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In the framework of the Spare Radio Frequency Quadrupole Project [1] the layout of an enhanced performance RFQ has been explored. In this note we report the results of the study, including the three options retained and we compare the performance of the different RFQ in relation to the LINAC4 overall performance.

The number of cells in a π-mode standing wave (SW) accelerating structure for the Compact linear Collider (CLIC) project is limited by mode overlap with nearby modes. The distributed coupling scheme avoids mode overlap by treating each cell as independent. Designs of cells suitable for distributed coupling with strong wakefield damping have not previously been studied. In this paper we develop a SW cell to be used in a distributed coupling structure that can satisfy the CLIC transverse wakepotential limit. From the middle cell of the CLIC-G travelling wave (TW) structure, a SW cell is designed. The cell is adapted to be suitable for distributed coupling. Its wakepotentials in an ideal case of open boundaries are reduced to satisfy the wakepotential threshold. An electric boundary is added to the model to simulate total reflection at the distribution network. A horizontal coupler cell that connects to the distribution network such that the reflected wakefields remain similar to the open boundary case is simulated. A triplet module which takes advantage of cell-to-cell coupling to reduce reflected wakepotential is presented.

A high-pulse-current injector followed by a short high-gradient X-band linac is considered as a driver for a compact Inverse Compton Scattering source. We show that using a high-power ultrashort pulse laser operating in burst mode and a Fabry-Pérot enhancement cavity, X-rays with flux values over 10¹³ ph/s and photon energies up to MeV are achievable. The resulting high-intensity and high-energy X-rays allow for various applications, including cancer therapy, tomography, and nuclear waste management. A preliminary conceptual design of such a compact ICS source is presented, together with simulations of the expected performance., Proceedings of the 31st International Linear Accelerator Conference, LINAC2022, Liverpool, UK

Multipactor discharges in dielectric accelerating structures are a major limitation on the performance of this otherwise very promising technology for future high energy physics machines and other applications. Multipactor occurs when the Secondary Emission Yield (SEY) of the dielectric material used in accelerating structures is significantly higher than 1. In this work, SEY coefficient versus energy of primary electrons is measured for different dielectric materials used for accelerating structure applications: MgTiO3 (D16) and MgTiO-based conductive ceramic. Furthermore, effect of SEY reduction by means of amorphous Carbon (a-C) and Diamond-Like Carbon (DLC) coatings is investigated experimentally. The results are presented and discussed.

The international FCC study group published in 2019 a Conceptual Design Report for an electron-positron collider with a centre-of-mass energy from 90 to 365 GeV with a beam currents of up to 1.4 A per beam. The high beam current of this collider create challenging requirements on the injection chain and all aspects of the linac need to be carefully reconsidered and revisited, including the injection time structure. The entire beam dynamics studies for the full linac, damping ring and transfer lines are major activities of the injector complex design. A key point is that any increase of positron production and capture efficiency reduces the cost and complexity of the driver linac, the heat and radiation load of the converter system, and increases the operational margin. In this paper we will give an overview of the status of the injector complex design and introduce the new layout that has been proposed by the study group working in the context of the CHART collaboration. In this framework, furthermore, we also present the preliminary studies of the FCC-ee positron source highlighting the main requirements and constraints., Proceedings of the 13th International Particle Accelerator Conference, IPAC2022, Bangkok, Thailand

During operation the LINAC4 RFQ (Radio-Frequency-Quadrupole) is exposed to high electric fields which can lead to vacuum breakdown. It is also subject to beam loss that can cause surface modification, including blistering, which can result in reduced electric field handling and an increased breakdown rate. An experimental study has been made to identify materials with high electric field capability and robustness to low-energy irradiation. In this paper we briefly discuss the selection criteria and we analyze these materials investigating their metallurgical properties using advanced microscopic techniques such as Scanning Electron Microscope, Electron Back Scattered Diffraction, Energy-dispersive X-ray Spectroscopy and conventional optical microscopy. These allow to observe and characterize the different materials on aμand a nano-scale, allowing us to compare results before and after irradiation and breakdown testing., Proceedings of the 31st International Linear Accelerator Conference, LINAC2022, Liverpool, UK

