Damping Of Higher Order Modes Research Articles

A low-frequency normal conducting cavity with higher-order mode damping for fourth-generation synchrotron radiation sources.

The utilization of a low-frequency (<200MHz) RF system in storage facilitates the attainment of ultra-low emittances in synchrotron light sources through on-axis injection. This paper focuses on the development of a low-frequency normal conducting (NC) cavity with higher-order mode (HOM) damping for fourth-generation synchrotron light sources. We propose a novel approach to achieve efficient HOM damping in a NC cavity by optimizing the lowest frequency HOM and implementing a beam-line absorber. Notably, unlike conventional NC cavities, the presence of a large beam tube for the beam-line absorber does not compromise the accelerating performance in a coaxial resonant cavity, enabling effective HOM damping while maintaining a high shunt impedance. Through simulations, the prototype design of a 166.6MHz HOM-damped cavity demonstrates a fundamental mode impedance of ∼8 MΩ, with longitudinal and transverse HOM impedances below 2.0 and 50 kΩ/m, respectively.

Optimal design and HOM damping of a 500 MHz 5-cell copper cavity for SAPS

In order to meet the requirements of the Southern Advanced Photon Source (SAPS), a slot-coupled π-mode 5-cell copper cavity with higher order mode (HOM) damping was proposed and developed. In this study, we present the optimal design of a 5-cell copper cavity and an investigation of HOM impedances. A prototype of 5-cell copper cavity based on this design was manufactured. The RF characteristics of 5-cell copper cavity have been tested. The results show that a flat accelerating electric field is obtained, with a coupling coefficient of 1.7% and a shunt impedance more than 19.5 MΩ/m. To damp these HOMs with high impedance, two inductive HOM couplers and two capacitive HOM couplers were designed. Finally, the impedances of all the HOMs are below the threshold of coupled-bunch instabilities (CBI) for SAPS.

Multipacting suppression of the HOM-damped 166.6-MHz β=1 quarter-wave superconducting cavities at HEPS

A higher-order-mode (HOM) damped 166.6-MHz β=1 quarter-wave superconducting cavity has been designed for the High Energy Photon Source project. Dangerous multipacting barriers have to be removed by geometry optimizations in superconducting cavity design while retaining favorable radiofrequency and mechanical properties as well as sufficient HOM damping. Multipacting analysis has been conducted on the cavity by using coupled simulation codes comprising electromagnetic calculations and particle tracking. A circular transition structure between the cavity and the beam pipe was proposed to remove the high-field multipacting band by eliminating the local minimum of the electric field thus preventing the field-induced electrons from being trapped into the potential well. The impacts of the geometric change on the cavities' electromagnetic and mechanical properties as well as HOM damping were examined. The deteriorated impedance of the first monopole HOM can be recovered by a slightly enlarged beam pipe, while the total loss factor has little change. The compact footprint of the cavity module was maintained as required for a limited straight section with a fixed length. Mechanical properties of the cavity were also retained with an improved tuning sensitivity and frequency stability. These constitute a first HOM-damped 166.6 MHz β=1 quarter-wave superconducting cavity with multipacting suppressed.

Design and Optimization of the High Order Modes Damper for a 1.5 GHz Superconducting Harmonic Cavity

The installation of a superconducting radio-frequency (SRF) cavity can improve the beam lifetime via increasing the electron bunch length. A prototype 1.5-GHz SRF passive harmonic cavity working at 4.5 K was designed for The Taiwan Photon Source (TPS). As the beam might become unstable because of the coupled-bunch instability resulting from Higher Order Modes (HOMs) of the SRF harmonic cavity, HOMs damping of this cavity is an important issue. Ferrite absorbers placed in the beam tubes are designed to damp the HOMs. A theoretical calculation for the instability threshold for the HOMs is given. A tracking method is used to simulate the instabilities for operation with this HOMs-damped SRF harmonic cavity. The design and optimization of the HOMs damper, as well as the results of instability studies for this SRF harmonics cavity, are presented here.

