RF Cavities Research Articles

Analysis of thermal grooving effects on vortex penetration in vapor-diffused Nb3Sn

Abstract While Nb3Sn theoretically offers better superconducting RF cavity performance (Q0 and Eacc) to Nb at any given temperature, peak RF magnetic fields consistently fall short of the ∼400 mT prediction. The relatively rough topography of vapor-diffused Nb3Sn is widely conjectured to be one of the factors that limit the attainable performance of Nb3Sn-coated Nb cavities prepared via Sn vapor diffusion. Here we investigate the effect of coating duration on the topography of vapor-diffused Nb3Sn on Nb and calculate the associated magnetic field enhancement and superheating field suppression factors using atomic force microscopy topographies. It is shown that the thermally grooved grain boundaries are major defects which may contribute to a substantial decrease in the achievable accelerating field. The severity of these grooves increases with total coating duration due to the deepening of thermal grooves during the coating process.

New operation analysis method for vacuum tube driving a tuned cavity

The rf system of China Spallation Neutron Source (CSNS) Rapid Cycling Synchrotron (RCS) utilizes vacuum tubes to drive the ferrite-loaded cavities. Ensuring stable tube operation in the rf system is crucial for the acceleration of high-intensity beams, as any instability may impede further increases in beam power. Therefore, the operation of the vacuum tube under heavy beam loading has attracted significant attention, particularly in the case of CSNS-II RCS, where the circulating beam current is expected to reach up to 15.25 A, imposing a substantial burden on the vacuum tube for beam loading compensation. Thus, a comprehensive analysis of tube operation is imperative. However, current methods for the tube operation analysis are developed based on wideband rf cavities, which are inadequate when the load of the tube is a narrowband ferrite-loaded cavity equipped with a tuning system. The presence of a tuning system renders the modeling method for wideband rf cavities inapplicable for ferrite-loaded cavities. Therefore, the development of a new method is necessary. This paper introduces a novel method for analyzing operation of vacuum tube driving a tuned cavity. The effectiveness of the method is validated by comparing the analysis results with measurements under beam loading conditions. Furthermore, an estimation of tube operation under 500 kW beam loading for CSNS-II is provided. Published by the American Physical Society 2024

Various Configurations for Improving the Efficiency of Metallic and Superconducting Photocathodes Prepared by Pulsed Laser Deposition: A Comparative Review.

This paper presents an innovative exploration of advanced configurations for enhancing the efficiency of metallic and superconducting photocathodes (MPs and SCPs) produced via pulsed laser deposition (PLD). These photocathodes are critical for driving next-generation free-electron lasers (FELs) and plasma-based accelerators, both of which demand electron sources with improved quantum efficiency (QE) and electrical properties. Our approach compares three distinct photocathode configurations, namely: conventional, hybrid, and non-conventional, focusing on recent innovations. Hybrid MPs integrate a thin, high-performance, photo-emissive film, often yttrium or magnesium, positioned centrally on the copper flange of the photo-injector. For hybrid SCPs, a thin film of lead is used, offering a higher quantum efficiency than niobium bulk. This study also introduces non-conventional configurations, such as yttrium and lead disks partially coated with copper and niobium films, respectively. These designs utilize the unique properties of each material to achieve enhanced photoemission and long-term stability. The novelty of this approach lies in leveraging the advantages of bulk photoemission materials like yttrium and lead, while maintaining the electrical compatibility and durability required for integration into RF cavities. The findings highlight the potential of these configurations to significantly outperform traditional photocathodes, offering higher QE and extended operational lifetimes. This comparative analysis provides new insights into the fabrication of high-efficiency photocathodes, setting the foundation for future advancements in electron source technologies.

On forced RF generation of CW magnetrons for accelerators

CW magnetrons, initially developed for industrial RF heaters, were suggested to power RF cavities of superconducting accelerators due to their higher efficiency and lower cost than traditionally used klystrons, IOTs or solid-state amplifiers. RF amplifiers driven by a master oscillator serve as coherent RF sources. CW magnetrons are regenerative RF generators with a huge regenerative gain. This causes regenerative instability with a quite large noise when a magnetron operates with the anode voltage above the threshold of self-excitation. Traditionally, an injection locking by a small signal is used for stabilization of magnetrons. In this case CW magnetrons with the injection-locked oscillations generate a high level of noise. This may preclude use of standard CW magnetrons in this operating mode in the Superconducting RF (SRF) accelerators. In this article we described a method developed for forced RF generation of CW magnetrons when the magnetron startup is provided by the injected forcing signal and the regenerative noise is suppressed. The method is most suitable for powering high Q-factor cavities.