Beam loss occurs in Radio Frequency Quadrupoles (RFQ), and has been observed in the H⁻ linear accelerator Linac4 (L4) at CERN. To determine if beam loss can induce breakdowns, and to compare the robustness of different materials, tests have been done using pulsed high-voltage DC systems. Electrical breakdown phenomena and conditioning processes have been studied using these systems. Cathodes of different materials were irradiated with 1.2x10¹⁹ H⁻ p/cm², the estimated beam loss of the L4 RFQ over 10 days. The irradiated electrodes were installed in a system to observe if the irradiated area coincided with the breakdown locations, with pulsing parameters similar to the RFQ. Tests of irradiated and non-irradiated electrodes of the same material were done for comparison. The main difference observed was an increase in the number of breakdowns during the initial conditioning that returned to non-irradiated sample values with further running. Visual observations after irradiation show the beam centre and a halo the same diameter of the beam pipe. Breakdown clusters occur in the centre and halo regions, suggesting irradiation is not the only factor determining the breakdown probability., Proceedings of the 31st International Linear Accelerator Conference, LINAC2022, Liverpool, UK

The linacs of the FCCee (Future Circular Electron-Positron Collider) injector complex will both provide the drive beam for positron production and accelerate nominal electron and positron beams up to 6 GeV. Several linacs comprise different traveling-wave (TW) accelerating structures fulfilling the beam dynamics and rf constraints. Notably, high-phase advance large-aperture structures accelerate the positron beam at low energies. All TW structures are rotationally symmetric for easier production. Long-range wakes are damped by HOM detuning. Operating mode and HOM parameters were calculated based on lookup tables and analytic formulas, allowing for rapidly scanning large parameter spaces. In this paper, we present both methodology and realization of the rf design of the TW structures including their pulse compressors., Proceedings of the 31st International Linear Accelerator Conference, LINAC2022, Liverpool, UK

The X-Band test facilities at CERN have been running for some years now qualifying CLIC structure prototypes but also developing and testing high power general-purpose X-Band components, used in a wide range of applications. Driven by operational needs, several components have been redesigned and tested aiming to optimize the reliability and the compactness of the full system and therefore enhancing the accessibility of this technology inside and outside CERN. To this extent, a new high-power RF-window has been designed and tested aiming to avoid unnecessary venting of high-power sections already conditioned, easing the interventions, and protecting the klystrons. A new spiral load prototype has also been designed, built, and tested, optimizing the compactness, and improving the fabrication process. In these pages, the design and manufacturing for each component will be shortly described, along with the last results on the high-power testing., Proceedings of the 31st International Linear Accelerator Conference, LINAC2022, Liverpool, UK

Over the last few years CERN has successfully designed, built and commissioned the smallest RFQ to date, the one meter long PIXE-RFQ operating at 750 MHz. Its compactness offers a unique opportunity for education and public presentation of the accelerator community: A duplicate machine called ELISA-RFQ (Experimental Linac for Surface Analysis) will be exhibited in the Science Gateway, CERN’s upcoming scientific education and outreach center. It will allow the public to approach within a few centimeters a live proton beam injected into air, which is visible to the naked eye. The construction of the ELISA-RFQ has been completed in 2022. In this paper, we present the results of low-power RF measurements as well as field and frequency tuning., Proceedings of the 31st International Linear Accelerator Conference, LINAC2022, Liverpool, UK

The 352 MHz Linac4-RFQ is the first rf accelerating structure of the CERN accelerator complex, accelerating an H⁻ beam to 3 MeV. After successful commissioning in 2013, superficial vane damage has been observed in 2020. In view that the RFQ is a single point of failure, in parallel to the production of a near identical spare (RFQ2), design studies on a longer-term upgrade have been launched: Linac4-RFQ3. Main goals are to achieve a design with higher beam acceptance, reduced beam losses, and reduced RF breakdown rate. Two versions of RFQ are under study: a conventional RFQ built by brazing copper, as well as an RFQ with titanium vane tips (brazed on copper). High-gradient experiments suggest that titanium vane tips support higher surface fields compared to copper, up to 40 MV/m, and are more resistant against beam irradiation. In this paper, we present beam dynamics and rfdesign of both variants of RFQ3., Proceedings of the 31st International Linear Accelerator Conference, LINAC2022, Liverpool, UK

The Compact Linear Collider (CLIC) Damping Rings (DRs) need to generate ultra-low emittance bunches to achieve high luminosity in CLIC. This requires many wiggler magnets with big energy loss which is compensated by the Radio Frequency (RF) system. The resulting strong beam loading transients affect the bunch position. At the same time, there is a very tight specification for the bunch position along the bunch train, to maintain the CLIC luminosity at the nominal level. These conflicting specifications lead to a challenging design for the RF system. In this work, a novel cavity with an ultra-low R/Q parameter of below 1 Ω is proposed to minimize the transient beam loading effects below acceptable level. The design strategy, the achieved parameters and resulting overall performance of the RF system including both the high power and Low-Level RF (LLRF) aspects are presented and discussed.

