Nb3Sn is a promising next-generation material for superconducting radiofrequency cavities, with significant potential for both large scale and compact accelerator applications. However, so far, Nb3Sn cavities have been limited to cw accelerating fields <18 MV/m. In this paper, new results are presented with significantly higher fields, as high as 24 MV/m in single cell cavities. Results are also presented from the first ever Nb3Sn-coated 1.3 GHz 9-cell cavity, a full-scale demonstration on the cavity type used in production for the European XFEL and LCLS-II. Results are presented together with heat dissipation curves to emphasize the potential for industrial accelerator applications using cryocooler-based cooling systems. The cavities studied have an atypical shiny visual appearance, and microscopy studies of witness samples reveal significantly reduced surface roughness and smaller film thickness compared to typical Nb3Sn films for superconducting cavities. Possible mechanisms for increased maximum field are discussed as well as implications for physics of RF superconductivity in the low coherence length regime. Outlook for continued development is presented.
A deeply modulated, regular, continuous, oscillating current waveform is reliably and repeatably achieved during electropolishing of niobium single-cell elliptical radiofrequency cavities. Details of the technique and cavity test results are reported here. The method is applicable for cavity frequencies in the range 500 MHz to 3.9 GHz and can be extended to multicell structures.
Nb3Sn superconducting radiofrequency (SRF) cavities have substantial potential for enabling new performance capabilities for particle accelerators for high energy physics (HEP), as well as for RF cavity-based detectors for dark matter, gravitational waves, and other quantum sensing applications. Outside of HEP, Nb3Sn SRF cavities can also benefit accelerators for nuclear physics, basic energy sciences, and the industry. In this contribution to Snowmass 2021, we overview the potential, status, and outlook of Nb3Sn SRF cavities.
Transverse breathing mode oscillations in a particle beam can couple energy into longitudinal oscillations in a bunch of finite length and cause significant losses. We develop a model that illustrates this effect and explore the dependence on mismatch size, space-charge tune depression, longitudinal focusing strength, bunch length, and RF bucket length.
The temporal resolution of sub-relativistic ultrafast electron diffraction (UED) is generally limited by radio frequency (RF) phase and amplitude jitter of the RF lenses that are used to compress the electron pulses. We theoretically show how to circumvent this limitation by using a combination of several RF compression cavities. We show that if powered by the same RF source and with a proper choice of RF field strengths, RF phases and distances between the cavities, the combined arrival time jitter due to RF phase jitter of the cavities is cancelled at the compression point. We also show that…
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 <1.5 K and Q0 ~5x10^10 at 2.0 K. In one cavity, measurements of surface resistance versus temperature showed an extremely small residual resist…
As a result of a collaboration between Jefferson Lab and niobium manufacturer CBMM, ingot niobium was explored as a possible material for superconducting radiofrequency (SRF) cavity fabrication. The first single cell cavity from large grain high purity niobium was fabricated and successfully tested at Jefferson Lab in 2004. This pioneering work triggered research activities in other SRF laboratories around the world. Large grain niobium became not only an interesting alternative material for cavity builders, but also material scientists and surface scientists were eager to participate in the d…
Inspired by the development of over-moded RF component as an undulator, we explored another over-moded structure that could serve the variable coupling for SRF purpose. This application is to fulfill variation of S11 from 0 to -20db with CW power of 7 KW. The static heat loss in the coupler is trivial from calculation. An advantage of this coupler is that the thermal isolation between the 2K and 300K section is considerable by vacuum separation. Within this coupler, only a single propagation mode is allowed at each section, and thus, the fact that no energy is converted to high order mode brin…
Simulating beam loading in radiofrequency accelerating structures is critical for understanding higher-order mode effects on beam dynamics, such as beam break-up instability in energy recovery linacs. Full wave simulations of beam loading in radiofrequency structures are computationally expensive, while reduced models can ignore essential physics and can be difficult to generalize. We present a self-consistent algorithm derived from the least-action principle which can model an arbitrary number of cavity eigenmodes and with a generic beam distribution.