Beam halo studies of double-periodic focusing structures at the low-energy end of superconducting linac

This paper presents a comprehensive study on the beam halo evolution in the low-energy end of high-current superconducting linear accelerators (SLAs) with double-period focusing structures. The structure consists of cryogenic cooling modules (macro-period structures) and focusing units composed of RF cavities and solenoids within each module (micro-period structures). We utilized a three-dimensional particle-core model based on an ellipsoidal bunch to study the mechanism of halo formation with greater realism and reference value.Numerical simulations indicate that, in line with envelope instability theory, maintaining the zero current phase advance (σ0s) below 90° is critical for the formation of halo structures. Using 90° as the demarcation line, the resonance behavior between particles and the core shows significant differences. Fourier analysis of the envelope oscillation spectrum reveals that when the σ0s of each micro-period structure is less than 90°, the resonance types differ between emittance-dominated beams and space-charge-dominated beams. 2:1 resonance occurs between particles and the 3rd harmonic of the envelope oscillation in space-charge-dominated beams; while 4:1 resonance occurs in emittance-dominated beams. As σ0s increases, the influence of lower harmonics in the resonance becomes more significant. When σ0s is slightly greater than 90°, 4:1 resonance form with higher harmonics of the envelope oscillation (specifically related to the number of micro-period structures in each module). When σ0s reaches 110° or higher, 1:1 resonance form with the 2nd harmonic of the envelope oscillation. Unlike most previous studies based on two-dimensional particle-core models, we found that the longitudinal envelope significantly alters the charge density distribution of the core, leading to halo formation. This finding highlights the importance of three-dimensional effects in beam dynamics, especially under high current conditions.Additionally, we analyzed the impact of the number of micro-period structures within a module and their envelope modulation. Specifically, fewer micro-period structures within a module result in the envelope oscillation spectrum power to be concentrated at lower frequencies, making resonance with low harmonics more likely. Finally, we provide guidelines for the beam matching design at the low-energy end. By adjusting the focusing parameters of the matching structures, the likelihood of beam halo formation can be effectively suppressed.In summary, this study not only reveals the key mechanisms of halo formation in double-period focusing structures but also provides an effective approach to optimizing beam quality by adjusting structural parameters and matching methods. These findings offer clear guidance for beam design in the low-energy end of SLAs.

High Q and high gradient performance of the first medium-temperature baking 1.3 GHz cryomodule

The world’s first 1.3 GHz cryomodule containing eight 9-cell superconducting radio-frequency (rf) cavities treated by medium-temperature furnace baking (mid-T bake) was developed at the Institute of High Energy Physics, Chinese Academy of Sciences. The 9-cell cavities in the cryomodule achieved an unprecedented high average intrinsic quality factor (Q0) of 3.8×1010 at 16 MV/m and 3.6×1010 at 21 MV/m in the horizontal test. The cryomodule can operate stably up to a total continuous wave rf voltage greater than 193 MV, with an average cavity usable accelerating gradient of more than 23 MV/m. The results significantly exceed the specifications of Circular Electron Positron Collider and Dalian advanced light source and the other high repetition rate free electron laser facilities [Linac Coherent Light Source II (LCLS-II), LCLS-II-high energy, Shanghai High Repetition Rate X-ray FEL and Extreme Light Facility, Shenzhen Superconducting Soft X-Ray Free Electron Laser, etc.]. There is evidence that the mid-T bake cavity may not require fast cooldown or long processing time in the cryomodule. This paper reviews the cryomodule performance and discusses some important issues in cryomodule assembly and testing. Published by the American Physical Society 2024

S-band high-gradient low β single-periodic magnetically coupled standing-wave accelerating structure for a proton therapy linac

