We have performed the harmonic analysis of the steady-state coherent pulse-stacking process in a high-Q Fabry-Perot cavity. The expression for the stacked pulse shape is obtained as a function of both the laser cavity and pulse-stacking cavity parameters. We have also estimated the pulse power gains attainable in the laser-optical system of NESTOR storage ring, which is under development at Kharkov Institute of Physics and Technology. It is shown that high power gains (~10000) can be, in principle, achieved in a cavity, formed with low-absorption, high reflectivity (R~0.9999) mirrors, if the laser carrier frequency will be matched to the second harmonic frequency of the pulse-stacking cavity. This means a development of the sophisticated frequency stabilization loop for maintaining the cavity length constant within a sub-nanometer range.
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.
We consider acceleration of protons in a cavity with such dimensions as length l = 10 m, width b = 20 m, height a = 1.01 m, located between two bending (180)degree magnets with isochronous magnetic field. The cavity is loaded with rows of drift plates (tubes), being excited by a wave of E110 type at the frequency fo = 148.5 MHz, excitation power P1 = 5.65 MW. Additionally, the cavity is fed with the power P2 = 8 MW, transmitted into a beam of protons with such parameters as the final energy 0.8 GeV, pulse beam current I = 10 mA, pulse width 25 microseconds, repetition rate 10 Hz. We discuss th…
We develop a configuration of radio-frequency (rf) cavities to dramatically improve the performance of slip-stacking. Slip-stacking is an accumulation technique used at Fermilab to nearly double proton intensity by maintaining two beams of different momenta in the same storage ring. The two particle beams are longitudinally focused in the Recycler by two 53 MHz 100 kV rf cavities with a small frequency difference between them. We propose an additional 106 MHz 20 kV rf cavity with a frequency at the double the average of the upper and lower main rf frequencies. We show the harmonic rf cavity ca…
In the recent decades of particle accelerator R&D area, fixed field alternating gradient (FFAG) accelerator has become a highlight for some advantages of its higher beam intensity and lower cost, although there are still some technical challenges. In this paper, FFAG accelerator is adopted to accelerate helium ion beam on the one hand for the study of helium embrittlement on fusion reactor envelope material and on the other hand for promoting the conception research and design of FFAG accelerator and exploring the possibility of developing high power FFAG accelerators. The conventional period …
The Accelerator Test Facility 2 (ATF2) is a scaled demonstrator system for final focus beam lines of linear high energy colliders. This paper describes the high resolution cavity beam position monitor (BPM) system, which is a part of the ATF2 diagnostics. Two types of cavity BPMs are used, C-band operating at 6.423 GHz, and S-band at 2.888 GHz with an increased beam aperture. The cavities, electronics, and digital processing are described. The resolution of the C-band system with attenuators was determined to be approximately 250 nm and 1 m for the S-band system. Without attenuation the best r…
In the development of high efficiency and high gradient RF-accelerators, RF waveguides and cavities have been designed with Photonic Band Gap (PBG) and fishnet- metamaterial structures. The designed structures are comprised of a periodically corrugated channel sandwiched between two photonic crystal slabs with alternating high to low dielectric constants and a multi-cell cavity-resonator designed with fishnet-metamaterial apertures. The structural designs of our interest are intended to only allow an operating-mode or -band within a narrow frequency range to propagate. The simulation analysis …
The research note presents results of coupled RF and thermal simulation of a cryocooler conduction cooled 650 MHz SRF cavity made of bulk niobium and coated with Nb3Sn on the RF surface. The cavity is part of a particle accelerator design capable of producing 10 MeV, 1000 kW electron beam for application in wastewater treatment. More details of the accelerator design are presented in R.C. Dhuley et al. (2022) Phys. Rev. Accel. Beams 25, 041601.
