Multi-cell Cavity Research Articles

Experimental Search for Invisible Dark Matter Axions around 22 μeV.

The axion has emerged as the most attractive solution to two fundamental questions in modern physics related to the charge-parity invariance in strong interactions and the invisible matter component of our Universe. Over the past decade, there have been many theoretical efforts to constrain the axion mass based on various cosmological assumptions. Interestingly, different approaches from independent groups produce good overlap between 20 and 30 μeV. We performed an experimental search to probe the presence of dark matter axions within this particular mass region. The experiment utilized a multicell cavity haloscope embedded in a 12T magnetic field to seek for microwave signals induced by the axion-photon coupling. The results ruled out the KSVZ axions as dark matter over a mass range between 21.86 and 22.00 μeV at a 90% confidence level. This represents a sensitive experimental search guided by specific theoretical predictions.

Multi-objective shape optimization of TESLA-like cavities: Addressing stochastic Maxwell's eigenproblem constraints

In this paper, we present a robust formulation to address nonlinear equality-constrained and inequality-constrained multi-objective optimization problems for multi-cell accelerating cavities with TESLA-like geometry under consideration of uncertain parameters. We propose to explore a robust approach that employs the polynomial chaos expansion (PCE) to model the uncertainty. For the uncertainty quantification (UQ), the resulting random-dependent eigenvalue problem is solved using the stochastic collocation method (SCM) combined with PCE. As a result, the shape optimization of the studied multi-cell cavity is defined in terms of statistical moments, which serve as the cost functionals in the multi-objective (MO) steepest descent algorithm. Two versions of this method are developed and implemented. Finally, the optimization results and their implications for the operation of a multi-cell cavity are discussed.

A novel method to identify and correct asymmetry of dumbbells in a multi-cell elliptical superconducting cavity.

Raja Ramanna Centre for Advanced Technology (RRCAT) has an ongoing program to develop 650MHz, 5-cell elliptical superconducting RF (SCRF) cavities under the Indian Institutes and Fermilab Collaboration. The elliptical multi-cell SCRF cavity fabrication process involves forming of half-cells and their precise machining and joining by electron beam welding to form end groups and dumbbells, which are then joined to make the final cavity. To ensure that the final welded cavity achieves physical lengths and resonant frequencies within design tolerance and has good field flatness, the measurement and correction of resonant frequency are carried out for dumbbells and end groups. A novel method to identify the frequency of individual half-cells in a dumbbell cavity and a dedicated tuning fixture to correct them had been developed. The paper details the RF characterization and correction procedure employed during fabrication of the first six 650MHz cavities at RRCAT.

Development and Understanding of Nb3Sn films for radiofrequency applications through a sample-host 9-cell cavity

Nb3Sn is a promising advanced material under development for superconducting radiofrequency 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 Nb3Sn 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 observed anomalously large thin grains and uncovered niobium spots observed in the past by other authors (Trenikhina et al 2018 Supercond. Sci. Technol. 32 015004). A phenomenological model to explain how these anomalously large grains could form is proposed. This model is invoked to provide possible explanations for literature results from several groups and to guide key process parameters to achieve uniform vapor-diffusion coatings, when applied to complex structures as the multi-cell cavity under study.

Position-based cavity tuning

Knowledge of the longitudinal beam parameters is important for understanding beam dynamics in any machine. In a linear accelerator (linac), to guarantee a stable motion in the longitudinal plane, the settings for the radio frequency cavities must be established by determining the phase and amplitude witnessed by the beam. In most modern hadron linacs, the requirements on phase and amplitude accuracy are 1\ifmmode^\circ\else\textdegree\fi{} and 1% respectively, which is particularly challenging. Although systematic approaches for finding the phase and amplitude of cavities are available in many facilities with operational linacs, it is also vital to have a system in place when the standard time-of-flight based functionality is down or not yet available (as is often the case in the early stages of commissioning). Such a method should be relatively fast and simple, and should not depend on time-of-flight measurements or a rigorous phase calibration of the beam position monitors. This work presents a new phase- and amplitude-scan method which only requires information from transverse positions measured at beam position monitors downstream of the cavity being scanned. The scanning then relies on transverse trajectory correction, which has been developed extensively by the community. As a by-product, this method can also provide estimates for a cavity's transverse misalignment. The method has been developed and tested for a single-cell cavity; the simulated effectiveness for a multicell cavity is also presented.

