications and references added

Investigations into the microwave surface impedance of superconducting resonators have led to the development of single photon counters that rely on kinetic inductance for their operation. While concurrent progress in additive manufacturing, `3D printing', opens up a previously inaccessible design space for waveguide resonators. In this manuscript, we present results from the first synthesis of these two technologies in a titanium, aluminum, vanadium (Ti-6Al-4V) superconducting radio frequency resonator which exploits a design unattainable through conventional fabrication means. We find that Ti-6Al-4V has two distinct superconducting transition temperatures observable in heat capacity measurements. The higher transition temperature is in agreement with DC resistance measurements. While the lower transition temperature, not previously known in literature, is consistent with the observed temperature dependence of the superconducting microwave surface impedance. From the surface reactance, we extract a London penetration depth of $8\pm3{\mu}$m - roughly an order of magnitude larger than other titanium alloys and several orders of magnitude larger than other conventional elemental superconductors. This large London penetration depth suggests that Ti-6Al-4V may be a suitable material for high kinetic inductance applications such as single photon counting or parametric amplification used in quantum computing., Comment: 4 pages, 4 figures

In this letter the evolution of the surface topography of a niobium superconducting radio frequency cavity caused by different plasma etching modes (isotropic and anisotropic) is studied by the three-dimensional level set method. The initial rough surface is generated starting from an experimental power spectral density. The time dependence of the rms roughness is analyzed and the growth exponential factors β are determined for two etching modes (isotropic and anisotropic) assuming that isotropic etching is a much more effective mechanism of smoothing. The obtained simulation results could be useful for optimizing the parameters of the etching processes needed to obtain high quality niobium surfaces for superconducting radio frequency cavities.

Superconducting cavities have been operating routinely in a variety of accelerators with a range of demanding applications. With the success of completed projects, niobium cavities have become an enabling technology, offering upgrade paths for existing facilities and pushing frontier accelerators for nuclear physics, high-energy physics, materials science, and the life sciences. With continued progress in basic understanding of radio-frequency superconductivity, the performance of cavities has steadily improved to approach theoretical capabilities.

This paper describes a superconducting magnet performance continuing persistent current operation for 25 years. The central magnetic field is 0.5 T for whole-body magnetic resonance imaging (MRI). The observed stability of the magnetic field was extremely stable compared with the specification. Because of the ultrafine electrical resistances in persistent current mode operation, it was difficult to observe an accurate magnetic field stability at the factory test. By long-term operation, we are able to observe the magnetic field stability and the joint resistance precisely. The magnet was started to energize on January 1988 and has been operated consecutively without re-excitation. At that time, a superconducting whole-body MRI system was appeared in the world among a normal conducting magnet and a permanent magnet system. We made superconducting joints to connect superconducting conductors between superconducting coils. We carried out the measurement of the central magnetic field by the nuclear magnetic resonance method. Because the device is used for the medical examination at the hospital, the magnetic field data acquisition is in a limited situation. The stability of the last magnetic field becomes 0.0006 ppm/h and gets the good performance that is more than 100 times than the specification. The total resistance of the superconducting joints is 4 ×10-12 Ω and supports good superconducting connection.

A new calorimetric measurement set up based on the measurement of nitrogen gas flow rate that results from the liquid nitrogen boil-off was established to measure ac losses in prototype superconducting modules of an inductive fault current limiter (iSFCL). A copper coil, to serve as the primary winding of iSFCL, was used to induce currents in the superconducting modules and drive them to a quench. The set up is suitable for superconducting devices of up to 360 mm in diameter and 400 mm in height. AC losses and magnetic shielding characteristics of the superconducting modules, made from wide second-generation high-temperature superconducting (2GHTS) tapes, were studied. Measurements were carried out at primary currents up to the quench current at frequencies from 10 Hz to 200 Hz. Dependence of primary quench current and ac losses on the frequency were measured. Implications of the extent of ac losses on the operating costs for maintaining liquid nitrogen in a practical iSFCL are discussed. The sudden change in the inductance of the primary coil and the corresponding change in the voltage during a quench of the superconducting modules were demonstrated.