The small bandwidth of superconducting cavities makes the study of dynamic Lorentz force detuning and its compensation indispensable in case of pulsed mode operation of high gradient accelerators. In this paper, we present the study of this detuning and also propose an optimized design for five cell 650 MHz βg= 0.9 elliptic superconducting cavities, which will be used in the high energy section of the 1 GeV H- linear accelerator for the proposed Indian Spallation Neutron Source project, by suitably inserting the inter-cell stiffeners. The paper presents a sequential design methodology which st…
We present the calculations for electromagnetic design of a Beta_g = 0.61, multi-cell superconducting radiofrequency cavity for the Indian Spallation Neutron Source (ISNS) project. Geometry of the mid-cells is optimized using a step by step, one-dimensional optimization technique. This is followed by the optimization of end-cells, which is done to achieve the required field flatness, as well as 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 is also presented, which is f…
Field emission (FE) is one of the main limiting factors of superconducting radio-frequency (SRF) cavities operating in accelerators and it occurs whenever contaminants, like dust, metal flakes or even absorbates, are present on the surface of the cavity high electric field region. Field emission reduces the maximum achievable accelerating field and generates free electrons that may interact with the beam, damage or activate the beamline. One practical method that can be used to mitigate this problem is in-situ plasma cleaning, or plasma processing. The development of a processing that can be a…
It is well known that the tuning of a long radio-frequency quadrupole accelerator is demanding. This study investigated how to realize efficient long RFQ accelerators with multiple shorter and independent cavities. From the RF point of view, the use of shorter cavities has many advantages e.g. enlarged mode separation and simplified RF tuning, but the beam matching between cavities will become an issue, especially at high current and low energy. Taking a more than 9 m long, high current machine as an example, this paper presents the design concepts and methods leading to good beam quality and …
In dark matter searches using axion haloscopes, the search sensitivity depends on the quality factors ($Q_0$) of radiofrequency cavities immersed in multi-tesla magnetic fields. Increasing $Q_0$ would increase the scan rate through the parameter space of interest. Researchers developing superconducting radiofrequency cavities for particle accelerators have developed methods for obtaining extremely high $Q_0\sim10^{11}$ in $μ$T-scale magnetic fields. In this paper, we describe efforts to develop high Q cavities made from Nb$_3$Sn films using a technique developed for particle accelerator caviti…
The axion is a hypothetical particle which is a candidate for cold dark matter. Haloscope experiments directly search for these particles in strong magnetic fields with RF cavities as detectors. The Relic Axion Detector Exploratory Setup (RADES) at CERN in particular is searching for axion dark matter in a mass range above 30 $μ$eV. The figure of merit of our detector depends linearly on the quality factor of the cavity and therefore we are researching the possibility of coating our cavities with different superconducting materials to increase the quality factor. Since the experiment operates …
Nb$_3$Sn is a promising advanced material under development for superconducting radiofrequency (SRF) cavities. Past efforts have been focused primarily on small development-scale cavities, but large, often multi-celled cavities, are needed for particle accelerator applications. In this work, we report on successful Nb$_3$Sn coatings on Nb in a 1 m-long 9-cell Nb sample-host cavity at Fermilab. The geometry of the first coating with only one Sn source made it possible to study the influence of Sn flux on the microstructure. Based on these results, we postulate a connection between recently obse…
One of the most important issues of high-power hadron linacs is the choice of technology with respect to superconducting or room-temperature operation. The favour for a specific technology depends on several parameters such as the beam energy, beam current, beam power and duty factor. This contribution gives an overview of the comparison between superconducting and normal conducting cavities. This includes basic radiofrequency (RF) parameters, design criteria, limitations, required RF and plug power as well as case studies.