S-band high-gradient accelerating structures were proposed to accelerate protons from 30 to 230 MeV for a compact proton therapy linac in Institute of Modern Physics. A backward traveling wave structure and a single-periodic magnetically coupled standing wave structure were designed, and differences between these two structures were analyzed to select a more suitable one for the proton therapy linac. In this paper, a novel single-periodic magnetically coupled standing-wave accelerating structure was studied, with an optimized nose cone that related to a smaller peak surface electric field and higher shunt impedance, with the reduced number of coupling holes and mechanical von Mises stress thus making it possible for higher duty factor and longer rf pulse operation, also with larger coupling holes and improved cell-to-cell coupling thus making this structure suitable for operating in π mode. An 11-cell high-power prototype for β=0.26 particles was developed with an active length of 143 mm. This prototype cavity is supposed to operate at an accelerating gradient of 30 MV/m with 0.1% duty factor, corresponding to a maximum surface electric field of 105 MV/m and a peak rf power of 3.43 MW. Cavity design, optimization, manufacture, rf measurement, and high-power test will be discussed in this paper. Published by the American Physical Society 2024

High precision tuning of RF cavity for 6 MeV SKKU X-band medical LINAC

High precision tuning of RF cavity for 6 MeV SKKU X-band medical LINAC

Search for gravitational waves using a network of RF cavities

The concept of detecting gravitational waves using RF-cavities in strong magnetic fields has gained considerable interest, using setups currently running axion searches. We propose a novel analysis approach for detecting GHz-regime gravitational waves, potentially from primordial black hole mergers, through synchronous measurements from multiple, distant cavities. While individual cavities may detect gravitational wave signatures, isolating them from noise is challenging since the strain (and thereby power deposition in a cavity) of such mergers is expected to be very small and short-lived. By correlating signals from several geographically separated cavities, we can significantly improve the signal-to-noise ratio and potentially investigate the sources of these waves. A demonstration experiment with a superconducting cavity is currently underway, forming the basis for our data analysis methods and outlining the prospects for the GravNet project. This project should be seen as an effort to bring the axion community together and collaborate in the context of gravitational wave physics.

Forming limit diagram of annealed copper OFE thick sheets for optimized hydroforming of superconducting RF cavities

Coated superconducting radio-frequency (SRF) cavities are planned to be implemented in the Future Circular Collider (FCC) at the European Organization for Nuclear Research (CERN). The interest in industrialized and high-performance processes for fabrication of the corresponding copper substrates has thus risen. Tubular hydroforming is a well-known industrial process, but has not been thoroughly applied for ultra-pure oxygen-free electronic copper (Cu-OFE) thick products. To provide a feasibility analysis for the optimized fabrication process of copper substrate seamless cavities, a Forming Limit Diagram (FLD) of the material of interest was studied. In this paper, the methodology, test results and initial benchmark of the material model is presented, for 4 mm and 2 mm thick sheets.

Red and Green Laser Powder Bed Fusion of Pure Copper in Combination with Chemical Post-Processing for RF Cavity Fabrication

Linear particle accelerators (Linacs) are primarily composed of radio frequency cavities (cavities). Compared to traditional manufacturing, Laser Powder Bed Fusion (L-PBF) holds the potential to fabricate cavities in a single piece, enhancing Linac performance and significantly reducing investment costs. However, the question of whether red or green laser PBF yields superior results for pure copper remains a subject of ongoing debate. Eight 4.2 GHz single-cell cavities (SCs) were manufactured from pure copper using both red and green PBF (SCs R and SCs G). Subsequently, the surface roughness of the SCs was reduced through a chemical post-processing method (Hirtisation) and annealed at 460 °C to maximize their quality factor (Q0). The geometric accuracy of the printed SCs was evaluated using optical methods and resonant frequency (fR) measurements. Surface conductivity was determined by measuring the quality factor (Q0) of the SCs. Laser scanning microscopy was utilized for surface roughness characterization. The impact of annealing was quantified using Energy-Dispersive X-ray Spectroscopy and Electron Backscatter Diffraction to evaluate chemical surface properties and grain size. Both the SCs R and SCs G achieved the necessary geometric accuracy and thus fR precision. The SCs R achieved a 95% Q0 after a material removal of 40 µm. The SCs G achieved an approximately 80% Q0 after maximum material removal of 160 µm. Annealing increased the Q0 by an average of about 5%. The additive manufacturing process is at least equivalent to conventional manufacturing for producing cavities in the low-gradient range. The presented cavities justify the first high-gradient tests.