Quench detection on superconducting bus bars and other devices with a low normal zone propagation velocity and low voltage build-up is quite difficult with conventional quench detection techniques. Currently, on ATLAS superconducting bus bar sections, superconducting quench detectors (SQD) are mounted to detect quench events. A first version of the SQD essentially consists of an insulated superconducting wire glued to a superconducting bus line or windings, which in the case of a quench rapidly builds up a relatively high resistance that can be easily and quietly detected. We now introduce a new generation of drastically improved SQDs. The new version makes the detection of quenches simpler, more reliable, and much faster. Instead of a superconducting wire, now a superconducting thin film is used. The layout of the sensor shows a meander like pattern that is etched out of a copper coated 25 μm thick film of Nb-Ti glued in between layers of Kapton. Since the sensor is now much smaller and thinner, it is easier to install and build up a high resistance with a much shorter response time. The design of the sensors is explained. The test results of the new sensors in a few variants in a calibration setup as well as when mounted on the windings surface of a magnet are reported.

At the radiation source ELBE a superconducting radio-frequency photoinjector (SRF gun) was developed and put into operation. Since 2010 the gun has delivered beam into the ELBE linac. A new driver laser with 13 MHz pulse repetition rate allows now to operate the free-electron lasers (FELs) with the SRF gun. This paper reports on the first lasing experiment with the far-infrared FEL at ELBE, describes the hardware, the electron beam parameters and the measurement of the FEL infrared radiation output.

The main superconducting radio frequency (SRF) linacs of the international linear collider (ILC) operate at a frequency of 1.3 GHz with a π phase advance per cell in the standing wave mode. An option being considered to reduce the overall footprint and project cost is to enhance the cavity gradient. The present baseline design for the main linacs of ILC demands the cavities to be able to reach a gradient of 35 MV/m—although during commissioning and operation the gradient will be 31.5 MV/m. This research concerns itself with the new cavity design with a view to reaching higher gradients. This design is focussed on minimising the surface electromagnetic fields and maximising the bandwidth of the accelerating mode. This new shape, which is referred to as the New Low Surface Field (NLSF) design. A design of a complete nine-cell cavity, including power couplers and higher order mode damping couplers is presented.

CEBAF, the Continuous Electron Beam Accelerator Facility, has been actively serving the nuclear physics research community as a unique forefront international resource since 1995. This CW electron linear accelerator (linac) at the U.S. Department of Energy's Thomas Jefferson National Accelerator Facility (Jefferson Lab) has continued to evolve as a precision tool for discerning the structure and dynamics within nuclei. Superconducting RF (SRF) technology has been the essential foundation for CEBAF, first as a 4 GeV machine, then 6 GeV, and currently capable of 12 GeV. We review the development, implementation, and performance of SRF systems for CEBAF from its early beginnings to the commissioning of the 12 GeV era., Comment: 56 pages, 31 figures, accepted for publication in Physical Review Accelerators and Beams

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.

A comparative analysis of two popular spectroscopy techniques is conducted in a coaxial cylindrical capacitively coupled discharge designed for the plasma processing of superconducting radio frequency (SRF) cavities. The density of the metastable and resonant levels in Ar is measured in both Ar and Ar/Cl 2 discharges to properly characterize the unique discharge system and aid in the development of a cavity etching routine. The first method, deemed the “branching fraction method,” utilizes the sensitivity of photon reabsorption of radiative decay to measure the lower state (metastable and resonant) densities by taking ratios of spectral lines with a common upper level. This method has been gaining popularity as it does not require any a priori knowledge about the electron energy distribution. The second method is a tunable diode laser absorption spectroscopy technique that measures the thermal Doppler broadening of spectral lines, from which the neutral gas temperature and lower state density of the transition can be evaluated. The two methods were conducted in tandem, while external parameters that were empirically determined to be important to the etching mechanism of SRF cavities are varied. Relationships between the excited state densities and the external parameters are presented for both spectroscopy methods and conclusions about the effects of these parameters on the discharge are stated when appropriate.