We demonstrate practical accelerating gradients on a superconducting radiofrequency (SRF) accelerator cavity with cryocooler conduction cooling, a cooling technique that does not involve the complexities of the conventional liquid helium bath. A design is first presented that enables conduction cooling an elliptical-cell SRF cavity. Implementing this design, a single cell 650 MHz Nb3Sn cavity coupled using high purity aluminum thermal links to a 4 K pulse tube cryocooler generated accelerating gradients up to 6.6 MV/m at 100% duty cycle. The experiments were carried out with the cavity-cryocoo…
The static and dynamic structural behavior of superconducting cavities for various projects was determined by finite element structural analysis. The b = 0.61 cavity shape for the Neutron Science Project was studied in detail and found to meet all design requirements if fabricated from five millimeter thick material with a single annular stiffener. This 600 MHz cavity will have a Lorentz coefficient of -1.8 Hz/(Mv/meter)2 and a lowest structural resonance of more than 100 Hz. Cavities at b = 0.48, 0.61, and 0.77 were analyzed for a Neutron Science Project concept which would incorporate 7-cell…
Photonic crystal (PhC) defect cavities that support an accelerating mode tend to trap unwanted higher-order modes (HOMs) corresponding to zero-group-velocity PhC lattice modes at the top of the bandgap. The effect is explained quite generally from photonic band and perturbation theoretical arguments. Transverse wakefields resulting from this effect are observed in a hybrid dielectric PhC accelerating cavity based on a triangular lattice of sapphire rods. These wakefields are, on average, an order of magnitude higher than those in the waveguide-damped Compact Linear Collider (CLIC) copper cavit…
A Three-Fluid Model of Dissipation at Surfaces in Superconducting Radiofrequency Cavities
Laboratory of Atomic and Solid State Physics, Cornell University（康奈尔大学原子与固态物理实验室）；Cornell Laboratory for Accelerator-Based Sciences and Education, Cornell University（康奈尔大学加速器科学与教育实验室）
Field emission is one of the key issues in superconducting RF for particle accelerators. When present, it limits operating gradient directly or via induced heat load at 2K. In order to minimize particulate contamination of and thus field emission in the CEBAF SRF cavities during assembly, a cold ceramic RF window was placed very close to the accelerating cavity proper. As an unintended consequence of this, the window is charged by field-emitted electrons, making it possible to monitor and model field emission in the CEBAF cavities since in-tunnel operation began. From January 30, 1995, through…
In the field of particle accelerators the most common use of RF cavities is to increase the particle velocity of traversing particles. This feature makes them one of the core ingredients of every accelerator, and in the case of linear accelerators they are even the dominant machine component. Since there are many different types of accelerator, RF cavities have been optimized for different purposes and with different abilities, e.g., cavities with fixed or variable RF frequency, cavities for short or long pulses/CW operation, superconducting and normal-conducting cavities. This lecture starts …
We present the electromagnetic design study of a multi cell, β_g= 0.9, 650 MHz elliptic superconducting radiofrequency cavity, which can be used for accelerating H- particles in the linear accelerator part of a Spallation Neutron source. The design has been optimized for maximum achievable acceleration gradient by varying the geometry parameters of the cavity, for which a simple and general procedure is evolved that we describe in the paper. For the optimized geometry, we have studied the higher order modes supported by the cavity, and the threshold current for the excitation of the regenerati…
The low energy part of the FAST linear accelerator based on 1.3 GHz superconducting RF cavities was successfully commissioned. During commissioning, beam based model dependent methods were used to correct linear lattice and trajectory. Lattice correction algorithm is based on analysis of beam shape from profile monitors and trajectory responses to dipole correctors. Trajectory responses to field gradient variations in quadrupoles and phase variations in superconducting RF cavities were used to correct bunch offsets in quadrupoles and accelerating cavities relative to their magnetic axes. Detai…
The recently developed frequency extraction algorithm [G.R. Werner and J.R. Cary, J. Comp. Phys. 227, 5200 (2008)] that enables a simple FDTD algorithm to be transformed into an efficient eigenmode solver is applied to a realistic accelerator cavity modeled with embedded boundaries and Richardson extrapolation. Previously, the frequency extraction method was shown to be capable of distinguishing M degenerate modes by running M different simulations and to permit mode extraction with minimal post-processing effort that only requires solving a small eigenvalue problem. Realistic calculations for…
We describe the upgrades to diagnostic capabilities on the Fermilab Accelerator Science and Technology (FAST) electron linear accelerator that will allow investigations of the effects of high-order modes (HOMs) in SCRF cavities on macropulse-average beam quality. We examine the dipole modes in the first pass-band generally observed in the 1.6-1.9 GHz regime for TESLA-type SCRF cavities due to uniform transverse beam offsets of the electron beam. Such cavities are the basis of the accelerators such as the European XFEL and the proposed MaRIE XFEL facility. Preliminary HOM detector data, prototy…
Linear accelerators containing the sequence of independently phased cavities with constant geometrical velocity along each structure are widely used in practice. The chain of cavities with identical cell lengths is utilized within a certain beam velocity range, with subsequent transformation to the next chain with higher cavity velocity. Design and analysis of beam dynamics in this type of accelerator are usually performed using numerical simulations. A full theoretical description of particle acceleration in an array of independent phased cavities has not been developed. In the present paper,…
