Accelerator Cavities Research Articles

Integral Resonance Control In Continuous Wave Superconducting Particle Accelerators

Superconducting accelerating cavities for continuous wave low-current particle accelerators requires a tight resonance control to optimize the RF power costs and to minimize the beam delivery downtime. When the detuning produced by radiation pressure becomes comparable to the RF bandwidth, the monotonic instability starts to affect the cavity operation. When this instability is triggered by external vibrations or drifts, the accelerating field amplitude drops rapidly, and the beam acceleration has to be stopped. Past experiments showed that using an integral control of the piezoelectric tuners installed on the cavity prevents the adverse effects of the monotonic instability. This paper derives theoretically why an integral controller is an effective way to counteract the monotonic instability. To perform the study a linearized state-space model of the cavity is derived. Simulations and experiments in a superconducting test facility indicate that the use of this kind of control has the additional benefit of bringing the cavities to the resonance condition automatically.

Development of the radio frequency quadrupole proton linac for ESS-Bilbao

The Radio Frequency Quadrupole (RFQ) linear accelerator for ESS-Bilbao is described. This device will complete ESS-Bilbao injection chain after the ion source and the LEBT. The design was finished in 2015 and machining of the accelerator cavity started in 2016. The RFQ is a 4-vane structure, aimed to accelerate protons from 45 keV to 3:0MeV and operating at 352:2MHz in pulsed mode with a duty cycle up to 10%. Total length is about 3:1m divided in 4 segments. Each segment is itself assembled from four components, named vanes, by using polymeric vacuum gaskets with no brazing among them. Notable aspects of the design are the constant mean aperture R0, vane radius ρ and thus ρ/R0 ratio and also uniform inter vane voltage. Novel procedures for the design of the modulation and integrated beam dynamics and electromagnetic design have been developed for this task. In this paper, the complete design procedure and its results are presented, including beam dynamics, RF cavity design, field flatness and frequency tuning, cooling and thermo-mechanical design.

A new RF structure: bent-vane type RFQ

A new RFQ accelerator cavity structure with bent vanes is proposed to meet the miniaturization requirement of low frequency accelerators. The news structure has a downsized cross section by bending the vanes while maintaining a certain vane length. It also possesses the advantage of a simple cooling configuration. The new RFQ structure is presented in this paper.

Design Process for Synchrotron RF Cavities Loaded With Magnetic Ring Cores

Cavities loaded with magnetic ring cores are often used in synchrotrons if the desired operating frequency is in the lower megahertz range. The properties of such a cavity are strongly determined by the ring cores. However, other cavity components also have an important influence. During the design phase of such a cavity, full-wave simulations are not the first choice because computational modeling of parts like magnetic ring cores is complicated and because the influence of the ring core material data is often larger than that of the cavity geometry. In the past, these types of cavities have often been built without performing 3-D simulations. While standard acceleration cavities can be developed rather accurately by experienced designers, the coupling between amplifier and cavity or the development of more advanced cavity systems e.g., for barrier bucket operation or longitudinal feedback still bears some risks. In this article, at hand, we describe a new development process that was successfully used for the development of an MA-loaded broadband cavity. It is based on the broadband lumped-element circuits for single-ring cores. We used standardized ring-core measurements to obtain the required impedance data, but analytical methods can, in general, be used as well. The lumped-element circuits for the ring cores are used in a PSpice model of the whole cavity, allowing for simulations in frequency and time domain. We will show that the simulation results are in good agreement with measurements of the overall cavity.

Design and construction of an electron accelerator for a pulsed neutron facility at AIST

A new electron accelerator has been designed and is now under construction at AIST, Tsukuba, Japan. The accelerator will be used to produce a pulsed neutron beam for materials analysis. The design maximizes the yield of slow neutrons with high pulse temporal resolution as described in a previous publication [K. Kino et al., Nuclear Inst. and Methods in Physics Research, A 927 (2019) 407–418]. The accelerator is composed of a thermal cathode type electron gun with S-Band buncher/accelerator cavity followed by three, 2.9 m long S-Band acceleration cavities. High power RF is supplied by three klystron units, with klystron No.1 supplying both the electron gun and the 1st cavity, and klystrons No.2 and No.3 connected to the 2nd and 3rd cavities respectively. The design parameters of the accelerator are; pulse rate: 100 Hz, pulse length: 10 μs, electron current: 250 mA, electron energy: 40 MeV. Operating at these parameters a maximum beam power of around 10 kW will be delivered to the water-cooled Ta target for neutron production. In this report we will describe the design and construction of the electron accelerator.