The potential of Nb3Sn for superconducting radio frequency cavities is widely recognized, and renewed R&D efforts continue to bring new insights about the material's structure and properties. We have systematically coated niobium samples with Nb3Sn for different durations. Several samples were coated over (“overcoat”) multiple times to elucidate the growth kinetics of Nb3Sn during the tin vapor-diffusion process. Analysis of coated samples is consistent with the model in which tin diffuses via grain boundaries to the Nb3Sn-Nb interface, where the growth of Nb3Sn into the niobium bulk takes place. Similar scaling laws are found for Nb3Sn grain growth and layer thickness. Nonparabolic layer growth is consistent with significant grain growth, which reduces the number of Sn transport channels. An examination of patchy regions in Nb3Sn coating, revealing large single-crystalline grains, points to impeded Nb3Sn layer growth due to low grain boundary density, resulting in a significantly thinner coating in those areas.

Superconducting radio frequency cavities performance preservation is crucial, from vertical test to accelerator operation. Field emission is still one of the performance limiting factors to overcome, and plasma cleaning has been proven successful by the Spallation Neutron Source (SNS), in cleaning field emitters (hydrocarbon contaminants) and increasing the work function of Nb. The cleaning for Linac Coherent Light Source-II will follow the same plasma composition adopted at SNS, which allows in situ processing of cavities installed in cryomodules. A novel method for plasma ignition has been developed at the Fermi National Accelerator Laboratory: a plasma glow discharge is ignited using high order modes to overcome limitations imposed by the fundamental power coupler. The plasma can be easily ignited and tuned in each of the cavity cells using low radio frequency (RF) power, from 100 W to as low as 2 W depending on the gas and pressure. A method for RF plasma detection has been developed: the plasma location is identified within the cavity without the need of cameras. The presented method can be applied to other multicell cavity designs, even for accelerators where the coupling for the fundamental modes at room temperature is very weak.Superconducting radio frequency cavities performance preservation is crucial, from vertical test to accelerator operation. Field emission is still one of the performance limiting factors to overcome, and plasma cleaning has been proven successful by the Spallation Neutron Source (SNS), in cleaning field emitters (hydrocarbon contaminants) and increasing the work function of Nb. The cleaning for Linac Coherent Light Source-II will follow the same plasma composition adopted at SNS, which allows in situ processing of cavities installed in cryomodules. A novel method for plasma ignition has been developed at the Fermi National Accelerator Laboratory: a plasma glow discharge is ignited using high order modes to overcome limitations imposed by the fundamental power coupler. The plasma can be easily ignited and tuned in each of the cavity cells using low radio frequency (RF) power, from 100 W to as low as 2 W depending on the gas and pressure. A method for RF plasma detection has been developed: the plasma locat...

