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Muon spins precess in transversemagnetic fields and emit a positron preferentially in the spin direction at the instant of decay, enabling muon spin rotation (µSR) as a precise probe of localmagnetic fields in matter. µSR has been used to characterize superconducting radio-frequency (SRF)materials since 2010. At TRIUMF, a beamof 4.2MeV μ+ is implanted at a material-dependent depth of approximately 150 μm. A dedicated spectrometer was developed to measure the field of first vortex penetration and pinning strength in SRF materials in parallel magnetic fields of up to 300mT. A low-energy beam available at PSI implants μ+ at variable depth in the London layer allowing for direct measurements of the London penetration depth from which other material parameters relevant for SRF applications, such as the lower critical field and the superheating field, can be calculated. Beta-detected nuclear magnetic resonance (β-NMR) is a technique similar to low-energy µSR using beams of low-energy β radioactive ions. With a recent upgrade, it is capable of detecting the penetration of parallel magnetic vortices, depth resolved with nanometer resolution at applied fields of up to 200mT. In this paper, we reviewthe impact and capabilities of these techniques for SRF research. [ABSTRACT FROM AUTHOR]

Muon spins precess in transverse magnetic fields and emit a positron preferentially in the spin direction at the instant of decay, enabling muon spin rotation (μSR) as a precise probe of local magnetic fields in matter. μSR has been used to characterize superconducting radio-frequency (SRF) materials since 2010. At TRIUMF, a beam of 4.2 MeV μ+ is implanted at a material-dependent depth of approximately 150 μm. A dedicated spectrometer was developed to measure the field of first vortex penetration and pinning strength in SRF materials in parallel magnetic fields of up to 300 mT. A low-energy beam available at PSI implants μ+ at variable depth in the London layer allowing for direct measurements of the London penetration depth from which other material parameters relevant for SRF applications, such as the lower critical field and the superheating field, can be calculated. Beta-detected nuclear magnetic resonance (β-NMR) is a technique similar to low-energy μSR using beams of low-energy β radioactive ions. With a recent upgrade, it is capable of detecting the penetration of parallel magnetic vortices, depth resolved with nanometer resolution at applied fields of up to 200 mT. In this paper, we review the impact and capabilities of these techniques for SRF research.

Transfer of quantum states between distant nodes is one of the most important preconditions for realizing large-scale quantum information processing. We here propose two ways to transfer an arbitrary quantum state between two separated superconducting cavities coupled to an ensemble of nitrogen-vacancy centers (NV ensemble). By using the double-swap scheme and theory of the dispersive regime, quantum states can be deterministically transferred between two superconducting cavities. With current hybrid circuit QED technology, the numerical simulations are performed to demonstrate that the single-photon state and cat state can be high-fidelity transferred between two superconducting cavities.

Superconducting cavities are responsible for beam acceleration in superconducting linear accelerators. Challenging cavity control specifications are necessary to reduce radio frequency (RF) costs and to maximize the availability of the accelerator. Cavity detuning and bandwidth are two critical parameters to monitor when operating particle accelerators. Cavity detuning is strongly related to the power required to generate the desired accelerating gradient. Cavity bandwidth is related to the cavity RF losses. A sudden increase in bandwidth can indicate the presence of a quench or multipacting event. Therefore, calculating these parameters in real time in the low-level RF (LLRF) system is highly desirable. A real-time estimation of the bandwidth allows for a faster response of the machine protection system in the case of quench events, whereas the estimation of cavity detuning can be used to drive piezoelectric tuner-based resonance control algorithms. In this article, a new field programmable gate array (FPGA)-based estimation component is presented. Such a component is designed to be used either in continuous wave (CW) or pulsed operation mode with loaded quality factors between 106 and 108. Results of this component with free-electron LASer in Hamburg (FLASH), European X-ray free electron laser (EuXFEL), cryo module test bench (CMTB), and electron linac for beams with high brilliance and low emittance (ELBE) are presented. [ABSTRACT FROM AUTHOR]