Nb3Sn multicell cavity coating system at Jefferson Lab.

Superconducting radio frequency 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 Nb3Sn coating system that converts the inner surface of niobium cavities to an Nb3Sn film. The niobium surface, heated by radiation from the niobium retort, is exposed to Sn and SnCl2 vapor during the heat cycle, which results in about 2 μm Nb3Sn film on the niobium surface. Film composition and structure as well as radio frequency properties with 1-cell R&D cavities and 5-cell practical accelerator cavities are presented.

Digital self-excited vertical test system of superconducting cavity

Vertical test is an important method for characterizing the performance of superconducting cavities. We designed an superconducting cavity vertical test system based on digital self-excited algorithm and the technology of low-level radio frequency, which could improve the vertical test efficiency of superconducting cavities. The RF front-end and clock distribution system of the vertical test system adopts the second up-and-down conversion scheme. To some extent, the working frequency of the digital self-excited loop of the vertical test system can be set flexibly, and the working bandwidth of the test system is increased. The test results of the pass-band frequency of the 1.3 GHz 9-cell superconducting cavity show that the vertical test system has strong frequency resolution (＜800 kHz) to ensure the smooth progress of the multi-cell superconducting cavity pass-band test.

Plasma ignition and detection for in-situ cleaning of 1.3 GHz 9-cell cavities

Superconducting radio frequency cavities performance preservation is crucial, from vertical test to accelerator operation. Field emission 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 (hydrocarbon contaminants) and increasing the work function of Nb. The cleaning for Linac Coherent Light Source-II will follow the same plasma composition adopted at SNS, which allows in situ processing of cavities installed in cryomodules. A novel method for plasma ignition has been developed at the Fermi National Accelerator Laboratory: a plasma glow discharge is ignited using high order modes to overcome limitations imposed by the fundamental power coupler. The plasma can be easily ignited and tuned in each of the cavity cells using low radio frequency (RF) power, from 100 W to as low as 2 W depending on the gas and pressure. A method for RF plasma detection has been developed: the plasma location is identified within the cavity without the need of cameras. The presented method can be applied to other multicell cavity designs, even for accelerators where the coupling for the fundamental modes at room temperature is very weak.

Design of a High-Efficiency S-band Klystron with a Multi-cell Output Cavity

As large-scale scientific facilities develop, the klystron efficiency becomes an important issue to reduce high operating costs. We are developing high-efficiency klystrons for the Pohang Light Source-II (PLS-II) and the Pohang Accelerator Laboratory X-ray Free-Electron Laser (PAL-XFEL) at the Pohang Accelerator Laboratory (PAL). The newly developed klystrons should have the same specifications, including perveance and overall length, to replace existing ones. Under these limited conditions, we designed the klystrons based on a multi-cell output cavity to improve the power-conversion efficiency significatly. We simulated the beam-dynamics using the Field Charge Interaction (FCI) code with a MATLAB script to find the optimum design parameters for high efficiency. We have determined the design parameters, such as cell tuning frequencies, inter-cell distances, couplings between cells and the Qe of the last cell, of the multi-cell output cavity.