Superconducting radiofrequency (SRF) cavities that could provide a higher quality factor as well as a higher operational accelerating gradient at an affordable cost are highly demanded for future generation of particle accelerators. This study aims to demonstrate the potential of Nb$_3$Sn as material of choice for such SRF applications. Due to its brittle nature, the only way to produce an Nb$_3$Sn SFR cavity is to synthesise a thin layer inside a cavity made of niobium or copper. In this work, direct current magnetron sputtering (DCMS) using a stoichiometric target of Nb$_3$Sn was employed to produce films on copper samples. Assessment of the morphology, microstructure and superconducting properties were performed in order to ensure that this approach is suitable for SRF applications. The potential of the method is proven by obtaining films, which exhibit a crack-free surface, dense morphology and critical temperatures ($T_c$) up to 16K. The essential properties of the films have also been investigated with respect to the deposition and annealing conditions. The use of krypton as working gas during deposition increases the atomic percent of Sn in the film compared to argon. However, in contrast to argon, higher krypton pressures reduce the atomic percent of Sn. It was also found that long-lasting high temperature annealing leads to higher superconducting critical temperatures due to an increased crystallographic order. Particular attention was given to the influence of the copper substrate on the film growth as well as the microstructural and superconducting characteristics. We discuss the main constraints introduced by the copper substrate, such as copper interdiffusion during annealing, lattice mismatch and difference in thermal expansion coefficients and methods to overcome them. Superconducting radiofrequency (SRF) cavities that could provide a higher quality factor as well as a higher operational accelerating gradient at an affordable cost are in high demand for the future generation of particle accelerators. This study aims to demonstrate the potential of Nb3Sn as material of choice for such SRF applications. Due to its brittle nature, the only way to produce an Nb3Sn SFR cavity is to synthesise a thin layer inside a cavity made of niobium or copper. In this work, direct current magnetron sputtering using a stoichiometric target of Nb3Sn was employed to produce films on copper samples. Assessment of the morphology, microstructure and superconducting properties were performed in order to ensure that this approach is suitable for SRF applications. The potential of the method is proven by obtaining films, which exhibit a crack-free surface, dense morphology and critical temperatures (T c) up to 16 K. The essential properties of the films have also been investigated with respect to the deposition and annealing conditions. The use of krypton as working gas during deposition increases the atomic percent of Sn in the film compared to argon. However, in contrast to argon, higher krypton pressures reduce the atomic percent of Sn. It was also found that long-lasting high-temperature annealing leads to higher superconducting critical temperatures due to an increased crystallographic order. Particular attention was given to the influence of the copper substrate on the film growth as well as the microstructural and superconducting characteristics. We discuss the main constraints introduced by the copper substrate, such as copper interdiffusion during annealing, lattice mismatch and difference in thermal expansion coefficients and methods to overcome them.

Quench propagation analysis of the superconducting outsert for the 45-T hybrid magnet being built at the High Magnetic Field Laboratory, Chinese Academy of Sciences was performed. The superconducting outsert was wound with a Nb3Sn cable-in-conduit conductor cabled in a 316LN jacket cooled with supercritical helium at 4.5 K. The superconducting outsert can produce 11.0-T central magnetic field at an operating current of 13410 A. An active protection circuit with a dump resistor of 0.272 Ω was adopted to transfer the stored magnetic energy during a dump operation. In this paper, the magnetic field and strain of the superconducting outsert were analyzed. During a quench, the hot spot temperature of the cable and the helium pressure inside a jacket for different quench delay times were introduced.

The electrical properties of a close-ended coil made of REBCO-coated conductor wire are studied via the measurement of its induced currents upon exposure to a sinusoidal magnetic field at different frequency. Results show a similarity to the behavior of currents inside an RLC circuit where the peak amplitude is dependent on the contact resistance of the solder joining the two ends of the coil and the resonant frequency, i.e., a function of the inductance of the coil and penetration rate of fluxons into the superconducting layers. Our findings suggest highly feasible applications stemming from such experimental setup in the field of linear and rotating machinery and low-frequency high-quality-factor resonators.

For the extension of the existing 100-MeV PEFP (Proton Engineering Frontier Project) proton linac up to a 1-GeV machine, a study of the RF linac based on RF superconductivity technology is underway. The operating frequency of the PEFP SCL (superconducting linac) is 700 MHz, which is determined by the fact that the frequency of the existing normal conducting linac is 350 MHz. A preliminary study of the beam dynamics shows that two types of cavities with geometrical betas of 0.50 and 0.74 can cover the entire energy range from 100 MeV to 1 GeV. The baseline design of the SRF (superconducting radio frequency) cavity is an elliptical shape with a reduced beta. The reduced-beta elliptical cavity is vulnerable to structural instability; therefore, mechanical analyses, such as vacuum pressure deformation, static Lorentz force detuning and natural mode frequency analyses, were performed by using a finite-element-method code. The mechanical analysis procedures and the results will be given in this paper.