Development and vertical tests of 650 MHz 2-cell superconducting cavities with higher order mode couplers

650 MHz 2-cell superconducting cavities are proposed for the main ring of the Circular Electron–Positron Collider (CEPC). The cavity is designed with two detachable double-notch higher order mode (HOM) couplers to suppress the fundamental mode and extracted the HOM power. The present paper describes the design, fabrication, surface treatment and vertical tests of the cavities and HOM couplers. The performances of the cavity at 2 K is not affected after the installation of the HOM coupler. The maximum intrinsic quality factor Q0 of the cavity dressed with the HOM coupler reached 3.1×1010 at operation accelerating gradient. The vertical tests results showed that the fundamental mode external quality factor Qe of all HOM couplers is an order of magnitude larger than Q0 of the cavity. The HOM damping results for the 650 MHz 2-cell cavity were also measured at cryogenic temperature and compared with the simulated and measured results at room temperature. Two 650 MHz 2-cell cavities each with two HOM couplers will be assembled in the CEPC 650 MHz test cryomodule after helium vessel dressing.

Realization and high power test of damped C -band accelerating structures

The linac of the European project Extreme Light Infrastructure-Nuclear Physics (ELI-NP) foresees the use of 12 traveling wave $C$-band accelerating structures. The cavities are 1.8 m long, quasiconstant gradient, and have a field phase advance per cell of $2\ensuremath{\pi}/3$. They operate at 100 Hz repetition rate, and, because of the multibunch operation, they have been designed with a dipole higher-order mode (HOM) damping system to avoid beam breakup. The structures have symmetric input and output couplers and integrate, in each cell, a damping system based on silicon carbide (SiC) rf absorbers coupled to each cell through waveguides. An optimization of the electromagnetic and mechanical design has been done to simplify the fabrication and to reduce the costs. The cavities have been fabricated, and the first full-scale prototype has been also successfully tested at the nominal gradient of $33\text{ }\text{ }\mathrm{MV}\mathrm{/m}$, repetition rate of 100 Hz, and pulse length of 820 ns. It represents, to our knowledge, the first full-scale linac structure with HOM damping waveguides and SiC absorbers tested at this high gradient. In the paper, we illustrate the realization process of such a complicated device together with the low and high power test results.

Higher order mode coupler for the circular electron positron collider

A conceptual design report (CDR) for the Circular Electron Positron Collider (CEPC) was published in September 2018. CEPC will use a 650 MHz radiofrequency (RF) system with 240 two-cell cavities. The collider will be a double ring with shared cavities for Higgs operation and separate cavities for W and Z operations. The higher order modes (HOMs) excited by the intense beam bunches must be damped to avoid additional cryogenic loss and multi-bunch instabilities. This study examined the impedance budget and HOM damping requirements for the CEPC ring to keep the beam stable, and a coaxial HOM coupler was considered for extracting the beam-induced HOM power. A double-notch coupler was chosen because of its wide bandwidth for the fundamental mode. Each cavity has two detachable coaxial HOM couplers mounted on the cavity beam pipe with a HOM power handling capacity of 1 kW. This paper presents the design process of the HOM coupler: the RF design, thermal analysis, and mechanical aspects. To validate the design, a prototype of the HOM coupler and a coaxial line test bench were fabricated and tested, and the test results agreed well with the simulation results. The HOM damping results for a real 650 MHz two-cell cavity were also measured and compared with the damping requirements.

HOM damping requirements and measurements for CEPC 650-MHz 2-cell cavity

The circular electron–positron collider (CEPC) will use a 650-MHz RF system with 240 two-cell cavities for the collider. The collider is a double ring with shared cavities for Higgs operation and separate cavities for W and Z operations. The higher-order modes (HOM) excited by the intense beam bunches must be damped to avoid additional cryogenic loss and multi-bunch instabilities. To get the real damping results, two prototypes of HOM coupler have been fabricated and installed on the 650-MHz two-cell cavity. The HOMs have been verified by bead pulling method. A test bench with two 2-cell cavities is used to measure the real damping results and study HOM propagating properties for a cavity string. In this paper, the impedance budget, HOM damping and HOM power requirements for the CEPC collider ring are given. The damping results measured for the fundamental mode and HOMs seem good compared with the simulated results. The absorbing efficiency of the absorber and the extraction power efficiency of HOM couplers were also achieved.