Electromagnetic Simulations of Mechanical Imperfections for Accelerator Cavities

Effects of surface roughness and transversal cell misalignments on the performance of elliptical accelerator cavities are studied in this article. A high- $\beta $ , 9-cell elliptical cavity, whose $\pi $ -mode resonates at 3.9 GHz, is designed to investigate imperfections. The considered frequency is chosen to observe variations of fundamental accelerating cavity parameters, wake potentials, and wake impedances more clearly by using relatively small structures. Moreover, 3-cell elliptical cavities having $\pi $ -mode at 2 and 3.9 GHz are designed to confirm the 9-cell cavity results. The undesired effects caused by the considered mechanical imperfections are simulated for an ultra-relativistic bunch in the parameter scope of a realistic scenario. In particular, Huray’s snowball model, which is a scattering-based surface roughness approach developed for microstrip lines, is employed to determine the effects of the surface roughness on the accelerator cavities. Surface roughness due to the fabrication process is expressed as a surface impedance, and the required equivalence between the surface roughness and surface impedance concept is achieved. Significant computational efficiency is observed by using the surface impedance concept with Huray’s snowball model in the simulations. Experimental verification of certain parameters is included for an elliptical cavity having high cell-to-cell coupling at 3.9 GHz.

Design of the elliptical superconducting cavities for the JAEA ADS

The superconducting CW proton linear accelerator for an Accelerator Driven Subcritical System (ADS) proposed by Japan Atomic Energy Agency (JAEA) employs elliptical cavities for the final acceleration of 180 MeV to 1.5 GeV. Since this energy region implies a changed of β from 0.55 to 1, two cavity models were developed using the geometrical betas of 0.68 and 0.89 to improve the acceleration efficiency. The study of the electromagnetic design was simulated using SUPERFISH (SF) code and python program to do variable scan, the results were benchmarked with CST Microwave Studio program (CST).

Simulations of Beam Loading Compensation in a Wideband Accelerating Cavity Using a Circuit Simulator Including a LLRF Feedback Control

Simulations of Beam Loading Compensation in a Wideband Accelerating Cavity Using a Circuit Simulator Including a LLRF Feedback Control

Cylindrical cavity design and particle-tracking simulation in cyclotron auto-resonance accelerator

The Cyclotron Auto-Resonance Accelerator (CARA) is a novel concept of accelerating continuous-wave (CW) charged-particle beams. This type of accelerator has applications in environment improvement area and generation of high-power microwaves. In CARA, the CW electron beam follows a gyrating trajectory while undergoing the interaction with a rotating TE11-mode RF field and tapered static magnetic field. The cylindrical cavity operating at TE11p-mode is adapted to accelerate electron beam. The cavity size is optimized to obtain a beam with designed energy, then a design method of the TE11p-mode acceleration cavity is described here. Moreover, regardless of space charge effect, several particle-tracking simulations of CARAs are showed.

Electromagnetic design and characterization of an S-band 3-cell rf acceleration cavity

An S-Band (2998 MHz) Radio Frequency (RF) cavity to accelerate electrons was developed taking into account the beam space charge, the relativistic change in velocity of the low energy beam particle distribution through the cavity, and the emittance growth. The electromagnetic design and geometry optimization were done using the codes Poisson Superfish (PSF) and CST Studio (CST). In addition, beam dynamics simulations were done using the program Travel to optimize the emittance and take into account the space charge effect.

Niobium near-surface composition during nitrogen infusion relevant for superconducting radio-frequency cavities