A concept for a two-stage injection-locked CW magnetron intended to drive Superconducting Cavities (SC) for intensity-frontier accelerators has been proposed. The concept considers two magnetrons in which the output power differs by 15-20 dB and the lower power magnetron being frequency-locked from an external source locks the higher power magnetron. The injection-locked two-stage CW magnetron can be used as an RF power source for Fermilab's Project-X to feed separately each of the 1.3 GHz SC of the 8 GeV pulsed linac. We expect output/locking power ratio of about 30-40 dB assuming operation i…
Nb$_3$Sn is currently the most promising material other than niobium for future superconducting radiofrequency cavities. Critical fields above 120 mT in pulsed operation and about 80 mT in CW have been achieved in cavity tests. This is large compared to the lower critical field as derived from the London penetration depth, extracted from low field surface impedance measurements. In this paper direct measurements of the London penetration depth from which the lower critical field and the superheating field are derived are presented. The field of first vortex penetration is measured under DC and…
Even when cooled through its transition temperature in the presence of an external magnetic field, a superconductor can expel nearly all external magnetic flux. This Letter presents an experimental study to identify the parameters that most strongly influence flux trapping in high purity niobium during cooldown. This is critical to the operation of superconducting radiofrequency cavities, in which trapped flux degrades the quality factor and therefore cryogenic efficiency. Flux expulsion was measured on a large survey of 1.3 GHz cavities prepared in various ways. It is shown that both spatial …
SRF niobium cavities are the building blocks of modern accelerators for scientific applications. Lower surface resistance, higher fields, and high operating temperatures advance the reach of the future accelerators for scientific discovery as well as potentially enabling cost-effective industrial solutions. We describe the design and performance of an Nb$_{3}$Sn coating system that converts the inner surface of niobium cavities to Nb$_{3}$Sn film. Niobium surface, heated by radiation from the niobium retort, is exposed to Sn and SnCl$_{2}$ vapor during the heat cycle, which results in about 2 …
Transverse deflecting cavities are used for a number of applications in modern accelerators. In this paper we discuss the fields of these cavities, some of their applications, and some important aspects of their design.
The European XFEL is a new research facility currently under construction at DESY in the Hamburg area in Germany. From 2015 on, it will generate extremely intense X-ray flashes that will be used by researchers from all over the world. The superconducting XFEL linear accelerator consists of 100 accelerator modules with more than 800 RF-cavities inside. The accelerator modules, superconducting magnets and cavities will be tested in the accelerator module test facility (AMTF). This paper gives an overview of the design parameters and the commissioning of the vertical insert, used in two cryostats…
The FREIA Laboratory at Uppsala University focuses on superconducting technology and accelerator development. It actively supports the development of the European Spallation Source, CERN, and MAX IV, among others. FREIA has developed test facilities for superconducting accelerator technology such as a double-cavity horizontal test cryostat, a vertical cryostat with a novel magnetic field compensation scheme, and a test stand for short cryomodules. Accelerating cavities have been tested in the horizontal cryostat, crab-cavities in the vertical cryostat, and cryomodules for ESS on the cryomodule…
Front end of a CW linac of the Project X contains an H source, an RFQ, a medium energy transport line with the beam chopper, and a SC low-beta linac that accelerates H- from 2.5 MeV to 160 MeV. SC Single Spoke Resonators (SSR) will be used in the linac, because Fermilab already successfully developed and tested a SSR for beta=0.21. Two manufactured cavities achieve 2.5 times more than design accelerating gradients. One of these cavities completely dressed, e.g. welded to helium vessel with integrated slow and fast tuners, and tested in CW regime. Successful tests of beta=0.21 SSR give us a con…
In this Letter, we present a study of the confinement properties of point-defect resonators in finite-size photonic-bandgap structures composed of aperiodic arrangements of dielectric rods, with special emphasis on their use for the design of cavities for particle accelerators. Specifically, for representative geometries, we study the properties of the fundamental mode (as a function of the filling fraction, structure size, and losses) via 2-D and 3-D full-wave numerical simulations, as well as microwave measurements at room temperature. Results indicate that, for reduced-size structures, aper…
Controlling trapped magnetic flux in superconducting radiofrequency (RF) cavities is of crucial importance in modern accelerator projects. In order to study flux trapping efficiency and sensitiv- ity of surface resistance, dedicated experiments have been carried out on different types of low-\b{eta} 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 res…
In storage rings, horizontal dispersion in the rf cavities introduces horizontal-longitudinal (xz) coupling, contributing to beam tilt in the xz plane. This coupling can be characterized by a "crabbing" dispersion term ζa that appears in the normal mode decomposition of the 1-turn transfer matrix. ζa is proportional to the rf cavity voltage and the horizontal dispersion in the cavity. We report experiments at the Cornell Electron Storage Ring Test Accelerator (CesrTA) where xz coupling was explored using three lattices with distinct crabbing properties. We characterize the xz coupling for each…
High-beta superconducting radiofrequency (SRF) elliptical cavities are being developed for several accelerator projects including Project X, the European XFEL, and the International Linear Collider (ILC). Fermilab has recently established an extensive infrastructure for SRF cavity R&D for future accelerators, including cavity surface processing and testing and cavity assembly into cryomodules. Some highlights of the global effort in SRF R&D toward improving cavity performance, and Fermilab SRF cavity R&D in the context of global projects are reviewed.