The results of high-gradient tests of a tapered X-band traveling-wave accelerator structure powered in reversed direction are presented. Powering the tapered structure from the small aperture, normally output, at the end of the structure provides unique conditions for the study of gradient limits. This allows high fields in the first cell for a comparatively low input power and a field distribution that rapidly falls off along the length of the structure. A maximum gradient of 130 MV/m in the first cell at a pulse length of 100 ns was reached for an input power of 31.9 MW. Details of the conditioning and operation at high-gradient are presented. Various breakdown rate measurements were conducted at different power levels and rf pulse widths. The structure was standard T24 CLIC test structure and was tested in Xbox-3 at CERN., Proceedings of the 12th International Particle Accelerator Conference, IPAC2021, Campinas, SP, Brazil

It has been technically challenging to efficiently couple external radiofrequency (RF) power to cylindrical dielectric-loaded accelerating (DLA) structures. This is especially true when the DLA structure has a high dielectric constant. This contribution presents a novel design of a matching section for coupling the RF power from a circular waveguide to an X-band DLA structure with a dielectric constant ε_{r}=16.66 and a loss tangent \tanθ = 3.43× 10⁻⁵. It consists of a very compact dielectric disk with a width of 2.035 mm and a tilt angle of 60 degrees, resulting in a broadband coupling at a low RF field which has the potential to survive in the high-power environment. To prevent a sharp dielectric corner break, a 45-degree chamfer is added. Moreover, a microscale vacuum gap, caused by metallic clamping between the thin coating and the outer thick copper jacket, is studied in detail. Based on simulation studies, a prototype of the DLA structure with the matching sections was fabricated. Results from preliminary bench measurements and their comparison with design values will also be discussed., Proceedings of the 12th International Particle Accelerator Conference, IPAC2021, Campinas, SP, Brazil

A new X-band high-power rotating mode SLAC Energy Doubler (SLED)-type rf pulse compressor is proposed. It is based on a novel cavity type, a single open bowl-shape energy storage cavity with high Q₀ and compact size, which is coupled to the waveguide using a compact rotating mode launcher. The novel cavity type is applied to the rf pulse compression system of the main linac rf module of the klystron-based option of the Compact Linear Collider (CLIC). Quasi-spherical rotating modes of \rm{TE}_{1,2,4} and \rm{TE}_{1,2,13} are proposed for the correction cavity and storage cavity of the rf pulse compression system respectively. The storage cavity working at \rm{TE}_{1,2,13} has a Q₀ of 240000 and a diameter less than 33 cm. The design of the pulse compressor and in particular of the high-Q cavity will be presented in detail., Proceedings of the 12th International Particle Accelerator Conference, IPAC2021, Campinas, SP, Brazil

The CLIC main linac uses X-band traveling-wave normal conducting accelerating structures. The RF power not used for beam acceleration nor dissipated in the resistive wall is absorbed in two high power RF loads that should be as compact as possible to minimize the total footprint of the machine. In recent years, CERN has designed, fabricated and successfully tested several loads produced by additive manufacturing. With the current design, only one load can be produced in the 3D printing machine at a time. The aim of this study is optimizing the internal cross-section of loads in order to create a stackable design to increase the number of produced parts per manufacturing cycle and thus decrease the unit price. This paper presents the new design with an optimization of the internal vacuum part of the so-called RF spiral load. In this case, RF and mechanical designs were carried out in parallel. The new cross section has showed good RF reflection reaching less than -30 dB in simulations. The final load is now ready to be manufactured and high-power tested. This new load will not only provide cost saving but also faster manufacturing for mass production., Proceedings of the 12th International Particle Accelerator Conference, IPAC2021, Campinas, SP, Brazil

CERN has built a 750 MHz radio-frequency quadrupole (RFQ), called PIXE-RFQ, for proton-induced X-ray emission analysis (PIXE) of cultural heritage artwork. The PIXE-RFQ is part of the MACHINA collaboration (Movable Accelerator for Cultural Heritage In-situ Non-destructive Analysis) between CERN and INFN, and the second RFQ worldwide to operate at this high frequency. The standalone PIXE-RFQ accelerates protons to 2 MeV over a length of one meter. It features two individually brazed modules, sixteen slug tuners, and a single power coupler. By mid-2020, RF measurements of the RFQ and its components, as well as field and frequency tuning have been completed. Results and the novel tuning procedure are reported in this note.