Preliminary Research of HOM for 100MHz Superconducting Cavity in the Pre-research Project of HALS

A 100MHz QWR superconducting cavity is researched in the pre-research project of Hefei Advanced Light Source (HALS). Higher order modes (HOM) damping is a big challenge for synchrotron radiation light source. In this paper, we first apply the novel choke mode structure to the 100MHz QWR (quarter wave resonator) cavity in order to damp the HOM. We identify the main harmful higher order modes. The HOMs in the QWR cavity are suppressed by optimizing the choke dimensions. The broadband HOM impedance spectrum of the cavity was also evaluated by calculating the beam induced wake potential in time domain. The results show that choke mode structure has a good HOM damping effect on the QWR cavity.

Design studies of a compact superconducting rf crab cavity for future colliders using Nb/Cu technology

The design of a compact crab cavity using Nb/Cu technology is presented. The cavity shape is based on a ridged waveguide resonator with wide-open apertures to provide access to the inner surface of the cavity and facilitate coating. It also provides natural damping for higher-order modes (HOMs) and comparatively low longitudinal and transverse impedances. A first prototype is being fabricated, originally within the HL-LHC framework and now for the Future Circular Collider study. The optimized shape is characterized and compared with respect to peak surface field balance, multipolar momentum components, and longitudinal and transverse impedances. Further investigations include the identification of multipacting barriers, the frequency sensitivity to pressure fluctuations in the helium bath, the radio frequency (rf) design of a fundamental mode coupler, and considerations of HOM damping. Along these topics, several methods have been established serving a wider range of applications, in particular, for the rf loss evaluation, the postprocessing of multipacting simulations, and a detailed error analysis of the pressure sensitivity simulations.

Higher order mode damper for low energy RHIC electron cooler superconducting radio frequency booster cavity

To improve RHIC luminosity for heavy ion beam energies below 10 GeV/nucleon, the Low Energy RHIC electron Cooler (LEReC) is currently under commissioning at BNL. The Linac of LEReC is designed to deliver a 1.6 MeV to 2.6 MeV electron beam, with rms dp/p less than 5e-4. A 704 MHz superconducting radio frequency (SRF) booster cavity in this Linac provides up to 2.2 MeV accelerating voltage. With such a low energy and very demanding energy spread requirement, control of Higher Order Modes (HOMs) in the cavities becomes critical and needs to be carefully evaluated to ensure minimum impact on the beam. In this paper, we report the multiphysics design of the HOM damper for this cavity to meet the energy spread requirement, as well as experimental results of the cavity with and without the HOM damper.

A higher-order mode coupler design for HEPS 166.6 MHz superconducting accelerating cavities

The 166.6 MHz superconducting radio frequency cavities have been proposed for High Energy Photon Source (HEPS) storage ring. HEPS is a 6 GeV, 200 mA diffraction-limited synchrotron light source to be built in Beijing. Higher-order modes (HOMs) excited in 166.6 MHz SRF cavities have to be sufficiently damped in order to prevent coupled-bunch instabilities and to limit parasitic mode losses. A hybrid HOM damping scheme has been proposed. An enlarged beam pipe allows HOMs above beam pipe cutoff frequencies to propagate and subsequently be absorbed by dampers installed on the downstream beam pipe, while a petal-shaped coaxial structure extracts HOMs below beam pipe cutoff frequencies and subsequently damped by the specially designed coaxial filter. HOM damping in cavities has been demonstrated by 3D simulations. The impedance of HOMs has been successfully reduced to be lower than the coupled-bunch instability threshold. In addition, no hard barrier multipaction has been observed and acceptable heat loss is obtained. In this paper, the hybrid HOM damping scheme has been demonstrated. Based on these studies, a prototype petal–loop structure and its associated filter are being fabricated.