Fermilab is executing a technology development program to develop a compact yet powerful electron accelerator. We are leveraging R&D breakthroughs in SRF cavities, cost-effective radio-frequency sources, modern cryo-coolers, and high average current electron guns. We show that a single accelerator module can deliver average beam power as high as 1 MW through detailed thermal, RF, and particle simulations.
Superconducting niobium film cavities deposited on copper substrates (Nb/Cu) have suffered from strong field-dependent surface resistance, often referred to as the Q-slope problem, since their invention. We argue that the Q-slope may not be an intrinsic problem, but rather originates from a combination of factors which can be revealed in appropriate environmental conditions. In this study, extrinsic effects were carefully minimized in a series of experiments on a seamless cavity. The origin of the Q-slope in low frequency cavities is traced back to two contributions with different temperature …
RF superconductivity has become a major subfield of accelerator science. There has been an explosion in the number of accelerator applications and in the number of laboratories engaged. The first lecture at this meeting of the CAS presented a review of fundamental design principles to develop cavity geometries to accelerate velocity-of-light particles ($β$ = v/c ~ 1), moving on to the corresponding design principles for medium-velocity (medium-$β$) and low-velocity (low-$β$) structures. The lecture included mechanical design topics. The second lecture dealt with input couplers, higher-order mo…
The radiofrequency resonant four-quadrant mechanical structure of a 4-vane Radio Frequency Quadrupole (RFQ) has a high quality factor and narrow bandwidth, resulting in high sensitivity to frequency detuning caused by thermal expansion under high power operation, so minimization of frequency errors and affording dynamic tuning are very important design issues. Here we describe an optimization approach to figure out a suitable cooling design for a 4-vane RFQ in steady state. Besides, we investigate how the accelerator responds in transient thermal analysis that could help to guide commissioning…
Ambient magnetic field, if trapped in the penetration depth, leads to the residual resistance and therefore sets the limit for the achievable quality factors in superconducting niobium resonators for particle accelerators. Here we show that a complete expulsion of the magnetic flux can be performed and leads to: 1) record quality factors $Q > 2\times10^{11}$ up to accelerating gradient of 22 MV/m; 2) $Q\sim3\times10^{10}$ at 2 K and 16 MV/m in up to 190 mG magnetic fields. This is achieved by large thermal gradients at the normal/superconducting phase front during the cooldown. Our findings op…
A new surface treatment technique for niobium (Nb) Superconducting Radio Frequency (SRF) cavities called Buffered Electropolishing (BEP) has been developed at JLab. It was found that BEP could produce the smoothest surface finish on Nb samples ever reported in the literature. Experimental results revealed that the Nb removal rate of BEP could reach as high as 4.09 um/min. This is significantly faster1 than that of the conventional electropolishing technique employing an acid mixture of HF and H2SO4. An investigation is underway to determine the optimum values for all relevant BEP parameters so…
Particle accelerators have enabled forefront research in high energy physics and other research areas for more than half a century. Accelerators have directly contributed to 26 Nobel Prizes in Physics since 1939 as well as another 20 Nobel Prizes in Chemistry, Medicine and Physics with X-rays. Although high energy physics has been the main driving force for the development of the particle accelerators, accelerator facilities have continually been expanding applications in many areas of research and technology. For instance, active areas of accelerator applications include radiotherapy to treat…
The dependence of the Q-value on the RF field (Q-slope) is actively studied in various accelerator laboratories. Although remedies against this dependence have been found, the physical cause still remains obscure. A rather straightforward two-fluid model description of the Q-slope in the low and high field domains is presented with emphasis on the recently experimentally identified improvement of the Q-value by so-called "N-doping".
The Tera Electronvolt Superconducting Linear Accelerator TESLA is the only linear electron-positron collider project based on superconductor technology for particle acceleration. In the first stage with 500 GeV center-of-mass energy an accelerating field of 23.4 MV/m is needed in the superconducting niobium cavities which are operated at a temperature of 2 K and a quality factor Q0 of 10 to the 10. This performance has been reliably achieved in the cavities of the TESLA Test Facility (TTF) accelerator. The upgrade of TESLA to 800 GeV requires accelerating gradients of 35 MV/m. Using an improve…
