The pursuit of optimal beam quality and stability in linear accelerators (Linacs) stands as a cornerstone of accelerator physics. However, the presence of High Order Modes (HOMs) within Linacs, particularly in the context of energy recovery (ERLs), presents formidable challenges to beam quality and stability. In response to this challenge, the development of the Compact HOMEN (High Order Mode Evolution based on Energy budget) model has emerged, providing precise prediction and analysis of HOM effects on beam dynamics within superconducting cavities. This model facilitates meticulous optimization strategies, guiding researchers towards unprecedented advancements in high-brightness accelerated electron beam technology. By comprehensively understanding and managing HOMs, Linacs can achieve enhanced performance and efficiency, crucial for a myriad of scientific and industrial applications. Through this study, we underscore the constraints posed by high currents and high repetition rate to ensure an optimal energy recuperation. Our findings not only deepen the understanding of ERL facilities but also underscore their transformative potential in shaping the forefront of accelerator technology.

This paper reports the status and recent progress of INFN LASA’s in-kind contribution to the PIP-II project at Fermilab, with updates on key activities and major procurements. Production efforts for the 38 INFN LASA-designed, 5-cell cavities (β=0.61) for the LB650 section of the linac are underway and two pre-series prototypes are being realized as a first step to validate the manufacturing and treatment sequence. Concurrently, preliminary testing on existing prototypes is progressing to gain a deeper understanding of the surface preparation and qualification procedures, including cross-validation at different infrastructures. Series LB650 cavities will be industrially produced and surface-treated to achieve the stringent performance targets, qualified via vertical cold tests at DESY AMTF, and delivered installation-ready for string assembly.

Muons offer a unique opportunity to build a compact high-energy electroweak collider at the 10 TeV scale. A Muon Collider enables direct access to the underlying simplicity of the Standard Model and unparalleled reach beyond it. It will be a paradigm-shifting tool for particle physics representing the first collider to combine the high-energy reach of a proton collider and the high precision of an electron-positron collider, yielding a physics potential significantly greater than the sum of its individual parts. A high-energy muon collider is the natural next step in the exploration of fundamental physics after the HL-LHC and a natural complement to a future low-energy Higgs factory. Such a facility would significantly broaden the scope of particle colliders, engaging the many frontiers of the high energy community. The last European Strategy for Particle Physics Update and later the Particle Physics Project Prioritisation Panel in the US requested a study of the muon collider, which is being carried on by the International Muon Collider Collaboration. In this comprehensive document we present the physics case, the state of the work on accelerator design and technology, and propose an R&D project that can make the muon collider a reality.

Multi-alkali antimonide photocathodes, particularly potassium–cesium-antimonide, have gained prominence as photoemissive materials for electron sources in high-repetition-rate FEL applications due to their properties, such as low thermal emittance and high sensitivity in the green wavelength. To explore the potential of these materials in high-gradient RF guns, a collaborative effort was undertaken between DESY PITZ and INFN-LASA to develop and study multi-alkali photocathode materials. A batch of three KCsSb photocathodes and one NaKSb(Cs) photocathode was grown on molybdenum substrates using a sequential deposition method in the new preparation system at INFN LASA. These cathodes were successfully transferred and tested in the high-gradient RF gun at PITZ. Following the tests, a post-operational optical study was conducted on all the cathodes. Based on these findings, efforts are underway to optimize the fabrication recipes for KCsSb and NaKSb(Cs) photocathodes to achieve lower field emission and longer lifetimes. This contribution summarizes the experimental results of the production, operational performance, and post-usage analysis of the current batch of cathodes.

This document is comprised of a collection of consolidated parameters for the key parts of the muon collider. These consolidated parameters follow on from the October 2024 Preliminary Parameters Report. Attention has been given to a high-level consistent set of baseline parameters throughout all systems of the complex, following a 10 TeV center-of-mass design. Additional details of the designs contributing to this baseline design are featured in the appendix. Likewise, explorative variations from this baseline set can be found in the appendix. The data is collected from a collaborative spreadsheet and transferred to overleaf.

As part of its ongoing and future contributions to high-Q/high-G activities in major international projects such as PIP-II, ILC Technology Network, and the European Strategy for Particle Physics, INFN-LASA is upgrading its experimental facility for vertical cold tests of superconducting cavities. This upgrade will enable cavity performance characterization in a low residual magnetic field environment and with dedicated diagnostics for understanding possible performance limitation. In parallel, state-of-the-art surface treatments aimed at achieving high-Q and high-G performance are being developed and applied to single and multicell cavities at different frequencies. This paper presents the current status of the facility, its key features, an overview of cavities currently in production, and the experimental results obtained to date.