A detailed study of the near-surface structure and composition of Nb, the material of choice for Superconducting Radio Frequency accelerator (SRF) cavities, is of great importance in order to understand the effects of different treatments applied during cavity production. By means of surface-sensitive techniques such as grazing incidence diffuse X-ray scattering, X-ray reflectivity and X-ray photoelectron spectroscopy, single-crystalline Nb(100) samples were investigated in and ex-situ during annealing in UHV as well as in nitrogen atmospheres with temperatures and pressures similar to the ones employed in real Nb cavity treatments. Annealing of Nb specimens up to 800{\deg}C in vacuum promotes partial reduction of the natural surface oxides (Nb2O5, NbO2, NbO) into NbO. Upon cooling to 120{\deg}C, no evidence of nitrogen-rich layers was detected after nitrogen exposure times of up to 48 hours. Oxygen enrichment below the Nb/oxide interface and posterior diffusion of oxygen species towards the Nb matrix, along with a partial reduction of the natural surface oxides was observed upon a stepwise annealing up to 250{\deg}C. Nitrogen introduction to the system at 250{\deg}C neither promotes N diffusion into the Nb matrix nor the formation of new surface layers. Upon further heating to 500{\deg}C in a nitrogen atmosphere, the growth of a new subsurface Nb$_x$N$_y$ layer was detected. These results shed light on the composition of the near-surface region of Nb after low-temperature nitrogen treatments, which are reported to lead to a performance enhancement of SRF cavities.

European XFEL Superconducting Cryomodules Characterization Toward Modules Acceptance and Future LLRF Operation

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.

Physics and Challenges of Superconducting Cavities for Particle Accelerators and Theoretical Implication towards Higher Performance:

We discuss the physics of superconducting resonant cavities for particle accelerators, including the basics of a superconducting cavity, Meissner state stability, and surface resistance together with related scientific and technological challenges. Ongoing research and some proposals for solving the challenges are also discussed.

Optimum multilayer coating of superconducting particle accelerator cavities and effects of thickness dependent material properties of thin films

We revisit the field limit of a superconductor–insulator–superconductor multilayer structure for particle accelerator cavities (BML), taking into account thickness (d)-dependent critical temperature, normal resistivity, and normal density of states seen in many thin films. Resultant d-dependent thermodynamic critical field and penetration depth lead to the appearance of a peak in BML(d) which has been missed in the previous studies. The procedure shown in this note would be useful to evaluate BML based on properties of one’s own films.

Optimization of tailored multilayer superconductors for RF application and protection against premature vortex penetration

Superconducting radiofrequency (SRF) cavities for accelerators is the only superconductivity application that operates in the Meissner state. For decades, bulk niobium, which exhibits the highest 1st critical field among all known superconductors, has been the only material to achieve high accelerating fields in combination with a high quality factor. Specific nanostructures tailored for SRF applications, in the form of superconducting/insulating/superconducting multilayers, were proposed to surpass Nb performances that could improve SRF performance. In this paper, we present the study of a series of NbN/MgO/Nb tri-layers including standard material characterization and specific superconducting characterization (local magnetometry) along with a comparison to the current state-of-the-art theoretical modeling. This study shows that such structures are effective to enhance the penetration field compared to bare niobium, even in the presence of numerous defects.

Effects of Ageing on the Microwave Surface Resistance of MgB2 Superconductor Films Stored in Low Vacuum

Nb is a very popular material in superconductive electronic devices such as superconducting single-photon detectors and radio-frequency (RF) cavities for particle accelerators. Before MgB2 can be used as a substitute material, the stability of its properties over time must be confirmed. We studied how ageing over a period of 514 weeks affected the microwave surface resistance (RS) and the normal-state resistivity (ρ) of epitaxially-grown MgB2 films. MgB2 films with thicknesses of 1.0 μm and 1.7 μm were prepared using the hybrid physical CVD method, and exhibited a critical temperature (TC) of 39 K. They were stored in low-vacuum to prevent reactions with humidity, except when the films were taken out of storage for measurement purposes. Variation in RS with the ageing period appeared to be significant at very low temperatures. The RS values of 1.0 μm-thick and 1.7 μm-thick MgB2 films increased by ~ 8 times and ~ 3 times, respectively, over ~ 10 years. The aged MgB2 films appeared to have reduced π-band gap energy values. The pristine MgB2 films value of 2.3 meV was reduced to 1.3–1.7 meV over ~ 10 years. The TC of the MgB2 films remained almost the same regardless of the ageing time. The enhanced RS and ρ in the aged MgB2 films were attributed to increased intraband scattering within the π-band and the σ-band.