We present an analysis of electromagnetic oscillations in a spherical conducting cavity filled concentrically with either dielectric or vacuum layers. The fields are given analytically, and the resonant frequency is determined numerically. An important special case of a spherical conducting cavity with a smaller dielectric sphere at its center is treated in more detail. By numerically integrating the equations of motion we demonstrate that the transverse electric oscillations in such cavity can be used to accelerate strongly relativistic electrons. The electron's trajectory is assumed to be ne…
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 …
A high-intensity continuous wave (CW) radio frequency quadrupole (RFQ) accelerator is designed for boron neutron capture therapy (BNCT). The transmission efficiency of a 20-mA proton beam accelerated from 30 keV to 2.5 MeV can reach 98.7% at an operating frequency of 200 MHz. The beam dynamics has good tolerance to errors. By comparing the high-frequency parameters of quadrilateral and octagonal RFQ cross-sections, the quadrilateral structure of the four-vane cavity is selected owing to its multiple advantages, such as a smaller cross section at the same frequency and easy processing. In addit…
For proton linear accelerators used in applications such as accelerator-driven systems, due to the nature of the operation, it is essential for the beam failure rate to be several orders of magnitude lower than usual performance of similar accelerators. A fault-tolerant mechanism should be mandatorily imposed in order to maintain short recovery time, high uptime and extremely low frequency of beam loss. This paper proposes an innovative and challenging way for compensation and rematch of cavity failure using fast electronic devices and Field Programmable Gate Arrays (FPGAs) instead of embedded…
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 …
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…
Superconducting radio-frequency cavities are commonly used in modern particle accelerators for applied and fundamental research. Such cavities are typically made of high-purity, bulk Nb and are cooled by a liquid helium bath at a temperature of ~2 K. The size, cost and complexity of operating a particle accelerator with a liquid helium refrigerator makes the current cavity technology not favorable for use in industrial-type accelerators. We developed a multi-metallic 1.495~GHz elliptical cavity conductively cooled by a cryocooler. The cavity has a ~2 $μ$m thick layer of Nb$_3$Sn on the inner s…
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…
For NOvA and future experiments requiring high intensity proton beams, Fermilab is in the process of upgrading the existing accelerator complex for increased proton production. One such improvement is to reduce the Main Injector cycle time, by performing slip stacking, previously done in the Main Injector, in the now repurposed Recycler Ring. Recycler slip stacking requires two new RF cavities operating at slightly different frequencies (df = 1260Hz). These are copper, coaxial, quarter wave cavities with R/Q =13 ohms. They operate at a peak gap voltage of 150 kV with 150 kW peak drive power (6…
Superconducting radio-frequency (SRF) cavities cooled by superfluid helium-4 (He II) are building blocks of many modern particle accelerators due to their high quality factor. However, Joule heating from sub-millimeter surface defects on cavities can lead to cavity quenching, which limits the maximum acceleration gradient of the accelerators. Developing a non-contacting detection technology to accurately locate these surface defects is the key to improve the performance of SRF cavities and hence the accelerators. In a recent proof-of-concept experiment (Phys. Rev. Applied, 11, 044003 (2019)), …
We present the technical and engineering design of a medium energy (10 MeV) and high average power (1 MW) electron beam accelerator intended for irradiation treatment of high volume industrial and municipal wastewater. The accelerator uses superconducting radiofrequency (SRF) cavity technology for producing the high average beam power with $>$90% RF to beam efficiency. The design of the accelerator is simplified and tailored for industrial settings by adopting the cryocooler conduction-cooling technique for the SRF cavity instead of a conventional liquid helium bath cryo-system. The technical …
As part of the R&D toward the production of high flux of polarised Gamma-rays we have designed and built a non-planar four-mirror optical cavity with a high finesse and operated it at a particle accelerator. We report on the main challenges of such cavity, such as the design of a suitable laser based on fiber technology, the mechanical difficulties of having a high tunability and a high mechanical stability in an accelerator environment and the active stabilization of such cavity by implementing a double feedback loop in a FPGA.
During the operation of the China ADS Front End Superconducting Demo Linac of the China initiative Accelerator Driven System (CiADS), it was found that there was flashover happened on the surface of vacuum ceramic window in the pick-up coupler of superconducting cavity, which is uncommon under low temperature conditions. That seriously affects on the stable operation of the superconducting linear accelerator. By comparing the flashover trajectory on the electrical aging windows from the accelerator, the simulation results of the field emission electrons trajectory in the superconducting cavity…
Multipactor discharges are widely observed in the accelerator field, and their suppression has been studied to improve accelerator performance. A dielectric-assist accelerating (DAA) cavity is a standing-wave accelerating cavity that attains a Q-value of over 100,000 at room temperature by using the reflection of the dielectric layer; the DAA cavity is expected to be used with a small RF power source and high-duty operation. Thus far, the maximum accelerating field of DAA cavities has been limited to a few MV/m by the multipactor. By applying a diamond-like carbon (DLC) coating to reduce the s…
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,…
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…
We suggest a program to establish theoretical performance limits of srf cavities using modern theories of nonequilibrium superconductivity under a strong electromagnetic field. These theories will be used to calculate the main parameter of merit of srf cavities: the quality factor Q and its dependencies on the field amplitude, temperature and frequency, which would allow us to understand how far the srf cavity performance could be pushed from the current state of the art. Given that the quality factor is determined by multiple mechanisms operating on very different length scales, we will addre…
The high-power RF coupler is the connecting part between the RF transmission line and the RF cavity and provides the electromagnetic power to the cavity and the particle beam. In addition to this RF function it also has to provide the vacuum barrier for the beam vacuum. High-power couplers are one of the most critical parts of the RF cavity system in an accelerator. A good RF and mechanical design as well as a high quality fabrication are essential for efficient and reliable operation of an accelerator.