Development of an HF Free Electropolishing Process for Nb Srf Cavities

The International Linear Collider (ILC) requires ~16,000 RF superconducting radio frequency niobium cavities that must be a) polished to a microscale roughness, and b) cleaned to be free of impurities that would degrade performance of the ILC. The DOE is seeking new commercially viable, fabrication technologies for SRF cavities, specifically new or improved bulk processing technologies. Current methods, such as buffered chemical polishing (BCP), use high viscosity electrolytes containing hydrofluoric acid, which is not conducive to low-cost, high volume manufacturing and is potentially harmful to workers. Therefore, Faraday is developing an electropolishing process for niobium SRF cavities, based on a new paradigm of non-viscous dilute acid processing, enabled by a pulse reverse or bipolar electric field. The use of this electrolyte combined with the sophisticated electric field changes the electropolishing mechanism by which material is removed and the surface profile reduced. The design of the waveform is such that the niobium is preferentially removed from the peaks on the surface structure, thus smoothing the surface to an Ra of less than 1 nm has been obtained in coupon studies. Furthermore, when used as a Final electropolishing step, the cavity testing performance, according to our colleagues at Fermilab, was at least equivalent to conventional electropolishing and in fact exhibited outstanding performance. The overall objective of the program is to develop, optimize and validate a low-cost, high throughput vertical cavity electropolishing process for eco-friendly Bulk Processing of single- and nine-cell SRF cavities at the alpha/beta scale. To achieve this objective Faraday, along with subcontractor Cornell University are focusing on: 1) Designing and building alpha-scale Bipolar EP Cell based on vertical cavity orientation without rotation for single and multi-cell cavity processing, 2) Optimizing the Bipolar EP waveform parameters and cathode geometry to improve electropolishing performance and test in single-cell cavities, and 3) Appling the optimized Bipolar EP process to multi-cell cavities and determine performance. During this talk we will specifically discuss recent results from our button cell cavity study studies. The button cell cavity, as shown in Figure 2, was designed and built by Cornell for their own cavity finishing process optimization using the conventional sulfuric acid HF approach. This tool enables rapid analysis of the effect of the electrofinishing applied parameter on the finish produced as a function of anode to cathode gap and active area ratio. Our recent studies that demonstrate an effect on the position of the button, orientation, gravity, electrofinishing conditions, and flow rate will specifically be discussed. Figure 2, shows the difference in removal rate and material removal ratio from the beam tube to the iris, as a function of half-cell orientation. Greater uniformity is demonstrated using the P/PR Electropolishing process as compared to the conventional H2SO4/HF process. Acknowledgements: The financial support of DOE Contract No. DE-SC0011342is acknowledged. Figure 1

Development of Infrastructure Facilities for Superconducting RF Cavity Fabrication, Processing and 2 K Characterization at RRCAT

An extensive infrastructure facility is being established at Raja Ramanna Centre for Advanced Technology (RRCAT) for a proposed 1 GeV, high intensity superconducting proton linac for Indian Spallation Neutron Source. The proton linac will comprise of a large number of superconducting Radio Frequency (SCRF) cavities ranging from low beta spoke resonators to medium and high beta multi-cell elliptical cavities at different RF frequencies. Infrastructure facilities for SCRF cavity fabrication, processing and performance characterization at 2 K are setup to take-up manufacturing of large number of cavities required for future projects of Department of Atomic Energy (DAE). RRCAT is also participating in a DAE’s approved mega project on “Physics and Advanced technology for High intensity Proton Accelerators” under Indian Institutions-Fermilab Collaboration (IIFC). In the R&D phase of IIFC program, a number of high beta, fully dressed multi-cell elliptical SCRF cavities will be developed in collaboration with Fermilab. A dedicated facility for SCRF cavity fabrication, tuning and processing is set up. SCRF cavities developed will be characterized at 2K using a vertical test stand facility, which is already commissioned. A Horizontal Test Stand facility has also been designed and under development for testing a dressed multi-cell SCRF cavity at 2K. The paper presents the infrastructure facilities setup at RRCAT for SCRF cavity fabrication, processing and testing at 2K.

Control of field flatness based on FPGA for multi-cell cavity

Control of field flatness based on FPGA for multi-cell cavity

Stability analysis of a C-band 500-kW klystron with a multi-cell output cavity

A progogype 5-GHz 500-kW CW klystron (model E3762 provided by Toshiba Electron Tubes & Devices Co. Ltd.) has been operating as the RF source for the lower hybrid current drive (LHCD) system in the KSTAR tokamak. In order to investigate how the efficiency of the 5-GHz 500-kW CW klystron prototype could be enhanced, the cavity design study is being carried out with simulation code based on the main klystron operation parameters. This is being done by simulating klystron performances for various cavity parameters including the number of cavities, inter-cavity distance, and cavity tuning frequencies. The simulation has been done with the FCI (field charge interaction) code aided by a matlab script for scanning input parameters. Initial set of scan parameters was obtained by benchmarking the E3762 klystron. It was possible to obtain optimized design parameters with better efficiency for a cavity system adopting a multi-cell output cavity. However, the multi-cell output cavity is prone to produce a self-oscillation due to the prolonged (several half RF periods) beam-field interaction along its multiple gaps. We have checked the feasibility of the optimization by evaluating the stability of output cavity system. The stability is given by the ratio of a beam-loading conductance to the circuit conductance.