Conceptual design of a high real-estate gradient cavity for a SRF ERL

The term “real-estate gradient” is used to describe the energy gain provided by an accelerating structure per actual length it takes in the accelerator. given that the length of the tunnel available for the accelerator is constrained, the real-estate gradient is an important measure of the efficiency of a given accelerator structure. When designing an accelerating cavity to be efficient in this sense, the unwanted Higher Order Mode (HOM) fields should be reduced by suitable HOM dampers. This is a particularly important consideration for high current operation. The additional RF components might take longitude space and reduce the total accelerating efficiency. We describe a new high efficiency 5-cell cavity with the dampers included. The total length of the cavity is reduced by 13% as compared to a more conventional design without compromising the cavity fundamental-mode performance. In addition, the HOM impedance is reduced for a higher Beam-Break-Up (BBU) threshold of operating current. In this paper, we consider an example, a possible application at the eRHIC Energy Recovery Linac (ERL).

Design of high gradient, high repetition rate dampedC-band rf structures

The gamma beam system of the European Extreme Light Infrastructure--Nuclear Physics project foresees the use of a multibunch train colliding with a high intensity recirculated laser pulse. The linac energy booster is composed of 12 traveling wave $C$-band structures, 1.8 m long with a field phase advance per cell of $2\ensuremath{\pi}/3$ and a repetition rate of 100 Hz. Because of the multibunch operation, the structures have been designed with a dipole higher order mode (HOM) damping system to avoid beam breakup (BBU). They are quasiconstant gradient structures with symmetric input couplers and a very effective damping of the HOMs in each cell based on silicon carbide (SiC) rf absorbers coupled to each cell through waveguides. An optimization of the electromagnetic and mechanical design has been done to simplify the fabrication and to reduce the cost of the structures. In the paper, after a review of the beam dynamics issues related to the BBU effects, we discuss the electromagnetic and thermomechanic design criteria of the structures. We also illustrate the criteria to compensate the beam loading and the rf measurements that show the effectiveness of the HOM damping.

An energy method for flutter analysis of wing using one-way fluid structure coupling

In this paper, an energy method for flutter analysis of wing using one-way fluid structure coupling was developed. To consider the effect of wing vibration, Reynolds-averaged Navier–Stokes equations based on the arbitrary Lagrangian Eulerian coordinates were employed to model the flow. The flow mesh was updated using a fast dynamic mesh technology proposed by our research group. The pressure was calculated by solving the Reynolds-averaged Navier–Stokes equations through the SIMPLE algorithm with the updated flow mesh. The aerodynamic force for the wing was computed using the pressure on the wing surface. Then the aerodynamic damping of the wing vibration was computed. Finally, the flutter stability for the wing was decided according to whether the aerodynamic damping was positive or not. Considering the first four modes, the aerodynamic damping for wing 445.6 was calculated using the present method. The results show that the aerodynamic damping of the first mode is lower than the aerodynamic damping of higher order modes. The aerodynamic damping increases with the increase of the mode order. The flutter boundary for wing 445.6 was computed using the aerodynamic damping of the first mode in this paper. The calculated flutter boundary is consistent well with the experimental data.

Higher order mode damping in a five-cell superconducting rf cavity with a photonic band gap coupler cell

We present a study of higher order mode (HOM) damping in the first multicell superconducting radio-frequency (SRF) cavity with a photonic band gap (PBG) coupler cell. Achieving higher 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 breakup instability, caused by parasitic 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 five-cell cavity with a PBG cell was designed and optimized for HOM damping. Monopole and dipole HOMs were simulated. The SRF cavity was fabricated and tuned. External quality factors for some HOMs were measured in a cold test. The measurements agreed well with the simulations.

Analysis of Higher Order Modes Damping Techniques in 9 Cell Cavity with Modified Drift Tubes

Electrodynamic characteristics (EDC) of higher order modes (HOM) were calculated for a superconducting 9-cell accelerating cavity of eLinac accelerator with operating frequency of 1300MHz. Several HOM damping techniques including damping with grooved, fluted and ridged beam pipes were analyzed and compared. The influence of the parameters of the drift tube on the HOM damping and on the parameters of the fundamental wave was analyzed.