Impact of beam coupling impedance on crab cavity noise induced emittance growth

Crab cavities will be deployed as a part of the High Luminosity Large Hadron Collider (HL-LHC) upgrade to mitigate the luminosity reduction induced by the crossing angle at the main experiments (ATLAS and CMS). Two prototype crab cavities have been installed in the CERN Super Proton Synchrotron (SPS) in 2018 for studies with proton beams. An issue of concern is the transverse emittance growth induced by noise in the crab cavity radio frequency (rf) system, which is anticipated to limit the performance of the HL-LHC. In measurements conducted in the SPS in 2018, the crab cavity noise-induced emittance growth was measured to be a factor of 4 lower than predicted from the existing analytical models. In this paper, it is shown that the observed discrepancy is explained by damping effects from the beam coupling impedance, which were not included in the models up to now. Using the van Kampen mode approach, a new theory is developed, suggesting that the impedance can separate the coherent tune from the incoherent spectrum leading to an effective reduction of the crab cavity rf noise-induced emittance growth. This mechanism is validated in tracking simulations using the SPS impedance model as well as in dedicated experimental measurements conducted in the SPS in 2022. The implications for the HL-LHC project are discussed. Published by the American Physical Society 2024

Beam dynamics framework incorporating acceleration to define the minimum aperture in two focusing schemes for proton radiotherapy linac

In this paper, a self-consistent transverse beam dynamics framework is demonstrated that incorporates acceleration into the transverse beam dynamics studies for a proton linac machine. Two focusing schemes are developed and discussed: the FODO-like scheme and the minimum aperture scheme. The FODO-like scheme is a simple scheme, requiring only one quadrupole per cavity. The scheme is analytically solved to minimize the beam size at the cavity entrance/exit and ensures a constant beam size along the lattice, with respect to adiabatic damping due to longitudinally accelerating rf cavities. The minimum aperture scheme describes the regime that matches the beam ellipse to the acceptance ellipse of a cavity, allowing for the smallest possible aperture, for a given cavity length. A simple approximation of an rf cavity map is determined to allow changes in particle energy along a lattice, and acceleration is assumed only in the longitudinal direction. Published by the American Physical Society 2024

Compact HOM-damping structure of a beam-accelerating TM020 mode rf cavity

A compact HOM-damping beam-accelerating rf cavity was developed to mitigate coupled-bunch instabilities in electron storage rings. This cavity operates in the TM020 mode, characterized by a high Q-value and a high shunt impedance. The new HOM-damping mechanism leverages the unique field distributions of the TM020 mode, allowing direct embedding of rf absorbing materials into the cavity body. This novel structure underwent validation through rf simulations and measurements using a model cavity resonating at 508.58 MHz in the TM020 mode with a shunt impedance of 6.8 MΩ and designed to have HOM impedances of less than 0.1 MΩ longitudinally and 0.5 MΩ/m transversely. We also fabricated and tested a prototype cavity, showcasing its HOM-damping efficacy and high-power functionality at 120 kW, generating an accelerating voltage of 900 kV. This prototype operated successfully at full power, confirming the viability of both the damping structure and its high-power application.

MgB$_{2}$ Coating by HPCVD for 1.3-GHz Superconducting RF Cavities

MgB$_{2}$ Coating by HPCVD for 1.3-GHz Superconducting RF Cavities

Cryogenic Distribution System for Polish Free Electron Laser Facility

Polish Free Electron Laser facility (PolFEL), presently under construction at the National Centre for Nuclear Research in Warsaw, will consist of an electron gun and four cryomodules each housing two 9-cell superconducting TESLA RF cavities. The cryomodules comprising the cavities will be supplied with superfluid helium at 2 K. Other PolFEL cooling power requirements result from the demand of the power couplers for the accelerating cryomodules (5 K) and thermal shields (40 K – 80 K). The machine will make use of several helium thermodynamic states like two-phase superfluid HeII, supercritical helium and low-pressure helium vapours supplied to cold compressors. A Cryogenic Distribution System (CDS) will provide supercritical helium to the valve-boxes where thermodynamic processing of the helium to a superfluid state will take place. The paper presents the CDS architecture and discusses the possible design options like methods of the power couplers cooling, optional use and location of cold compressors. The second law of thermodynamics has been used for the optimization of the CDS configuration. A generalized approach to the design of helium distribution systems, taking into account thermomechanical aspects and the consequences of the second law of thermodynamics, is presented.