Landau damping is a powerful mechanism to suppress impedance-driven coherent instabilities in circular accelerators. In the transverse planes it is usually introduced by means of magnetic octupoles. We will discuss a method to generate the required incoherent betatron frequency spread through detuning with the longitudinal rather than the transverse amplitudes. The approach is motivated mainly by the high-brightness, low transverse emittance beams in future colliders where detuning with the transverse amplitudes from magnetic octupoles becomes significantly less effective. Two equivalent methods are under study: a radio-frequency quadrupole cavity and the nonlinear chromaticity. The underlying beam dynamics mechanisms are explained based on a recently extended Vlasov theory and relevant results are discussed for dierent longitudinal beam distributions under specific approximations. Finally, the analytical studies are benchmarked against numerical simulations employing a circulant matrix and a macroparticle tracking model., CERN Yellow Reports: Conference Proceedings, Vol. 9 (2020): Proceedings of the ICFA mini-Workshop on Mitigation of Coherent Beam Instabilities in Particle Accelerators, Zermatt, Switzerland, 23–27 September 2019

Scientific reports 9(1), 19912 (2019). doi:10.1038/s41598-019-56433-8, Transverse deflection structures are useful devices for characterizing the longitudinal properties of bunches in electron accelerators. With efforts to produce ever-shorter bunches for applications such as external injection into novel accelerator structures, e.g. plasma cells or dielectric structures, the applicability of deflection structures to measuring ultrashort bunches has been considered. In this paper, charge-density and bunch-length measurements of femtosecond and subfemtosecond bunches at the ARES linac at the SINBAD facility at DESY are studied with simulations and the limitations discussed in detail. The novel polarizable X-band transverse deflection structure (PolariX-TDS) will allow the streaking of bunches at all transverse angles, making a 3D charge-density reconstruction of bunches possible, in addition to the standard 1D charge-density reconstruction and bunch-length measurements. These various measurements of the charge-density distributions of bunches have been simulated, and it is shown that useful information about ultrashort bunches down to subfemtosecond lengths may be obtained using the setup planned for the ARES linac., Published by Macmillan Publishers Limited, part of Springer Nature, [London]

A reliable impedance model of a particle accelerator can be built by combining the beam coupling impedances of all the components. This is a necessary step to be able to evaluate the machine performance limitations, identify the main contributors in case an impedance reduction is required, and study the interaction with other mechanisms such as optics nonlinearities, transverse damper, noise, space charge, electron cloud, beam-beam (in a collider). The main phases to create a realistic impedance model, and verify it experimentally, will be reviewed, highlighting the main challenges. Some examples will be presented revealing the levels of precision of machine impedance models that have been achieved., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

The tolerances are essential for defining the fabrication shape accuracy specification as well as the structure temperature control. In this paper, the tolerance study for a single accelerating structure of the CLIC main linac is described. The change in the nominal accelerating voltage caused by systematic temperature errors, systematic geometrical errors and random geometrical errors has been investigated. The tolerances related to the change in the field distribution and the reflections in the structure have also been analysed.

The three X-band test facilities currently at CERN aim at qualifying CLIC structures prototypes but are also exten- sively used to qualify X-band components operation at high power. In order to upgrade one of the facilities from a single test line to a double test line facility, a high power variable splitter and variable phase shifter have been designed and manufactured at CERN. They have been power tested, first in a dedicated test and also in their final configuration, to en- sure stable power operation before installing them together with an accelerating structure. In this paper, we broadly describe the RF and mechanical design, manufacturing and low power measurements agreement with simulations. We report the high power qualification of both components and their suitability to be used in existing and planned X-band facilities., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

This paper describes the concept of a primary electron beam facility at CERN, to be used for dark gauge force and light dark matter searches. The electron beam is produced in three stages: A Linac accelerates electrons from a photo-cathode up to 3.5 GeV. This beam is injected into the Super Proton Synchrotron, SPS, and accelerated up to a maximum energy of 16 GeV. Finally, the accelerated beam is slowly extracted to an experiment, possibly followed by a fast dump of the remaining electrons to another beamline. The beam parameters are optimized using the requirements of the Light Dark Matter eXperiment, LDMX, as benchmark., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

39th International Free-Electron Laser Conference, FEL19, Hamburg, Germany, 26 Aug 2019 - 30 Aug 2019; JACoW Publishing, Geneva, Switzerland 396-399 (2019). doi:10.18429/JACOW-FEL2019-WEP036, A collaboration between DESY, PSI and CERN has been established to develop and build an advanced modular X- band transverse deflection structure (TDS) system with the new feature of providing variable polarization of the deflecting force. The prototype of the novel X-band TDS, the Polarizable X-band (PolariX) TDS, was fabricated at PSI following the high-precision tuning-free production process developed for the C-band Linac of the SwissFEL project. Bead-pull RF measurements were also performed at PSI to verify, in particular, that the polarization of the dipole fields does not have any rotation along the structure. The high-power test was performed at CERN and now the TDS is at DESY and has been installed in FLASHForward, where the first streaking experience with beam will be accomplished. We summarize in this paper the status of the project, the results of the bead-pull measurements and the high power test., Published by JACoW Publishing, Geneva, Switzerland