Oxygen dissolution and surface oxide reconstructions on Nb(100)

The ever-present native oxide layer on niobium plays a fundamental role in the performance of Nb superconducting radio frequency (SRF) cavities for particle accelerators and light sources. Using Nb(111) and Nb(100) as model systems, oxygen dissolution and surface structural evolution as a function of thermal treatments in ultrahigh vacuum (UHV) were studied using combined Auger electron spectroscopy (AES) and scanning tunneling microscopy (STM), providing novel, real space information regarding the complex evolution of the Nb surface oxide. The surface crystallographic orientation was shown to influence oxide surface structure; Nb(111) displayed disordered surface oxide domains while Nb(100) was comprised of well-ordered, (n × 1)-O domains. The temperature-dependent dissolution of the native oxide layer on Nb(100) was characterized to ascertain the energetics for oxygen dissolution, and evolution of the surface oxide superlattice structure following thermal annealing was observed with STM. By understanding the kinetic behavior of oxygen dissolution during thermal annealing and subsequent structural evolution of the Nb(n × 1)-O superlattice on Nb(100), a more thorough understanding of the complex interactions driving chemical composition and cavity surface structure under Nb SRF cavity processing and operating conditions may be understood.

Quench-Spot Detection for Superconducting Accelerator Cavities Via Flow Visualization in Superfluid Helium-4

Superconducting ratio-frequency (SRF) cavities, cooled by superfluid helium-4 (He II), are key components in modern particle accelerators. Quenches in SRF cavities caused by Joule heating from local surface defects can severely limit the maximum achievable accelerating field. Existing methods for quench spot detection include temperature mapping and second-sound triangulation. These methods are useful but all have known limitations. Here we describe a new method for surface quench spot detection by visualizing the heat transfer in He II via tracking He$_2^*$ molecular tracer lines. A proof-of-concept experiment has been conducted, in which a miniature heater mounted on a plate was pulsed on to simulate a surface quench spot. A He$_2^*$ tracer line created nearby the heater deforms due to the counterflow heat transfer in He II. By analyzing the tracer-line deformation, we can well reproduce the heater location within a few hundred microns, which clearly demonstrates the feasibility of this new technology. Our analysis also reveals that the heat content transported in He II is only a small fraction of the total input heat energy. We show that the remaining energy is essentially consumed in the formation of a cavitation zone near the heater. By estimating the size of this cavitation zone, we discuss how the existence of the cavitation zone may explain a decades-long puzzle observed in many second-sound triangulation experiments.

Efficient hybrid approach to the electromagnetic design of resonant cavities for side‐coupled linacs

A novel hybrid analytical/numerical approach, implemented via homemade code, is exploited to design the microwave cavities of a linear accelerator (linac) characterised by a very complex geometry. A finite element method-based three-dimensional investigation is integrated with a multi-objective particle swarm optimisation technique in order to automatically design and optimise the geometry of the linac cavities. The cavity optimisation takes into account the typical figures of merit of a linac design. The hybrid approach is used to design a 27 MeV 3 GHz standing-wave side-coupled linac tank of 35 cavities including the end cells. The promising simulated results suggest that a global search approach is a useful and flexible tool for the electromagnetic design of accelerator cavities as well as other complex resonating multicavity structures.

Influence of crystalline structure on rf dissipation in superconducting niobium

Bulk niobium is the most common material used in the fabrication of rf superconducting cavities for accelerators. Predicting and reducing the rf surface dissipation in these cavity structures is mandatory, since it has a tremendous cost impact on the large accelerator projects. In this paper the author hopes to demonstrate that sources of dissipation usually attributed to external causes (mainly flux trapping during cooldown and hydrides precipitates) are related to the same type of crystalline defects that affect the local superconducting properties and can be at the source of early vortex penetration at the surface. We also want to show how these types of defects can explain some of the discrepancies observed from other laboratories in niobium cavity doping experiments. Understanding the origin and the role of these defects could provide direction for improving material specifications as well as improving fabrication control from sheet material to completed cavity. In particular, we will demonstrate that dislocation entanglements, due to the fabrication damage layer, have the strongest impact for the pinning behavior of trapped flux, as well as hydrogen segregation in cavity niobium. The author wishes to present to the superconducting radio frequency (SRF) accelerator community the synthesis of experimental results scattered in the literature, completed with some personal results. The results of this effort provide a new perspective on recently published work in the domain of SRF cavity doping and sensitivity to trapped flux during cooldown. I will also try to draw whenever possible, some conclusions about other types of superconductors used for SRF applications including $\mathrm{Nb}/\mathrm{Cu}$ thin films and to discuss their possible change of behavior with field or frequency. I will concentrate on surface and material science aspects since the experimental results on rf cavities have already been treated elsewhere.