Cryogenic testing of the 2.1 GHz five-cell superconducting RF cavity with a photonic band gap coupler cell

We present results from cryogenic tests of the multi-cell superconducting radio frequency (SRF) cavity with a photonic band gap (PBG) coupler cell. Achieving high average beam currents is particularly desirable for future light sources and particle colliders based on SRF energy-recovery-linacs (ERLs). Beam current in ERLs is limited by the beam break-up instability, caused by parasitic higher order modes (HOMs) interacting with the beam in accelerating cavities. A PBG cell incorporated in an accelerating cavity can reduce the negative effect of HOMs by providing a frequency selective damping mechanism, thus allowing significantly higher beam currents. The multi-cell cavity was designed and fabricated of niobium. Two cryogenic (vertical) tests were conducted. The high unloaded Q-factor was demonstrated at a temperature of 4.2 K at accelerating gradients up to 3 MV/m. The measured value of the unloaded Q-factor was 1.55 × 108, in agreement with prediction.

The simulation and optimization of a room-temperature Cross-Bar H-Type Drift-Tube Linac

The room-temperature Cross-Bar H-Type Drift-Tube Linac (CH-DTL) is one of the candidate accelerating structures, working at 325 MHz in CW mode, for the Chinese ADS project. In this study the multi-cell cavity geometry has been optimized using the method of “parameter sweeping with constraint variable” which is found superior to parameter sweeping with a single variable. To facilitate manufacture, a spherical drift tube shape is adopted. It can be concluded that a cavity having cylindrical end cups has higher shunt impedance than one with cone-shaped end cups.

Status of Vertical Electroplishing at Cornell University

Cornell's SRF group has continued R&D on vertical electropolishing (VEP) of Niobium SRF Cavity with electrolyte of HF(40w%):H2SO4(96w%) in Volume proportions 1: 9. Compared with conventional horizontal EP system for SRF cavities, VEP could eliminate rotary acid seals, sliding electrical contact, cavity vertical/horizontal position control fixtures. And also, the acid plumbing/containment could be simplified. So, the cost reduction of capital equipment is highly expectable. Many of single- and multi-cell cavities have been processed with VEP and performed RF test at Cornell. VEP’ed multi-cell cavity performance was limited by too much Qo degradation above 25MV/m before. But in the analysis of removal vs. field gradient against previous VEP’ed cavity tests, we have found a tendency that small removal as final etches could provide high gradient without Qo degradation. The ILC 9-cell cavity “ACCEL9” with TESLA shape processed by VEP at Cornell achieved Eacc of about 40MV/m by minimizing final removal. This is the first 9-cell cavity that achieved ILC base-line specification (Qo=1.0e10 at 31.5MV/m, Qo=8.0e9 at 35MV/m) by VEP in the world. We have also fabricated demountable half-cell coupon cavity. This cavity consists of Nb half-cell and dummy cavity assemblies. This coupon cavity provides us more information about VEP process details, such as the difference of I-V curve between cell orientations. Four demountable Nb coupons with diameter of 11mm are attached on half-cell and processed. These coupons are very strong tool for further surface analysis to optimize process parameters. We will present our current status of VEP R&D, cavity test results, and fundamental study results with Nb coupons.