HOMs of the SRF Electron Gun Cavity in the BNL ERL

The Brookhaven Energy Recovery Linac (ERL) is operated as an R&D test bed for high-current, low emittance electron beams. It comprises a superconducting five-cell cavity and a half-cell superconducting RF photo-injector electron gun. The ERL is undergoing commissioning with focus on the performance of the electron gun, not the least on the cavity Higher Order Modes (HOM). Among the various alternative solutions, a beam tube damper based on a layer of ferrite tiles was adopted for the five-cell accelerator cavity. For the gun, a ceramic-ferrite damper consisting of a lossless ceramic cylinder surrounded by damping ferrite tiles has been investigated. This design is innovative in its damper approach and combines a variety of goals including broadband HOM damping and protection of the superconducting cavity vacuum from potential damage by the separately cooled absorber. In this paper the empirical performance of an installed ceramic-ferrite damper is described by the Q reduction of a few selected gun cavity resonances. The theoretical coupling impedance presented to a traversing beam is numerically analyzed in terms of radial waveguide modes in the damper section. Strong damping of the gun cavity HOMs by the fundamental power coupler (FPC) is found and discussed. Finally, the measured Q-values of the operational gun cavity without the ceramic-ferrite damper at superconducting temperatures are presented

Origin and reduction of wakefields in photonic crystal accelerator cavities

Photonic crystal (PhC) defect cavities that support an accelerating mode tend to trap unwanted higher-order modes (HOMs) corresponding to zero-group-velocity PhC lattice modes at frequencies near the top of bandgaps. The effect is explained quite generally by photonic band and perturbation theoretical arguments. Transverse wakefields resulting from this effect are observed (via simulation) in a 12 GHz hybrid dielectric PhC accelerating cavity based on a triangular lattice of sapphire rods. These wakefields are, on average, an order of magnitude higher than those in the 12 GHz waveguide-damped Compact Linear Collider copper cavities. The avoidance of translational symmetry (and, thus, the bandgap concept) can dramatically improve HOM damping in PhC-based structures.

Development of the superconducting rf 2-cell cavity for cERL injector at KEK

An injector cryomodule for the compact energy recovery linac (cERL) is under development at KEK. This injector cryomodule has 3 L-band 2-cell superconducting rf cavities. The cERL is required to accelerate a 10-mA CW electron beam to 5MeV. The required accelerating gradient per cavity is 7.5–12.5MV/m at ∼30kW input power to the cavity and the beam. The operational frequency is 1300MHz at 2K and the mode of operation is CW. In this application, the critical hardware components are not the cavities, but the rf input couplers and higher-order-mode (HOM) dampers. Initially, a TESLA-style coaxial HOM coupler was chosen for HOM damping of the injector cavities. However, this HOM coupler had a heating problem at low gradients (a few MV/m) in CW operation. The components heated in the accelerating mode were the HOM body and the feedthrough that extracts HOM power from the cavity. To control the heating problem, a new HOM coupler was designed based on a TESLA-style coaxial HOM coupler, and the feedthrough was also modified based on a Kyocera N-R type connector to have better thermal conductivity. A prototype 2-cell cavity and 3 other 2-cell cavities with 5 new HOM couplers for actual operation were fabricated through May 2011. Vertical tests of these cavities were carried out after standard surface preparation at the KEK Superconducting Accelerator Test Facility (KEK-STF) through March 2012. The accelerating gradient achieved exceeded 50MV/m without quenching during the vertical test using the prototype 2-cell cavity and feedthroughs. The magnetic field at the cell equator was 2127Oe. Three 2-cell cavities passing the criteria of the High Pressure Gas Safety Institute of Japan exceeded 25MV/m without field emissions. The cavities with the best performance were prepared in March 2012 for the cERL injector. The designs of the HOM couplers and feedthroughs and the results of the vertical tests to evaluate their performance are reported here.