Bridging Metal Additive Manufacturing and RF Accelerator Design: Development of a 704.4 MHz Crossbar H-Mode Linac for Efficient Beam Acceleration
GSI Helmholtzzentrum für Schwerionenforschung（GSI亥姆霍兹重离子研究中心）；Institute for Applied Physics, Goethe-University Frankfurt（法兰克福歌德大学应用物理研究所）；Helmholtz Research Academy Hesse for FAIR（黑森亥姆霍兹FAIR研究院）
We present a topology for linear accelerators (linacs) that permits larger degrees of freedom for the optimization of individual cavity shapes. The power is distributed to the cavities through a waveguide with periodic apertures that guarantees the correct phases and amplitudes along the structure. This topology optimizes the power consumption and efficiency and allows the manipulation of the surface fields for high gradient operation. It also provides a possibility for low-temperature manufacturing techniques for use with novel materials. This greatly enhanced performance for both normal and …
In laser illuminated dielectric accelerators (DLA) high acceleration gradients can be achieved, due to high damage thresholds of the materials at optical frequencies. This is a necessity in developing more compact particle accelerator technologies. The Accelerator on a CHip International Program funded by the Gordon and Betty Moore Foundation is researching such devices. Means to manipulate the beam, i.e. focusing and deflection, are needed for the proper operation of such devices. These means should rely on the same technologies for manufacturing and powering like the accelerating structures.…
Superconducting Radio Frequency (SRF) cavities performance preservation is crucial, from vertical test to accelerator operation. Field Emission (FE) 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 and increasing the work function of Nb. A collaboration has been established between Fermi National Accelerator Laboratory (FNAL), SLAC National Accelerator Laboratory and Oak Ridge National Laboratory (ORNL) with the purpose of applying plasma processing to the Linac Coheren…
The commissioning of two cryomodule components is underway at Fermilab's Superconducting Radio Frequency (SRF) Accelerator Test Facility. The research at this facility supports the next generation high intensity linear accelerators such as the International Linear Collider (ILC), a new high intensity injector (Project X) and other future machines. These components, Cryomodule #1 (CM1) and Capture Cavity II (CC2), which contain 1.3 GHz cavities are connected in series in the beamline and through cryogenic plumbing. Studies regarding characterization of ground motion, technical and cultural nois…
The untuned magnetic alloy (MA) loaded cavity will be adopted in the recent proposed projects based on compact proton and heavy-ion synchrotron in China. The MA loaded cavity has been operated successfully in many accelerators, but there is no construction and operation experience in China. The small-size MA cores with outer diameter >100 mm have been supplied by Chinese firms, and the properties of cores are tested and show good consistency with Hitachi material. Based on the MA cores testing results, the schematic design of a cavity is conducted, which could obtain 1 kV gap voltage with less…
The research is aimed to address issues of analysis and mitigation of high repetition rate effects in Dielectric Wakefield Accelerators, and more specifically, to study charging rate and charge distribution in a thin walled dielectric wakefield accelerator from a passing charge bunch and the physics of conductivity and discharge phenomena in dielectric materials useful for such accelerator applications. The issue is the role played by the beam halo and intense wakefields in charging of the dielectric, possibly leading to undesired deflection of charge bunches and degradation of the dielectric …
Use of a traveling wave (TW) accelerating structure with a small phase advance per cell instead of standing wave may provide a significant increase of accelerating gradient in a superconducting linear accelerator. The TW section achieves an accelerating gradient 1.2-1.4 larger than TESLA-shaped standing wave cavities for the same surface electric and magnetic fields. Recent tests of an L-band single-cell cavity with a waveguide feedback demonstrated an accelerating gradient comparable to the gradient in a single-cell ILC-type cavity from the same manufacturer. This article presents the next st…
Accelerator-based terahertz (THz) radiation has been expected to realize a high-power broadband source. Employing a low-emittance and short-bunch electron beam at a high repetition rate, a scheme of coherent diffraction-radiation in an optical cavity layout is proposed. The scheme's stimulated radiation process between bunches can greatly enhance the efficiency of the radiation emission. We performed an experiment with a superconducting linac constructed as an energy recovery linac (ERL) test facility. The electron beam passes through small holes in the cavity mirrors without being destroyed. …
The FLASH L-band (1.3 GHz) superconducting accelerator facility at DESY has a Low Level RF (LLRF) system that is similar to that envisioned for ILC. This system has extensive monitoring capability and was used to gather performance data relevant to ILC. Recently, waveform data were recorded with both beam on and off for three, 8-cavity cryomodules to evaluate the input RF and cavity gradient stability and study the rf overhead required to achieve constant gradient during the 800μ\s pulses. In this paper, we present the recent experimental results and discuss the pulse-to-pulse input rf and cav…
There's presently a growing demand for cw high current proton and deuteron linear accelerators based on superconducting technology to better support various fields of science. Up to now, high order modes (HOMs) studies induced by ion beams with current higher than 10 mA and even 100 mA accelerated by low beta non-elliptical Superconducting rf (SRF) cavities are very few. Peking University has recently designed and fabricated a beta=0.09 162.5 MHz HWR cavity to study the key physics problems in accelerating beam with current of about 100mA. This paper focuses on the study of the HOM-induced pow…
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…
A 3.9 GHz deflecting mode (pi, TM110) cavity has been long used for six-dimensional phase-space beam manipulation tests at the A0 Photo-Injector Lab (16 MeV) in Fermilab and their extended applications with vacuum cryomodules are currently planned at the Advanced Superconducting Test Accelerator (ASTA) user facility (> 50 MeV). Despite the successful test results, the cavity, however, demonstrated limited RF performance during liquid nitrogen (LN2) ambient operation that was inferior to theoretical prediction. We have been performing full analysis of the designed cavity by analytic calculation…