Resonant-frequency discharge in a multi-cell radio frequency cavity

We are reporting experimental results on a microwave discharge operating at resonant frequency in a multi-cell radio frequency (RF) accelerator cavity. Although the discharge operated at room temperature, the setup was constructed so that it could be used for plasma generation and processing in fully assembled active superconducting radio-frequency cryo-module. This discharge offers a mechanism for removal of a variety of contaminants, organic or oxide layers, and residual particulates from the interior surface of RF cavities through the interaction of plasma-generated radicals with the cavity walls. We describe resonant RF breakdown conditions and address the issues related to resonant detuning due to sustained multi-cell cavity plasma. We have determined breakdown conditions in the cavity, which was acting as a plasma vessel with distorted cylindrical geometry. We discuss the spectroscopic data taken during plasma removal of contaminants and use them to evaluate plasma parameters, characterize the process, and estimate the volatile contaminant product removal.

Coupling power into accelerating mode of a three-dimensional silicon woodpile photonic band-gap waveguide

Silicon woodpile photonic crystals provide a base structure with which to build a three-dimensional dielectric waveguide system for high-gradient laser-driven acceleration. To realize an on-chip woodpile laser accelerator, a key component is the power coupler to deliver laser power to the fundamental accelerating mode. The woodpile waveguide is periodically loaded in the longitudinal direction; therefore simple cross-sectional mode profile matching is not sufficient to launch the accelerating mode appropriately and will result in significant scattering loss. Several traveling-wave coupler design schemes developed for multicell radio frequency cavity accelerators can be adapted to the woodpile accelerator coupler design. This paper presents design procedures and results using these methods. We present simulations indicating near 100% power transmission between the transverse electric mode of a silicon-guide side coupler and the transverse--magnetic-like accelerating mode of a woodpile waveguide. The coupler launches a full traveling-wave propagation of the accelerating mode, which maintains its propagation quality over long waveguide structures, and provides better tolerance on the structure fabrication uncertainty and material breakdown than standing-wave coupling.

Integrated system modeling analysis of a cryogenic multi-cell deflecting-mode cavity resonator

A deflecting mode cavity is the integral element for six-dimensional phase-space beam control in bunch compressors and emittance transformers at high energy beam test facilities. RF performance of a high-Q device is, however, highly sensitive to operational conditions, in particular in a cryo-cooling environment. Using analytic calculations and RF simulations, we examined cavity parameters and deflecting characteristics of TM110,π mode of a 5 cell resonator in a liquid nitrogen cryostat, which has long been used at the Fermilab A0 Photoinjector (A0PI). The sensitivity analysis indicated that the cavity could lose 30%–40% of deflecting force due to defective input power coupling accompanying non-uniform field distribution across the cells with 40 ∼ 50 MeV electron beam and 70–80 kW klystron power. Vacuum-cryomodules of the 5 cell cavity are planned to be installed at the Fermilab Advanced Superconducting Test Accelerator facility. Comprehensive modeling analysis integrated with multi-physics simulation tools showed that RF loading of 1 ms can cause a ∼5 K maximum temperature increase, corresponding to a ∼4.3 μm/ms deformation and a 1.32 MHz/K maximum frequency shift. The integrated system modeling analysis will improve design process of a high-Q cavity with more accurate prediction of cryogenic RF performance under a high power pulse operation.

The main linac cavity for Cornell's energy recovery linac: Cavity design through horizontal cryomodule prototype test

Future particle accelerators will require continuous wave operation of SRF cavities capable of supporting high beam currents. An example of this is the Energy Recovery Linac (ERL) at Cornell University, a next generation light source designed to run high currents (100mA) with a high bunch repetition rate (1.3GHz). Obtaining the beam emittance necessary to meet design specification requires strong damping of higher-order modes that can lead to beam breakup. We discuss the optimization and verification of the accelerating cavity. Next we show that an ERL constructed from the optimized cavity geometry – including realistic shape errors – can support beam currents in excess of 300mA while still maintaining beam stability. A niobium prototype 7-cell cavity was fabricated and tested in a horizontal cryomodule. We show that the prototype cavity exceeds quality factor and gradient specifications of 2×1010 at 16.2MV/m at 1.8K by 50%, reaching Q=(3.0±0.3)×1010. The prototype cavity also satisfies all design constraints and has a higher order mode spectrum consistent with the optimized shape geometry. At 1.6K, the cavity set a record for quality factor of a multicell cavity installed in a horizontal cryomodule reaching Q=(6.1±0.6)×1010.