This paper reports the research results of the effect of cathode shape during vertical buffered electropolishing (BEP) by employing a demountable single cell niobium (Nb) superconducting radio frequency (SRF) cavity. Several different cathode shapes such as, for instance, bar, ball, ellipsoid, and wheels of different diameters have been tested. Detailed electropolishing parameters including I-V characteristic, removal rate, surface roughness, and polishing uniformity at different locations inside the demountable cavity are measured. Similar studies are also done on conventional electropolishing (EP) for comparison. It is revealed that cathode shape has dominant effects for BEP especially on the obtaining of a suitable polishing condition and a uniform polishing rate in an Nb SRF single cell cavity. EP appears to have the same tendency. This paper demonstrates that a more homogeneous polishing result can be obtained by optimizing the electric field distribution inside the cavity through the modification of the cathode shape given the conditions that temperature and electrolyte flow are kept constant. Electric field distribution and electrolyte flow patterns inside the cavity are simulated via Poisson-Superfish and Solidworks respectively. With the optimal cathode shape, BEP shows a much faster polishing rate of similar to 2.5 mu m/min and is able to produce a smoother surface finish in the treatments of single cell cavities in comparison with EP.

Introduce the basic concepts of the superconducting and superconducting magnetic separation, the species and properties of superconducting material. Through the current use situation of superconducting magnetic separation machine to explain the advantages and disadvantages about superconducting magnetic separation machine than conventional magnetic separation machine. The paper analyzes the main problems of the superconducting magnetic separation. With a clear direction of superconducting magnetic separation research, improve the superconducting magnet of the critical temperature, especially in liquid nitrogen temperature, even at room temperature, which is a very important research subject.

Recent research has shown that choline chloride (vitamin B4)-based solutions can be used as a greener alternative to acid-based electrochemical polishing solutions. This study demonstrated a successful method for electrochemical deposition of niobium compounds onto the surface of copper substrates using a novel choline chloride-based ionic liquid. Niobium ions present in the ionic liquid solution were dissolved into the solution prior to deposition via electrochemical polishing of solid niobium. A black coating was clearly visible on the surface of the Cu following deposition. This coating was analyzed using scanning electron microscopy (SEM), electron dispersive X-ray spectroscopy (EDX), atomic force microscopy (AFM), and X-ray fluorescence spectroscopy (XRF). This ionic liquid-based electrochemical deposition method effectively recycles previously dissolved niobium from electrochemical polishing of superconducting radio frequency (SRF) cavities.

Modern particle accelerators require minimal interior surface roughness for Niobium superconducting radio frequency (SRF) cavities. Polishing of the Nb is currently achieved via electrochemical polishing with concentrated mixtures of sulfuric and hydrofluoric acids. This acid-based approach is effective at reducing the surface roughness to acceptable levels for SRF use, but due to acid-related hazards and extra costs (including safe disposal of used polishing solutions), an acid-free method would be preferable. This study focuses on an alternative electrochemical polishing method for Nb, using a novel ionic liquid solution containing choline chloride, also known as Vitamin B{sub 4} (VB{sub 4}). Potentiostatic electrochemical impedance spectroscopy (EIS) was also performed on the VB4-based system. Nb polished using the VB4-based method was found to have a final surface roughness comparable to that achieved via the acid-based method, as assessed by atomic force microscopy (AFM). These findings indicate that acid-free VB{sub 4}-based electrochemical polishing of Nb represents a promising replacement for acid-based methods of SRF cavity preparation.

Imaging the three-dimensional atomic-scale structure of complex interfaces has been the goal of many recent studies, due to its importance to technologically relevant areas. Combining atom-probe tomography and aberration-corrected scanning transmission electron microscopy (STEM), we present an atomic-scale study of ultrathin (~5 nm) native oxide layers on niobium (Nb) and the formation of ordered niobium hydride phases near the oxide/Nb interface. Nb, an elemental type-II superconductor with the highest critical temperature (T(c) = 9.2 K), is the preferred material for superconducting radio frequency (SRF) cavities in next-generation particle accelerators. Nb exhibits high solubilities for oxygen and hydrogen, especially within the RF-field penetration depth, which is believed to result in SRF quality factor losses. STEM imaging and electron energy-loss spectroscopy followed by ultraviolet laser-assisted local-electrode atom-probe tomography on the same needle-like sample reveals the NbO(2), Nb(2)O(5), NbO, Nb stacking sequence; annular bright-field imaging is used to visualize directly hydrogen atoms in bulk β-NbH.