Close to 780 superconducting 1.3-GHz accelerating cavities made of bulk niobium have been installed in the European X-ray free electron laser (E-XFEL) linear accelerator. The linac can operate at a mean gradient (Eacc) of ca. 23 MV/m in the nominal short pulse (SP) operation mode with 1.4-ms pulses and a repetition rate of 10 Hz. An R&D program at Deutsches Elektronen-Synchrotron (DESY) is ongoing since 2011 on the feasibility of a continuous wave (CW) upgrade of the E-XFEL accelerator. In this publication, a modification of the low-level radio frequency (LLRF) control system to the CW mode is presented. Currently, the control system successfully stabilizes accelerating field for the SP mode. The demanding E-XFEL specifications of 0.01% for the field amplitude and 0.01° for the phase stability are required to keep the photon beam quality for all experimental stations. The proposed modification has been implemented and tested to verify the system versatility for SP and CW operation modes. The tests were conducted for a prototype E-XFEL cryomodule XM-3. As for the series cryomodules, this prototype contains eight superconducting 1.3-GHz cavities. The results presented here confirm that the same LLRF system can be used with minor software modifications for either operation mode (CW or SP). However, the CW mode requires a more complex RF-power management and more precise control, especially when cavities operate with very high loaded quality factor (Ql) of the order of 6E7. The achieved stability for such high-Ql is also presented and discussed. [ABSTRACT FROM AUTHOR]

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

This paper describes some achievements of the low-level radio frequency (LLRF) tests performed to evaluate the superconducting cryomodules in preparation of their installation in the European X-ray Free Electron Laser (XFEL) accelerator. The software developed to characterize the cryomodules tested at the Accelerator Module Test Facility (AMTF) will be presented. The purpose of these tests is to evaluate some of the key parameters of the eight Tera Electronvolt Superconducting Linear Accelerator cavities, housed in every cryomodule. These parameters include: cavity quench gradients, cavity fundamental resonant modes, performance of the slow and fast frequency tuners, and characteristics of the input power coupler. These results impact the final decision on the cryomodule acceptance and its installation inside the linear accelerator (linac). In this approach, the MicroTCA.4-based LLRF system is used to control and assess the cryomodule performance. Middle layer servers are developed to verify the tests initial conditions, carry the test, and log the results in a dedicated database. The tests results provide a basis for the cryomodule validation, but also serve as a reference useful during the commissioning and operating phase of the accelerator. [ABSTRACT FROM AUTHOR]

Since flux vortices are easily pinned by defects in superconducting radio frequency (SRF) cavities, trapped magnetic flux increases the surface resistance of the SRF cavity. It has been experimentally demonstrated that trapped magnetic flux can be expelled by cooling down a superconducting cavity under a large spatial temperature gradient. In this paper, we use the time-dependent Ginzburg–Landau (TDGL) theory to theoretically investigate the effect of defects on the flux expulsion ability of pure niobium superconducting cavities with an applied spatial temperature gradient. It is shown that the residual flux vortex density displays a notable stepwise decreasing trend with increasing temperature gradient at smaller residual fields, while it shows a stepwise rising with increasing defect size. Both larger defect number and size make vortices difficult to be expelled. Moreover, long cooling time is beneficial for vortices to be expelled, and the cooling time has nearly no significant effect on flux expulsion under conditions of large cooling times. The results in this paper provide some theoretical guidelines for improving the performance of superconducting cavities in practical situations. [ABSTRACT FROM AUTHOR]

In this article a scientific prove for a new technology to maximize the accelerating gradient in superconducting cavities by processing on higher order mode frequencies is presented. As dominant energy source the heating of field emitters by an induced rf current (rf-heating) is considered. The field emitter structure is assumed to be a chain of conductive particles, which are formed by attractive forces. [ABSTRACT FROM AUTHOR]

A f = 162 . 5 MHz superconducting half-wave resonator (HWR) with β = 0 . 12 is one of the four superconducting cavities being developed for the heavy ion linac of the Rare Isotope Science Project (RISP). The linac will accelerate various ions ranging from proton to uranium with beam power of about 400 kW. In particular, the HWR’s will accelerate the ion beam in low–medium energy range, i.e., from 1.6 to 18 MeV for the case of uranium. In this paper, we describe design, fabrication, surface treatment, and vertical test of the 1st prototype of the cavity in detail. We also discuss some issues on the performance enhancement of the cavity. The Q 0 values at 2 K surpassed the target performance, Q 0 = 1 . 1 × 1 0 9 at E a c c = 6 . 3 MV ∕ m . [ABSTRACT FROM AUTHOR]