Plasma treatment has proven effective in recovering and reducing field emission in the affected superconducting radiofrequency (SRF) cavities. A joint effort is underway between CEA, ESS and INFN to apply this technique to the treatment of elliptical cavities in the ESS linac. This paper presents the work done so far, which aims at both the development of the plasma process for cavities in the cryomodule and the treatment of cavities in the vertical test configuration. The peculiarity of ESS cavities compared with typical cavities at 1.3 GHz is the absence of couplers for higher orders.

The Circular Electron Positron Collider (CEPC) is a large scientific project initiated and hosted by China, fostered through extensive collaboration with international partners. The complex comprises four accelerators: a 30 GeV Linac, a 1.1 GeV Damping Ring, a Booster capable of achieving energies up to 180 GeV, and a Collider operating at varying energy modes (Z, W, H, and ttbar). The Linac and Damping Ring are situated on the surface, while the Booster and Collider are housed in a 100 km circumference underground tunnel, strategically accommodating future expansion with provisions for a Super Proton Proton Collider (SPPC). The CEPC primarily serves as a Higgs factory. In its baseline design with synchrotron radiation (SR) power of 30 MW per beam, it can achieve a luminosity of 5e34 /cm^2/s^1, resulting in an integrated luminosity of 13 /ab for two interaction points over a decade, producing 2.6 million Higgs bosons. Increasing the SR power to 50 MW per beam expands the CEPC's capability to generate 4.3 million Higgs bosons, facilitating precise measurements of Higgs coupling at sub-percent levels, exceeding the precision expected from the HL-LHC by an order of magnitude. This Technical Design Report (TDR) follows the Preliminary Conceptual Design Report (Pre-CDR, 2015) and the Conceptual Design Report (CDR, 2018), comprehensively detailing the machine's layout and performance, physical design and analysis, technical systems design, R&D and prototyping efforts, and associated civil engineering aspects. Additionally, it includes a cost estimate and a preliminary construction timeline, establishing a framework for forthcoming engineering design phase and site selection procedures. Construction is anticipated to begin around 2027-2028, pending government approval, with an estimated duration of 8 years. The commencement of experiments could potentially initiate in the mid-2030s.

This document is comprised of a collection of updated preliminary parameters for the key parts of the muon collider. The updated preliminary parameters follow on from the October 2023 Tentative Parameters Report. Particular attention has been given to regions of the facility that are believed to hold greater technical uncertainty in their design and that have a strong impact on the cost and power consumption of the facility. The data is collected from a collaborative spreadsheet and transferred to overleaf.

FLASH at DESY (Hamburg, Germany) and the European XFEL photoinjectors are operated by laser driven RF-guns. For both user-facilities cesium telluride (Cs 2 Te) photocathodes are successfully used since several years. We present recent data on the lifetime and quantum efficiency (QE) of the current photocathode at FLASH #105.2, operated before and after a long shutdown. In addition, data for the cathodes that recently have been exchanged at the European XFEL will be presented.

A muon collider would enable the big jump ahead in energy reach that is needed for a fruitful exploration of fundamental interactions. The challenges of producing muon collisions at high luminosity and 10 TeV centre of mass energy are being investigated by the recently-formed International Muon Collider Collaboration. This Review summarises the status and the recent advances on muon colliders design, physics and detector studies. The aim is to provide a global perspective of the field and to outline directions for future work.

The International Muon Collider Collaboration (IMCC) [1] was established in 2020 following the recommendations of the European Strategy for Particle Physics (ESPP) and the implementation of the European Strategy for Particle Physics-Accelerator R D Roadmap by the Laboratory Directors Group [2], hereinafter referred to as the the European LDG roadmap. The Muon Collider Study (MuC) covers the accelerator complex, detectors and physics for a future muon collider. In 2023, European Commission support was obtained for a design study of a muon collider (MuCol) [3]. This project started on 1st March 2023, with work-packages aligned with the overall muon collider studies. In preparation of and during the 2021-22 U.S. Snowmass process, the muon collider project parameters, technical studies and physics performance studies were performed and presented in great detail. Recently, the P5 panel [4] in the U.S. recommended a muon collider R D, proposed to join the IMCC and envisages that the U.S. should prepare to host a muon collider, calling this their "muon shot". In the past, the U.S. Muon Accelerator Programme (MAP) [5] has been instrumental in studies of concepts and technologies for a muon collider.