Research and Design of the RF Cavity for an 11 MeV Superconducting Cyclotron

In contrast to the room temperature cyclotron, the superconducting cyclotron’s high operational magnetic field and small extraction radius lead to a magnet design with a reduced radius. This limits the space available for the RF cavity in the 11 MeV superconducting cyclotron, necessitating a more compact RF cavity design. By using the transmission line theory, the complex structure of the quarter-wavelength coaxial cavity can be represented as a microwave circuit. Through relevant theoretical analytical formulas, equivalent circuit parameters can be derived. The resonant frequency of the RF cavity is then determined using the equivalent circuit method. The optimization of the RF cavity structure was achieved by creating a numerical model and conducting finite element numerical calculations on the high-frequency resonant system. The comparative results between the equivalent circuit and numerical calculations indicate that the frequency error remains within 0.1%, validating the compact RF cavity design. A multiple linear regression analysis facilitates the prediction of resonance frequency across various parameter variables. By analyzing the fitting formula, RF cavity machining error requirements are established, ensuring a prediction error within 1%, thus meeting engineering design criteria.

GrAHal-CAPP for axion dark matter search with unprecedented sensitivity in the 1–3 μeV mass range

A collaboration between CNRS-Grenoble and IBS-CAPP Daejeon plans to build a Sikivie’s type haloscope for axion/ALPs dark matter search at the Dine-Fischler-Srednicki-Zhitnitskii sensitivity for the 300–600 MHz range. It will be based on the large-bore superconducting “outsert” coil of the Grenoble hybrid magnet, providing a central magnetic field up to 9 T in an 810-mm warm bore diameter. This magnet has recently been successfully powered up to 8.5 T, achieving the first step of the electrical commissioning phase. The design principles of the cryostat with its double dilution refrigerators to cool below 50 mK, the light Cu RF cavity of 700-mm diameter, and its tuning rod(s) and the first stages of the measurement chain are presented. Perspectives for the targeted sensitivity assuming less than a 2-year integration time are given.

Improving Fabrication and Performance of Additively Manufactured RF Cavities by Employing Co-Printed Support Structures and Their Subsequent Removal

The enormous potential of additive manufacturing (AM), particularly laser powder bed fusion (L-PBF), to produce radiofrequency cavities (cavities) has already been demonstrated. However, the required geometrical accuracy for GHz TM010 cavities is currently only achieved by (a) avoiding downskin angles <40∘, which in turn leads to a cavity geometry with reduced performance, or (b) co-printed support structures, which are difficult to remove for small GHz cavities. We have developed an L-PBF-based manufacturing routine to overcome this limitation. To enable arbitrary geometries, co-printed support structures are used that are designed in such a way that they can be removed after printing by electrochemical post-processing, which simultaneously reduces the surface roughness and thus maximizes the quality factor Q0. The manufacturing approach is evaluated on two TM010 single cavities printed entirely from high-purity copper. Both cavities achieve the desired resonance frequency and a Q0 of approximately 8300.

An advanced solid-state RF power source maximizing energy efficiency for optimal superconducting RF cavity charging

Abstract This paper outlines an experimental demonstration of an envelope tracking (ET) technique applied to a kilowatt-level single-ended solid-state power amplifier (SSPA), aimed at enhancing the charging efficiency of superconducting radio frequency (SRF) cavities by reducing reflection power while maintaining a high degree of efficiency. The technique is particularly designed for the pulsed operation of the European Spallation Source (ESS) at a nominal frequency of 352 MHz, with a 5% duty cycle and a pulse width of 3.5 ms. The study introduces an optimal charging scheme using a solid-state-based amplifier to maintain high efficiency, allowing for power ramp-up while minimizing reflections from SRF cavities and optimizing SSPA efficiency. A fast envelope tracking power supply (ETPS) system is implemented for the approximately 300 ms charging time required by the SRF cavities at ESS. The ETPS system, demonstrated on a single module as a proof-of-concept with scalability potential to a 400 kW power station, indicates an overall average efficiency of 70% and a 24% energy saving over traditional vacuum-tube based amplifiers. This demonstrates the ET technique’s effectiveness at the kilowatt level for efficient SRF cavity charging with reduced reflection, offering significant efficiency and energy savings.