The use of non-evaporable getter (NEG) pumps in particle accelerators has increased significantly over the past few years because of their large pumping speed, particularly for hydrogen, compared to the size of the pump. A concern about using such pumps in superconducting radio-frequency (SRF) accelerators is the possibility of shedding particulates which could then migrate into the SRF cavities and produce field emission, therefore degrading the cavity performance. One option to mitigate such issue is to use sintered getter materials which intrinsically offer superior mechanical and particle retention properties. In this article we present the results from cryogenic RF tests of a high-gradient SRF cavity after being evacuated several times with an NEG pump equipped with sintered getter disks and placed in close proximity to the cavity. The results showed that the cavity performance was not affected by the pump up to the quench gradient of 34 MV/m. As a result of this study, two such NEG pumps have been installed next to a cryomodule in the CEBAF accelerator to maintain ultra-high vacuum in the SRF cryomodule and two adjacent warm girder sections. [ABSTRACT FROM AUTHOR]

This article presents a novel design for a superconducting rf electron injector that incorporates energy recovery. This concept relaxes the demands of high power input couplers, improves essential beam parameters and energy efficiency and reduces the overall cost of a compact energy recovery linac machine. [ABSTRACT FROM AUTHOR]

Transfer of quantum states between distant nodes is one of the most important preconditions for realizing large-scale quantum information processing. We here propose two ways to transfer an arbitrary quantum state between two separated superconducting cavities coupled to an ensemble of nitrogen-vacancy centers (NV ensemble). By using the double-swap scheme and theory of the dispersive regime, quantum states can be deterministically transferred between two superconducting cavities. With current hybrid circuit QED technology, the numerical simulations are performed to demonstrate that the single-photon state and cat state can be high-fidelity transferred between two superconducting cavities. • We propose two ways to transfer an arbitrary quantum state between two separated superconducting cavities coupled to an NV ensemble. • By using the double-swap scheme and theory of the dispersive regime, quantum states can be deterministically transferred. • The states of cavities can be in arbitrary states such as continuous-variable states or discrete-variable states. • Numerical simulations show that the single-photon state and cat state can be high-fidelity transferred between two superconducting cavities. [ABSTRACT FROM AUTHOR]

Superconducting cavities have been widely used in high intensity proton accelerators. In this paper, we discuss parameters that affect energy bandwidth of the accelerating cavity and propose a new energy averaged transit time factor to help choose transition energy between different sections and cavity geometry parameters in the linac design. These wide energy bandwidth superconducting cavities can potentially be used to accelerate a proton beam multiple times. A time-pass condition is defined to attain fixed RF phases during multiple passes of the beam. Such a superconducting recirculating proton linac could significantly reduce the number of RF cavities in the accelerator and lower construction and operational costs of the facility. [ABSTRACT FROM AUTHOR]

Design and simulation of a superconducting gun cavity with 3½ cells have been studied in order to give the first push to the electron beam for the linear accelerating system at The Institute of Accelerator Technologies at Ankara University. Electrons are accelerated through the gun cavity with the help of the Radiofrequency power suppliers from cryogenic systems. Accelerating gradient should be as high as possible to accelerate electron beam inside the cavity. In this study, electron beam reaches to 9.17 MeV energy at the end of the gun cavity with the accelerating gradient; E c =19.21 MV/m. 1.3 GHz gun cavity consists of three TESLA-like shaped cells while the special designed gun-cell includes a cathode plug. Optimized important beam parameters inside the gun cavity, average beam current 3 mA, transverse emittance 2.5 mm mrad, repetition rate 30 MHz and other parameters are obtained for the SASE-FEL System. The Superfish/Poisson program is used to design each cell of the superconducting cavity. Superconducting gun cavity and Radiofrequency properties are studied by utilizing 2D Superfish/Poisson, 3D Computer Simulation Technology Microwave Studio, and 3D Computer Simulation Technology Particle Studio. Superfish/Poisson is also used to optimize the geometry of the cavity cells to get the highest accelerating gradient. The behavior of the particles along the beamline is included in this study. ASTRA Code is used to track the particles. [ABSTRACT FROM AUTHOR]

At present, applications of 800 MHz harmonic cavities in both bunch lengthening and shortening regimes are under consideration and discussion in the framework of the High Luminosity LHC project. In this paper we study electromagnetic characteristics of high order modes (HOMs) for a single cell 800 MHz superconducting cavity and arrays of such cavities connected by drifts tubes. Different techniques for the HOMs damping such as beam pipe grooves, coaxial-notch loads, fluted beam pipes etc. are investigated and compared. The influence of the sizes and geometry of the drift tubes on the HOMs damping is analyzed. The problems of a multipacting discharge in the considered structures are discussed and the operating frequency detuning due to the Lorentz force is evaluated. [ABSTRACT FROM AUTHOR]