Due to their excellent photoemissive properties, especially low thermal emittance and high sensitivity in the green wavelength, multi-alkali antimonide photocathodes, in particular, cesium–potassium–antimonide, emerged as prominent photoemissive materials for the electron sources of high-repetition-rate FEL applications. To explore its feasibility of operating in a high-gradient RF gun, DESY collaborated with INFN LASA to develop multi-alkali photocathode materials. In this report, we describe the recipe of K-Cs-Sb photocathodes, which were grown on a Mo substrate by varying the foundational Sb layer thickness using sequential deposition techniques. This report also illustrates the information regarding the film thickness, substrate temperature, deposition rate, and its possible effects on the photocathode’s properties. In addition, the influence of temperature on the cathode degradation is also summarized. Furthermore, in the framework of density functional theory (DFT), we investigated the electronic and optical properties of the K2CsSb material. The optical properties, such as dielectric function, reflectivity, refracting index, and extinction coefficient, were evaluated. The correlation between the calculated and measured optical properties, such as reflectivity, provides a better and more efficient strategy to rationalize and understand the photoemissive material’s properties.

This paper presents a quantitative description of field emission in superconducting rf cavities. The goal is to fully reconstruct the physics involved in the phenomenon, from electron current generation at the emitter to the radiation pattern measured by the x-ray detector. The field emission process in the cavity was reconstructed using CST Particle Studio, and then the generation of x-ray radiation and its propagation toward the detectors were modeled analytically. The entire model was then applied to the specific case of the European Spallation Source medium beta superconducting cavities, and the theoretical predictions were cross-checked with experimental data collected during cavity vertical tests.

This paper presents a new approach to space charge dominated beamline design using an artificial intelligence (AI)-based optimization code named giotto. The code incorporates advanced algorithms for multiobjective genetic optimizations in particle accelerators, allowing efficient exploration of the parameter space and improved beam quality. The study demonstrates the application of giotto in the design of a high-brightness injector for an Energy Recovery Linac (ERL) called BriXSinO. The optimized injector features a low energy (4.5 MeV) and relatively high bunch charge (100 pC) operation. The results show promising beam parameters comparable to other ERL projects. Furthermore, the paper introduces innovative techniques, including bunches back-rotation the use of Lorentzian distributions in the fitness function. The approach successfully achieves dispersion closure in a space charge dominated dogleg. Overall, this work contributes to the advancement of accelerator science, offering a powerful methodology for beamline design and optimization. The new techniques and methodologies introduced have the potential to enhance the performance and stability of particle accelerators in various applications.

The European Workshop on Photocathodes for (particle) Accelerator Applications (EWPAA) brings together experts in the field of photocathode based electron sources for use in particle accelerators, with the aim of sharing their knowledge and latest research and development progress in this crucial field of particle accelerator science. The workshop is convened every other year, and is thus complementary to the P3 workshop (Photocathode Physics for Particle accelerators) run in the USA. Consequently, there is a workshop focusing on photocathodes for particle accelerator applications convened every year, either in Europe or the USA. The EWPAA 2022 is the 4th meeting in this workshop series. The event was hosted by the INFN LASA Institute in Milan in collaboration with University of Milano between September 20th and 22nd. The programme was organised with 7 working groups, with each oral contribution assigned to the most appropriate group. This report presents, summarised by the work group conveners, the innovative ideas, the challenges and the main points raised by each of the speakers.

Electron photoinjectors provide high-brightness electron beams to numerous research applications in physics, chemistry, material, and life sciences. Semiconductor photocathodes are widely used here, as they enable the production of low-emittance beams with variable charge at high repetition rates. One of the key figures of merit of photocathodes is the minimum achievable bunch length. In semiconductor cathodes, this is dominated by scattering effects and varying penetration depths of the extracting photons, which leads to a characteristic electron emission function. We present a method to determine this cathode time response with resolution on the tens of femtoseconds level, breaking the resolution barrier encountered in previous studies. The method is demonstrated with cesium-telluride (Cs2Te) and gold cathodes, revealing response times of (184 ± 41) fs up to (253 ± 58) fs for the semiconductor and an upper limit of (93 ± 17) fs for the metal. Monte Carlo simulations of Cs2Te emission benchmarked to these results give detailed information about the cathode material.

We present the design study of an innovative scheme to generate high repetition rate (multi-MHz-class) THz radiation pulses by using an Energy Recovered Super Conducting Linac operating in Continuous Wave mode driving a Free-Electron Laser Oscillator. The FEL performance is illustrated for one and two color operation. Start-to-end simulations are presented to assess the capability of this scheme for typical values of wavelengths of interest in the 10-50 mu m (6-30 THz) range.