Abstract: Split resonances are a common appearance in superconducting cavities and are studied here on a specific example. A five cell superconducting cavity suitable for the envisioned electron-hadron collider eRHIC was designed and will be used in the upcoming “Coherent electron Cooling” Proof-of-Principle experiment. The superconducting cavity is intended for high-current applications, and beam stability depends on minimizing the Higher Order Modes (HOM). This was attempted in the design phase and will be done with appropriate mode dampers in operation. The design was implemented as a copper cavity and two Niobium cavities were fabricated. The availability of the copper model provided a convenient opportunity to confirm the design and to study potentially nefarious HOMs. Selected high-Q resonances were investigated via bead pull measurements. The appearance of split resonances impeded their study and suitable methods of HOM identification are presented in this report. [Copyright &y& Elsevier]

The well-established technology of superconducting niobium cavities (TESLA - TeV-Energy Superconducting Linear Accelerator) finds an increasing number of applications for linear accelerators in high-energy physics experiments. Together with cavity design and manufacturing, control systems for accelerating field parameters were developed. The digital control system developed for cryomodule operation at the Free Electron Laser in Hamburg (FLASH) is able to perform field amplitude and phase regulation with the precision requested for the multiple user experiments carried at FLASH and according to the required laser light parameters. While new experiments (like the International Linear Collider or the European X-FEL) are based on the same technology (and similar control systems), they require additional levels of controlling superconducting structures parameters in order to comply with tighter power budget overheads and finer field regulation requirements. Consequently, the possibility of controlling such parameters as cavity loaded quality factor or resonance frequency is becoming more attractive. The necessity and the benefits of regulating these parameters have been described in refid="ref1"/ and refid="ref2"/. In this paper, we describe the algorithm for feedback control of the loaded Q in TESLA cavities by means of motor control of the fundamental power coupler. [ABSTRACT FROM PUBLISHER]

Abstract: Nb films have been magnetron sputtered onto quartz sheets oriented with respect to the target at angles varying between 0° and 90°, with 15° steps. Impedance plots have been obtained by contacting these films with aqueous Na2SO4, either at the open circuit potential or at a potential where Nb is covered by an anodic passive Nb2O5 film. As the target–substrate angle θ increases, the shape of the impedance plots changes from that of a smooth electrode to that of a porous one, characterised in the high frequency range by a straight line forming a 45° angle with the real axis. The surface roughness of the Nb deposits, calculated from their double layer capacity, is low and constant at low θ, significantly increases at θ =45°, goes through a maximum in the range 60–75° and drops at θ =90°. AFM surface profiling confirms this trend, but estimates a lower surface roughness. Attempts to obtain Nb deposits with a surface roughness less strongly dependent on θ have been made by performing the depositions under pulsed conditions or by heating the substrates at 400–600°C. Heating at the higher temperature was a fairly effective method for decreasing the roughness of deposits formed at large θ. [Copyright &y& Elsevier]

Brazing is a process to join metallic and non-metallic materials, without any melting or plastic state of the base metals. The poor wettability of ceramic may be improved by the use of alloys containing an element capable of changing the chemistry of the ceramic surface (active braze). In this paper we propose the substitution of the Electron Beam welding with Vacuum Brazing in order to join the resonator components, Nb, Cu OFHC, and Al2O3, with stainless steel flanges, in superconducting resonant cavities. Several combinations of brazed joints have been tested. The characterization of the joints has been performed with SEM-EDS and MICROPIXE. All the joints fulfil the requirements. [ABSTRACT FROM AUTHOR]

• Superconducting Nb_3 Sn thin films are obtained by on copper substrate by dc magnetron co-sputtering technique. • The inhomogenous distribution of tin like forming tin island is observed. • The inhomogenous distribution of tin in Nb_3 Sn thin film is analyzed according to EDS and MPMS results. Superconducting Nb 3 Sn thin films have been produced on copper substrate by dc magnetron co-sputtering technique successfully. XRD and magnetic moment measurement demonstrate the exist of Nb 3 Sn crystal. By changing the ratio between niobium target sputtering current to tin target, the tin concentration in different films is varied. However, the experiment results indicate critical temperature T C of different Nb 3 Sn thin film are nearly invariable while the composition of Nb-Sn of those are changed. Highest T C of those films annealing at 650°C are around 12K, while 750°C annealing increase it to 15K. The SEM and element analysis results manifest inhomogenous distribution of tin atoms, where some tin islands appear over the film. The exist of tin-rich areas like tin islands is responsible for such particular performance of Nb 3 Sn films. [ABSTRACT FROM AUTHOR]